Below is a list of well understood mods you can do to your vehicle to squeeze more distance from each drop of fuel. They range from mild to wild, free to expensive. Some admittedly split hairs, while others can make a significant fuel economy difference. EcoModder members have done most, if not all of these.
You'll note a distinct lack of "magic bullet" or "fringe" category modifications here (particularly additives and infomercial gadgets). To understand why they're omitted, spend some time in our "Unicorn Corral".
Disclaimer: some of the modifications on this list may be illegal in some areas. Some could alter your vehicle's driveability or handling characteristics. Others may be potentially damaging, depending on the skill of the mechanic and the ability of the driver to monitor his/her vehicle.
So don't be stupid! Make safety your first priority.
A lot of vehicles (SUVs, crossovers, wagons and minivans particularly) come with roof racks that are rarely if ever used by their owners.
The aerodynamic penalty of roof racks at highway speeds can be significant: they increase both frontal area AND Cd. Removal is usually easy (aftermarket) to moderately difficult (OEM). Removed racks can be reinstalled when needed.
A compromise for OEM racks with difficult to remove side rails/mounts is to remove just the crossbars. Usually this is a simple job.
Aesthetics vs. aerodynamics... A cleaner vehicle vs. cleaner airflow... You decide!
Ironically, mud flaps can be useful when they're used as mounting points for aerodynamic mods such as boat tails behind the tires, or fender skirt attachment points.
Sure, the dealer or that go-fast kid on the corner might've convinced you that spoilers are good for downforce, but in reality, most are merely a cosmetic addition - one that is more likely to add drag than it is to do anything meaningful for handling.
Take it off and your car will not only look stock, but it'll have better aerodynamics.
It isn't always obvious whether a particular spoiler style is an aero help or hindrance. For discussion, see the thread, below.
Side mirrors are absent from most high efficiency concept vehicles for a reason: they increase both frontal area and Cd.
Many jurisdictions do not require a passenger side mirror, so that is most often the one deleted.
Some will replace the outside mirrors with smaller mirrors, or more aerodynamically shaped mirrors from similar models.
Others replace the outside mirror with convex, in-car mirrors or extra width rear view mirrors. Cameras and LCD screens are a favourite of the concept car circuit, though a somewhat costly option.
Radio antennas aren't the biggest things in the world, but they do stick out in the way, and you definitely won't find traditional "whip" style antennas on high-efficiency concept & production cars.
You can either remove it or relocate it to a more aerodynamically friendly location.
Yes, this falls into the "splitting hairs" category.
Shaving your door handles and smoothing out all the lines on your car isn't likely to boost your fuel economy a whole ton. However, if you're really motivated to create a the lowest drag form possible, this is what you'll need to do.
The effect of front wipers on airflow varies from vehicle to vehicle. Some people use RainX and remove their wipers completely, which is a bit risky and not advisable. Others, however, simply remove the blades and store them in their car. When rain threatens, you can easily throw the blades back on. If you're worried about airflow over the wipers, this is probably your best option.
Another hair-splitting option is to replace your blades with a lower profile style.
Moon caps or moon eyes, as they are often known, cut down on aero drag, and can even look pretty cool. The amount of drag reduction varies between different vehicle and wheel styles.
They can be made DIY style with things like pizza pans, or bought at various online retailers.
Some modders will also use clear tape over OEM hubcaps or wheels, though this is obviously less permanent.
Some older headlight assemblies (particularly those with the sealed beam bulbs), have a bucket-style scoop design. Often these can be swapped out for different style headlights, or modified to cut down on aero drag.
Some car models came with both bucket and aero style assemblies, so swapping is a plug 'n' play affair.
One EcoModder member even removed his significantly less aero pop-up style sealed beams with a lower profile flush fitting assembly from another vehicle.
Some different trim levels of the same vehicle model have more aerodynamic bumper styles than others. If your car has the possibility of swapping between multiple different bumper styles, consider the one that is more aerodynamic.
Another option is fabrication/modification of your existing bumper.
As mentioned in one of the previous tips, many cars come with unaerodynamic headlight assemblies. If no swap is easily available, clear covers can be made. These covers clean up the aero while leaving light from the headlights unfiltered.
Grill blocks have two purposes. The first is it cut down on aerodynamic drag by limiting the amount of air that enters the engine bay. The second is to allow the engine/transaxle (fwd vehicles) to warm up more quickly and retain more heat in the colder months by reducing excessive airflow through the engine compartment in colder months.
Grill blocks come in many forms, though most often they are made to be flush with the outside of the bumper for greatest benefit. Simply inserting a piece of cardboard in front of the radiator does not provide the same aerodynamic benefit.
While grill openings are usually oversized for "worst case" applications (towing a trailer through Death Valley with the A/C on), obviously care must be taken to monitor coolant temperature. If your cooling fan runs more after installing a block, you've gone too far. So, pay attention to your temp gauge and making your grill block easy to remove.
Properly designed tire spats or contoured deflectors direct flow around tires in such a way that drag is reduced without increasing outward flow enough to increase wake.
The underside of your car is one of its most aerodynamically dirty areas. Most vehicles will benefit by fitting a smooth undertray - something manufacturers are doing more and more where they want to squeeze the most efficiency from any particular model (eg. Jetta diesel vs. gas; Camry hybrid vs. non-hybrid).
Sheet aluminum or corrugated plastic will do the trick. Just make sure that you don't put any flammable materials near hot exhaust pipes.
Wheel arches & wheels can be a significant area of turbulence depending on the OEM design. Covering the back wheels to smooth airflow at the rear of the vehicle can make a measurable difference in fuel consumption.
This mod is seen on production cars like the 1st generation Honda Insight, GM EV1, and many other efficient concept cars. It's often one of the first attempted by ecomodders because it is rather easy to do.
Front wheel skirts have many of the same benefits as rear wheel skirts, but are a lot harder to make because the front wheels have to steer!
Front skirts have appeared on a few high MPG concept cars, and a few EcoModder members have tackled the project, but this mod is not for the faint of heart.
Radiusing the aft edge of wheel well arches allows air exiting the wheel well to more easily stay attached to/reattach to the side of the body.
While modifying the metalwork itself would be difficult, the mod could also be accomplished by adding & shaping material on the inside of the arch.
Examples of this design approach can be seen on the first generation Honda Insight, GM EV1 and Corvette, VW 1L concept, and Solectria Sunrise prototype EV (pictured).
This mod could be considered a partial wheel/fender skirt, which in combination with a smooth wheel cover may provide some of the benefits of a full skirt.
Manufacturers of high efficiency cars that do not use wheel skirts typically try to minimize the tire/wheel arch gap to assist airflow, in addition to using a wheel offset which has the outside wheel/tire surface close to the side plane of the vehicle. Examples are the 2nd generation Prius, GM Volt, Audi A2.
All those little gaps may not seem like much, but they add up. That's why all the bonneville race cars come sealed up like a coffin, and automakers are starting to pay attention to these areas on production cars.
There are many solutions here, including the use of clear tape, color matching silicone filler, or foam weatherstripping. Others will adjust body panels to minimize gaps.
One of the biggest aerodynamic problems with the majority of bluff body vehicles (most hatchbacks, vans, wagons) is the amount of pressure drag / size of the trailing wake. In other words, the rear of the vehicle is where the big gains are to be made.
That said, a hatchback configuration isn't automatically an aerodynamic death sentence. Several of the most slippery production cars in recent years (1st gen Insight, 2nd gen Prius, Audi A2) have used "Kammback" shapes, where the roofline tapers downward, following a particular shape (a "chopped" teardrop).
It's possible to retrofit this shape onto existing vehicles and gain a measurable improvement in fuel economy.
Taking the partial Kammback to its logical conclusion, this mod dramatically reduces rear pressure drag and minimizes trailing wake.
Of all aero mods, a full boat tail will probably have the single largest effect on reducing fuel consumption, though it is obviously more difficult to construct (and will result in more heads turned than anything other mod!).
This mod involves any radical modification that reduces frontal area (aside from simple removal of a mirror, roof racks, etc., or lowering). This may include chopped tops, or other reforming of the body.
Another radical modification requiring much fabrication. Decreasing the angle between the hood and windshield reduces the amount of pressure build-up at its base and can help maintain laminar flow at the windshield/roof transition.
Extending a dam to the level of the lowest underbody component (possibly lower depending on the vehicle) diverts air away from (around) the most aerodynamically "dirty" area of most vehicles.
Adding air dams (particularly on trucks & SUV's) is a common tactic used by auto makers looking for quick aero-fixes to improve efficiency.
Airflow along the side of a vehicle tends to follow the curvature of the tail lights and bumper part way around to the rear of the car, which can cause more drag than a sharp corner "crease" that promotes clean separation at the trailing edge.
GM designers integrated such a crease at the rear of the Chevy Volt "production" concept, a feature that could be retrofitted onto existing vehicles.
Lowering a vehicle is a common tactic employed by auto manufacturers to improve efficiency in production and concept vehicles.
While not a guaranteed improvement (much depends on the initial vehicle design - particularly the underbody), reduced ride height has multiple aerodynamic benefits, including:
- improved fineness ratio (length/height)
- slightly reduced frontal area (tires and possibly suspension components)
- reduced tire/wheel arch gaps
Lowering may also reduce body roll, and improve handling enough to permit comfortable/safe cornering at higher speeds, thus saving fuel.
Smaller engines get better fuel economy. Swap out that 'ol V6 for a 4 cylinder from the same or similar model for a boost in mileage. Also very useful if you can swap a lean burn engine into your favorite Honda vehicle.
Advancing timing can increase low rpm power which is where most ecodrivers spend their time driving. This can lead to improved BSFC at those rpms. You must be careful not to advance too far as you could encounter engine ping, which could lead to damage (assuming the engine does not have a knock sensor).
Saves your ignition switch from many cycles. It can also make Pulse and Glide and Engine-Off Coasting driving techniques easier to perform.
On some vehicles using the ISO OBD-II protocol (e.g. 1996 and up Geo Metro), switching the ignition off and back on again via the key will cause a ScanGauge to stop recording speed/distance until the unit reboots, which can take some time. A kill switch enables the engine to be stopped without interrupting power to the ScanGauge for uninterrupted data and more accurate readings.
Synthetic oils generally have more stable viscosities across a wide range of temperatures relative to conventional oils. This decreases resistance that the engine must overcome, particularly after cold starts until full operating temperature has been reached.
Economy camshafts are the opposite of "hot" cams favoured by racers. But rather than aiming to increase high RPM performance, economy cams are cut to enhance efficiency at lower engine speeds where ecodrivers tend to drive.
"Cool" cams may have lower lift, duration and more advanced timing than regular cams, resulting in more low end torque at the expense of high end power.
The Geo Metro XFi had a factory economy cam, and owners of garden variety Metros regularly swap it in place of the standard one.
Increasing compression increases the efficiency of an engine with all else being equal. However, this requires a head gasket swap, head shaving, block shaving, or new pistons. It's an involved process, but if you have the engine apart already its worth looking into.
Higher intake charge temperature has been found to increase the flame speed, the combustion reaction rate, the uniformity of the fuel-air mixture and reduce the heat transfer rate though the cylinder walls. This all adds up to the engine using more heat for physical movement and less being wasted.
The downside to this is that hotter air also tends to retard ignition timing and cause engine pinging. Different engines will react differently to warm air intakes and testing will need to be done to see if it will work for your specific vehicle. Saturns are known to react well to warm air intakes.
Alternator deletes have been shown to increase fuel economy as much as 10%. However, extra deep-cycle batteries (and/or another form of power) must be used as a replacement, and their cost may offset any economic savings of reduced fuel consumption.
That said, if your regular starting battery is in need of replacement anyway, it could be replaced with a good deep-cycle battery for not too much more money.
Hydraulic power steering systems continually pump fluid even when steering assist is not needed. The pump takes engine power to run and reduces mileage.
Manual racks have no power assist, and electric racks only use electricity when you actually turn the steering wheel.
By removing the A/C you easily cut 50 pounds of weight from the car, reduce parasitic belt & pulley losses when the system isn't being used, and free up some space in the engine bay for work.
There are alternatives out there for keeping cool without A/C:
Auto manufacturers are moving towards electric pumps for steering and cooling.
Aftermarket retrofits are available for some engines, though the gains are likely to be had only where they are computer controlled so power use is continually optimized based on engine temperature.
In many automatic cars the ECU controls the gear that the transmission is in at any time. People with an automatic transmission can make their car more efficient by manually controlling the gear the car is in making the car shift earlier when accelerating.
Lower engine RPM at a set speed means reduced friction losses and increased engine load to maintain a given power output. Higher engine load increases engine efficiency.
Reduces internal bearing friction leading to less energy being absorbed as heat. Also increases bearing life. Most effective for vehicles driven in cold climates where conventional lubricant viscosity increases as ambient temperatures fall.
Reduces friction between gears leading to less energy being absorbed as heat into the oil. Also increases parts life. Again, most effective for vehicles in cold climates, since the viscosity of conventional lubricants increases as ambient temperature drops.
This mod only applies to cars with automatic transmissions.
The torque converter can provide extra torque and allows the engine and drive shaft to rotate at different speeds. In performing this function the torque converter wastes energy and generates heat. Most if not all torque converters have a lock up ability where the engine shaft and drive shaft can be locked together. When they are locked together the torque converter waists very little energy. The lock up function is often performed by an electrically controlled solenoid.
The ECU will often lock up the torque converter when the car reaches a particular speed however ecomodders may prefer to lock up their torque converter at lower speeds for better efficiency.
Block heaters pre-warm the engine before starting. The reduced warm-up time can dramatically improve fuel economy, especially for short trips. It is recommended to put the heater on a timer that starts to heat the engine no more than about 3 hours before departure to avoid wasting electricity.
Helpful for cars with older batteries, or cars that sit for extended periods (batteries experience slow self-discharge when sitting). Can keep your battery topped off during the day to counter small electrical loads. Can reduce the amount of output needed from your alternator (and therefore fuel burned) by a very small amount.
Reduces rolling resistance which decreases the amount of power your car needs to move.
There are mixed anecdotal reports about improvements in other properties (steering response, wet traction, and tire wear). Some folks report improvements and some report degradations.
Narrow tires present a smaller frontal area and reduce wind resistance. In fact, narrower tires on the CRX HF model were what made its Cd .01 lower than the DX ans Si models.
Cost is high, but if you're planning to replace your tires anyway, this is worth considering.
Auto makers regularly raid the light rims bin when trying to improve a vehicle's fuel economy ratings. Lighter wheels mean less mass to accelerate, which means less energy used. Unit for unit, reducing rotational mass will show greater benefits than reducing static (non-rotating) mass.
This mod is more beneficial in sub/urban stop & go type driving than at constant speeds (highway driving).
Ecomodding Honda owners are particularly fond of the featherweight 13 inch rims originally spec'ed on the frugal Civic VX (pictured).
As auto manufacturers focus on efficiency, one of the first things they will do to save fuel is reduce vehicle mass. EG. Nissan, Toyota, Ford and Mazda have publicly announced plans to shed weight from their lineups.
Weight reduction can be mild or wild, free or expensive, depending on how far you take it: from simply ensuring you don't carry around unneeded junk in the trunk; to replacing the spare tire with an aerosol can of tire sealant and a roadside assistance plan; to removing unused seats and interior panels; to spending money on lightweight rims/panels; to replacing glass with lexan; to driving only with the fuel tank partially full.
Don't forget that the driver's weight is a factor too! If you're looking for another reason to drop those extra pounds, think of your fuel economy.
Adding electric assist or EV mode to an internal combustion vehicle is like the weather: it's one of those topics everyone likes to talk about, but nobody does anything about!
Well, almost nobody.
EcoModder member MD2000 designed a pneumatically actuated, electrically driven 5th wheel for his first generation Honda Insight.
Some companies sell kits to retrofit an electric motor inline with the drive shaft of a larger rear wheel drive vehicle.
And at least one person has built an electrically powered "pusher trailer".
But it must be said that the starting point for an efficient vehicle is one that's maintained in good condition. There's little point in building on a weak foundation.
Particularly if you're driving an older vehicle, there are a number of maintenance items that should be checked to ensure you're starting from the best possible position.
Most ecodrivers know that using cruise control will burn more gas in most situations compared to a smart right foot. The problem is that cruise reacts to inclines & descents in exactly the WRONG way for best efficiency. If only there were a clever way to control the throttle via engine load rather than speed... Well, EcoModder member jomelmaldonado has done just that.
OK, OK... so the practicality of converting an internal combustion vehicle to human power is debatable, but just imagine the fun you (and your team of stokers) will have!
Fuel economy instrumentation is one of the simplest and most effective mods you can make. The ScanGauge fuel economy computer (shameless plug: we sell these here) is the plug-and-play tool of choice for most people who own 1996 or newer North American market vehicles.
Having instant and resettable trip fuel consumption feedback is critical not only for improving driving habits, but for evaluating other mods.
Other aftermarket/commercial options similar to the ScanGauge include plug-in OBD-II scan tools (which may require a laptop to view output), and the PLX Kiwi.
"MPGuino" is a DIY, open source digital fuel economy display for cars with electronic fuel injection that can be made for about $40-50 from readily available components.
It's ideal for people with older cars, where a commercial OBD-II computer can't be used (or isn't affordable).
The SuperMID is a similar open source device (knowledge of the Japanese language for documentation may be useful).
The lowly vacuum gauge is the grand-daddy of fuel economy instrumentation.
Installing and using one remains a cost effective way to help a driver fine tune certain driving tasks for better efficiency, particularly "driving with load" (aka DWL - see the 100+ Hypermiling Tips).
As well as aiding efficiency, adding a vacuum gauge is useful as an indicator of the general health of an engine.
More than a few EcoModders are big fans of Honda's very efficient lean burn engines, the miserly 1992-1995 Civic VX in particular.
The key to driving one of these engines efficiently, not surprisingly, is keeping them in lean burn mode. Some drivers can "feel" the engine entering and exiting lean burn, but for those who are new at it, or whose butt-o-meters aren't as sensitive, instrumentation is your friend.
EM moderator TomO has written a how-to for using a digital multimeter to monitor lean burn. He's also possibly working on an LED indicator.
This is the same idea as swapping in a taller top gear or final drive in a car or truck transmission, except that on a motorcycle, it's far easier to do and much less expensive.
Swapping to a smaller rear sprocket is a commonly practiced mod to lower RPM and get better fuel economy in top gear cruising.
While motorcycles, scooters and bicycles have small projected areas relative to cars & trucks, their coefficients of drag are generally much higher.
For a motorcycle, a Cd of .50 is considered low, and .90 medium, and 1.00 high.
So the addition of aerodynamic fairings that can part the air flow more gently at the front and shepherd it back together at the rear will be apparent at the fuel pump.
A lot of vehicles (SUVs, crossovers, wagons and minivans particularly) come with roof racks that are rarely if ever used by their owners.
Aesthetics vs. aerodynamics... A cleaner vehicle vs. cleaner airflow... You decide!
Sure, the dealer or that go-fast kid on the corner might've convinced you that spoilers are good for downforce, but in reality, most are merely a cosmetic addition - one that is more likely to add drag than it is to do anything meaningful for handling.
Side mirrors are absent from most high efficiency concept vehicles for a reason: they increase both frontal area and Cd.
Radio antennas aren't the biggest things in the world, but they do stick out in the way, and you definitely won't find traditional "whip" style antennas on high-efficiency concept & production cars. 
Yes, this falls into the "splitting hairs" category. 
The effect of front wipers on airflow varies from vehicle to vehicle. Some people use RainX and remove their wipers completely, which is a bit risky and not advisable. Others, however, simply remove the blades and store them in their car. When rain threatens, you can easily throw the blades back on. If you're worried about airflow over the wipers, this is probably your best option.
Moon caps or moon eyes, as they are often known, cut down on aero drag, and can even look pretty cool. The amount of drag reduction varies between different vehicle and wheel styles.
Some older headlight assemblies (particularly those with the sealed beam bulbs), have a bucket-style scoop design. Often these can be swapped out for different style headlights, or modified to cut down on aero drag.
Some different trim levels of the same vehicle model have more aerodynamic bumper styles than others. If your car has the possibility of swapping between multiple different bumper styles, consider the one that is more aerodynamic.
As mentioned in one of the previous tips, many cars come with unaerodynamic headlight assemblies. If no swap is easily available, clear covers can be made. These covers clean up the aero while leaving light from the headlights unfiltered.
Grill blocks have two purposes. The first is it cut down on aerodynamic drag by limiting the amount of air that enters the engine bay. The second is to allow the engine/transaxle (fwd vehicles) to warm up more quickly and retain more heat in the colder months by reducing excessive airflow through the engine compartment in colder months.
Properly designed tire spats or contoured deflectors direct flow around tires in such a way that drag is reduced without increasing outward flow enough to increase wake.
Just as entire vehicle rear ends can benefit from boat tailing, so can individual wheels/tires. 
The underside of your car is one of its most aerodynamically dirty areas. Most vehicles will benefit by fitting a smooth undertray - something manufacturers are doing more and more where they want to squeeze the most efficiency from any particular model (eg. Jetta diesel vs. gas; Camry hybrid vs. non-hybrid). 
Wheel arches & wheels can be a significant area of turbulence depending on the OEM design. Covering the back wheels to smooth airflow at the rear of the vehicle can make a measurable difference in fuel consumption.
Front wheel skirts have many of the same benefits as rear wheel skirts, but are a lot harder to make because the front wheels have to steer!
Radiusing the aft edge of wheel well arches allows air exiting the wheel well to more easily stay attached to/reattach to the side of the body.
This mod could be considered a partial wheel/fender skirt, which in combination with a smooth wheel cover may provide some of the benefits of a full skirt.
All those little gaps may not seem like much, but they add up. That's why all the bonneville race cars come sealed up like a coffin, and automakers are starting to pay attention to these areas on production cars.
One of the biggest aerodynamic problems with the majority of bluff body vehicles (most hatchbacks, vans, wagons) is the amount of pressure drag / size of the trailing wake. In other words, the rear of the vehicle is where the big gains are to be made.
Taking the partial Kammback to its logical conclusion, this mod dramatically reduces rear pressure drag and minimizes trailing wake. 
This mod involves any radical modification that reduces frontal area (aside from simple removal of a mirror, roof racks, etc., or lowering). This may include chopped tops, or other reforming of the body.
Another radical modification requiring much fabrication. Decreasing the angle between the hood and windshield reduces the amount of pressure build-up at its base and can help maintain laminar flow at the windshield/roof transition.
Extending a dam to the level of the lowest underbody component (possibly lower depending on the vehicle) diverts air away from (around) the most aerodynamically "dirty" area of most vehicles.
Airflow along the side of a vehicle tends to follow the curvature of the tail lights and bumper part way around to the rear of the car, which can cause more drag than a sharp corner "crease" that promotes clean separation at the trailing edge.
Lowering a vehicle is a common tactic employed by auto manufacturers to improve efficiency in production and concept vehicles. 
Pickup trucks have high coefficients of drag. Fortunately, they're also ideal platforms for adding this highly effective aerodynamic modification.
Not quite as effective as a full aeroshell, a tonneau cover can still provide a measurable fuel economy benefit vs. an open bed.
Thinner oil decreases internal resistance that the engine must overcome. This leaves you with a bit better fuel economy and power.
Smaller engines get better fuel economy. Swap out that 'ol V6 for a 4 cylinder from the same or similar model for a boost in mileage. Also very useful if you can swap a lean burn engine into your favorite Honda vehicle.
Advancing timing can increase low rpm power which is where most ecodrivers spend their time driving. This can lead to improved BSFC at those rpms. You must be careful not to advance too far as you could encounter engine ping, which could lead to damage (assuming the engine does not have a knock sensor).
Saves your ignition switch from many cycles. It can also make Pulse and Glide and Engine-Off Coasting driving techniques easier to perform. 
Synthetic oils generally have more stable viscosities across a wide range of temperatures relative to conventional oils. This decreases resistance that the engine must overcome, particularly after cold starts until full operating temperature has been reached.
Economy camshafts are the opposite of "hot" cams favoured by racers. But rather than aiming to increase high RPM performance, economy cams are cut to enhance efficiency at lower engine speeds where ecodrivers tend to drive.
Increasing compression increases the efficiency of an engine with all else being equal. However, this requires a head gasket swap, head shaving, block shaving, or new pistons. It's an involved process, but if you have the engine apart already its worth looking into.
Higher intake charge temperature has been found to increase the flame speed, the combustion reaction rate, the uniformity of the fuel-air mixture and reduce the heat transfer rate though the cylinder walls. This all adds up to the engine using more heat for physical movement and less being wasted.
Alternator deletes have been shown to increase fuel economy as much as 10%. However, extra deep-cycle batteries (and/or another form of power) must be used as a replacement, and their cost may offset any economic savings of reduced fuel consumption. 
Hydraulic power steering systems continually pump fluid even when steering assist is not needed. The pump takes engine power to run and reduces mileage. 
By removing the A/C you easily cut 50 pounds of weight from the car, reduce parasitic belt & pulley losses when the system isn't being used, and free up some space in the engine bay for work. 
Cars have used on-demand electric radiator fans for decades, but some trucks still have less efficient, belt-driven ones.
Auto manufacturers are moving towards electric pumps for steering and cooling.
Manual transmissions provide more efficient acceleration and give greater control of the car to its driver.
Lower engine RPM at a set speed means reduced friction losses and increased engine load to maintain a given power output. Higher engine load increases engine efficiency.
Reduces internal bearing friction leading to less energy being absorbed as heat. Also increases bearing life. Most effective for vehicles driven in cold climates where conventional lubricant viscosity increases as ambient temperatures fall.
Reduces friction between gears leading to less energy being absorbed as heat into the oil. Also increases parts life. Again, most effective for vehicles in cold climates, since the viscosity of conventional lubricants increases as ambient temperature drops.
Block heaters pre-warm the engine before starting. The reduced warm-up time can dramatically improve fuel economy, especially for short trips. It is recommended to put the heater on a timer that starts to heat the engine no more than about 3 hours before departure to avoid wasting electricity.
Placing a exhaust to coolant heat exchanger into the exhaust decreases warm up times. The 3rd gen Prius uses one of these to speed up warming up.
Helpful for cars with older batteries, or cars that sit for extended periods (batteries experience slow self-discharge when sitting). Can keep your battery topped off during the day to counter small electrical loads. Can reduce the amount of output needed from your alternator (and therefore fuel burned) by a very small amount.
Reduces rolling resistance which decreases the amount of power your car needs to move. 
Auto makers regularly raid the light rims bin when trying to improve a vehicle's fuel economy ratings. Lighter wheels mean less mass to accelerate, which means less energy used. Unit for unit, reducing rotational mass will show greater benefits than reducing static (non-rotating) mass.
As auto manufacturers focus on efficiency, one of the first things they will do to save fuel is reduce vehicle mass. EG. Nissan, Toyota, Ford and Mazda have publicly announced plans to shed weight from their lineups.
IF you're the type of driver who has trouble with a lead foot on the open road, an electronic throttle nanny will save you fuel. 
This is the ultimate engine mod: yank the ICE out and do a 100% battery electric conversion. 
Adding electric assist or EV mode to an internal combustion vehicle is like the weather: it's one of those topics everyone likes to talk about, but nobody does anything about!
OK, so technically this isn't a mod.
Most ecodrivers know that using cruise control will burn more gas in most situations compared to a smart right foot. The problem is that cruise reacts to inclines & descents in exactly the WRONG way for best efficiency. If only there were a clever way to control the throttle via engine load rather than speed... Well, EcoModder member jomelmaldonado has done just that. 
OK, OK... so the practicality of converting an internal combustion vehicle to human power is debatable, but just imagine the fun you (and your team of stokers) will have! 
Fuel economy instrumentation is one of the simplest and most effective mods you can make. The 
"MPGuino" is a DIY, open source digital fuel economy display for cars with electronic fuel injection that can be made for about $40-50 from readily available components.
The lowly vacuum gauge is the grand-daddy of fuel economy instrumentation.
More than a few EcoModders are big fans of Honda's very efficient lean burn engines, the miserly 1992-1995 Civic VX in particular.
This is the same idea as swapping in a taller top gear or final drive in a car or truck transmission, except that on a motorcycle, it's far easier to do and much less expensive.
While motorcycles, scooters and bicycles have small projected areas relative to cars & trucks, their coefficients of drag are generally much higher.