Glides like a Brick Wall (Jeep Cherokee, GMC Pick-up)
 

How large of a factor is vehicle aerodynamics as compared to engine size, driving style, gear ratio and all other factors that influence fuel economy?

I am thinking that Aerodynamics is indeed THE largest factor; ESPECIALLY when the vehicle is grossly underpowered as most 'fuel efficient' vehicles seem to be these days.

Between my wife and I, we have two vehicles:

1) a 1999 Jeep Cherokee Sport 4x4 w/ 4.0L inline 6 cylinder engine - 70k miles

2) a 1992 GMC Pickup, 4x4, 5.7L engine - 160k miles

The Jeep gets around 17mpg, whereas the old truck gets about 10.

It's tough out here in the sticks, because the nearest plowed road is about 3 miles away, creating some serious snow between us and town. I had to get rid of my precious Monte Carlo, that I had up to 32mpg (sticker only said 29 when it was new, AND even with 278,000 miles, I got 32mpg!).

Anyway, aside from those three miles, these vehicles suffer from "pushing a wall of wind" syndrome. The truck plows through it better I think, as it's engine is more appropriately scaled for the weight / size of the vehicle. The Jeep, though it has a lot of torque and power, has a tough time pushing the air efficiently when you get near freeway speeds (70-80mph in Michigan), and their fuel economy suffers. I think the last time I checked, the GMC pickup gets about 7mpg at those speeds.

Okay, I look forward to your responses!

Don

Brick walls actually glide a lot better than you'd think... the problem is it takes a lot of fuel to regain the speed after you've lost it.

Barn doors glide poorly, but you use less fuel on the pulse.

With the comments about vehicles being "underpowered", I have to ask if you are trolling. If you aren't, I'd ask if you consider a semi-trailer to be "underpowered", and your reasoning behind that.

It depends on the type of driving you do. If you're doing city driving and never get over 35 mph then aero mods won't help as much as driving 70 mph on the freeway. I think most people don't necessary spend most of there time on the highway so the biggest bank for the buck is driving style and gearing then aero. Some of the aero stuff should be standard on all cars though IMO. Belly pans, some form of grill blocks and wheel deflectors. Oh and something that all cars should have is some kind of mandatory FE instrumentations. :turtle:

Sorry for the confusion, Mr. Firstlaw- not trolling at all.

I don't have too much experience with sub-compacts, but my father in law is an enthusiast of Geo and Kia, and when I drove them, I felt like I really had to push the things to get them to do what I wanted.

So, not "underpowered" in the sense of "my vehicle is more powerful than yours", simply in the sense of "is it really economical to try to push a (relatively) heavy vehicle with the smallest engine possible? Would not make more sense to give a very light vehicle an engine that could essentially 'idle' down the road?"

For example, I know a guy who put a large V6 on a small tricycle type frame, and the unit itself got extremely high mileage. I do not recall the numbers, but I remember wondering if perhaps the unit would not have gotten such good mileage had the engineer chosen a smaller engine.

Okay, all for now.

Pat

Welcome to the site. I will chip in that I think the balance of what's most important depends on the car, but certainly for a truck (as far as physical mods go) I think aeromodding would give you the most bang for your buck.

I actually just downgraded the engine power on my car (in hopes of greater fuel economy). Sure, I can't accelerate as fast, but I can still go down the highway as fast as I need to with quite a bit extra power if I feel like downshifting, :p

Again, welcome. Would be interesting to see some crazy aeromods on the truck.

Ok then, welcome to ecomodder.com!

The answer is "it depends on how you drive it".

If driven correctly, a smaller engine will ALWAYS be more economical, provided that it is correctly geared and that power produced at peak BSFC is more than the load on the engine at constant desired cruising speed on level ground. This is the case with any car you care to name, geo and kia included, and including my car (660cc engine). The exception might be a 105cc unfaired postie bike, but only if driven at top speed.

For top fuel economy, giving it more than about 65% of available power in an Otto cycle engine at that rpm is wasting fuel. At about that power level, you get more bang for buck, i.e. more power at the wheels for a given quantity of fuel. Diesel is different, wide open throttle (WOT) is the most efficient, so as long as your engine is capable of cruising, it is not undersized wrt fuel economy.

Also, for hill climbing, even the smallest engine can move a truck up a hill, given the correct gearing. (I just know there has to be someone more nitpicky than I am who is about to bring up static friction, but I beat you to it. :D) "Give me a place to stand and I will move the earth" - Archimedes. It's all about leverage, not power. Getting there at the same pace you would cruise on the level is a different story however. But this is a site about fuel economy.

No. The reason for that is the engine map of a petrol engine. They are most efficient at load, and get less and less efficient the closer you get to idle. The reason for this is that in a larger engine, at idle, you have more mass to rotate, more surface to rub against, more air to pump, a more closed throttle plate that is choking off the flow through the engine.

You could get a good idea of what it's like to be an Otto cycle engine by forcing yourself to only breathe through a drinking straw every time you slowed down, and a smaller drinking straw when you stop. It's hard work, and you are putting out next to no power. If your aim was to go somewhere, it would be easier to walk or jog along.

It's not a great analogy, but it might help?

Remember not to confuse it with gearing issues. Low power at low rpm is more efficient than the same power at higher rpm, but it is still inefficient compared to a smaller engine at low rpm with a more moderate power loading.

WOW... this is fascinating...

Thank you guys, for the information- really.

My passion and obsession lies far from driving a fuel efficient vehicle, (to me, fuel efficiency is "miles per tank", therefor, my GMC is more efficient because I end up filling it up less than the Jeep), but I am VERY interested in this subject. I cringe at the thought of a country filled with tiny cars, as driving, to me, is an art. I do enjoy feathering my pedal, especially when I had the Monte, as I would try to top my record with each tank, but I ALWAYS have days where I'm all about hammering into and out of corners...

Anyway, I have company and it's very rude for me to be in here typing... I'd better head out!

Guys, I plan on coming back. I love all things mechanical, and I love not the man who goes out and buys a Prius with the notion in mind that he is saving the world, but the man, like you, who appears to be meticulously striving to get the most out of the machine.

Congrats to you, sir. I look forward to hearing more of what you have to say.

Pat

Well, I was going to chime in before reading the entire thread... But I just checked and someone else mentioned it :p Higher load is the way to go for a gasser :D

Here's a nugget to think about... Given a car's shape etc. lets say it takes 20hp to stay at speed on the highway. Weather or not your engine can make 30 or 300hp - you can only make 20hp to maintain a steady speed. So on an engine that makes less power, yes, you do need to push on the pedal a little further... But you're still going to make the same amount of power :thumbup:

Now think about this - the largest source of restriction in your intake is your throttle plate. If you have to keep your throttle open 30% in the smaller car versus 10% in the larger car - which one has more restriction (and therefore more pumping losses to overcome)? :thumbup: <-- this is a bit of an oversimplification, but a quick/easy thing to think about :thumbup::thumbup:



For my last bit... I wouldn't call any of the modern fuel sipper ecoboxes underpowered by any means. For what they are - they're grossly overpowered (my 20hp example above really isn't too far off for cruising at highway speeds :p).

Okay, so your saying my car shifting into it's highest gear while I glide with the pedal off is actually hurting my fuel economy? If It drops the RPM's from 1600 to 750 at 40mph I'm getting worse than pushing the pedal? That's confusing. Also, In MO, I drove my moms Dodge Caliber R/T which has the largest engine available for 07 and is rated at 29mpg highway. Well MO has lots of hills, and going up a hill, with the speed limit at 75 me going 80, you'd go up these hills that are over 2 miles up and the engine would do 3xxx rpm keeping it going 60, I would have to floor it to go atleast 70, whereas in my Jeep I could keep it at 2200 RPM and keep trudging up the hill with no problem. There's where the Underpowered part comes in. If you frequently drive in MO, your gonna have some problems keeping up with the flow without getting rear ended by a cell phone talker.

Do a grille block, I did one on my ZJ, and the results are feel-able. also if you do an belly pan, then it will still look like a Jeep without the other Jeeper's laughing at you. Also do as thin an oil as you can. I just switched from Mobil 1 10w-30 full syn High Mileage to Mobil 1 5w-30 full syn Truck and SUV and noticed, with the grille block, have so far done approx 26mpg this tank ^_^ really good results up from 21mpg! that's only judging of going 78 miles on aprox 3 gallons.

Is the Caliber's aero really that bad? I was able to climb the mountains through Albuquerque in my Jetta @70mph - smallest engine available (110hp versus the Caliber's 14?) - both cars are about the same weight I believe (awd?). Don't know if I could go faster - I just had my cruise control set the whole time. When I had an HPV on my roof - those same climbs were between 55 and 60mph floored (I was also loaded with a summer's worth of gear :p). I hung out in the semi-truck lane :D Going back down was awesome though :p

I know that's a comparison between a sedan and hatch - but that makes me even that much more angry at GM :p

In any case, I personally don't classify a car as a fuel sipper unless it gets >30mpg (my car is on the fence, morso not, without good technique :p). It's almost as if the mfr's break 100hp just for the sake of breaking 100 :/

Gliding is very different than cruising.... But, and I should have made this more clear (my fault), my comparison was between two different engines. One that can make more power versus one that makes less :)

Well, now that I've got 70 hp we'll see how I do up some of those bigger mountains in vermont in a week here, :p Think I might see some second gear action.

I had Cruise on, and to keep around 65 it went from 2000 rpm to about 3000rpm. To go 75 I'd probably have to floor it, or get close to it. The Caliber's aero isn't that bad, I think it's the CVT tranny, you can't glide in it. It's the R/T, so it's got a 2.4L that makes 176ish hp (don't remember) Had the whole car packed and had everyone in there, playing a nintendo gamecube in the back even. I think we went 300 odd miles per tank going there. It's 2wd. I was in the fastlane most of the time until a honda would come behind me at around 90-100 and I'd switch lanes.

Uh... Dodge is a Chrysler vehicle..

I think on that whole trip we got pretty good fuel mileage despite going that fast, I think it was the downhills that helped.

oh, I see.

Do'H

That was pretty bad of me :rolleyes:

What about swapping the GM six into the pickup with a tall enough transmission? It may even be possible to do it for free since SBCs aren't exactly cheap. My pickup is probably just as bad if not worse than yours aerodynamically but I can still pull ~30mpg@55mph because it has a 2.4L engine. I've read/heard ~25mpg with the TBI six is possible if you keep your foot out of it with stock gearing. :turtle:

I always wanted to know the same thing. Like my pickup truck with a 5.7 Hemi gets 16-28 But if you were to put a smaller engine in it would it get worst mpg. They offer it in a 6 cylinder and its is suppost geting worst gas millage. But they say thats because the hemi has mds.

Sorry for not replying to this earlier. I tried to find the thread, but had difficulty. I certainly hope you hadn't interpreted this the wrong way. It's extremely difficult to have a civil discussion about anything when your only tool is 26 letters of the alphabet and a few emoticons.

Thanks for the compliments, I will explain more about fuel economy as I get the time. And glad you are enjoying the site.

Ways to increase efficiency:

Cheap
increase gear (change 3.73 to 3.42) or get taller tires
highway type tires vs off road, narrower is better (tires with higher inflation rates have lower rolling resistance, usually)
free-flowing air filter, exhaust
remove any extra weight
lower the truck
add front air dam
plug any holes in front end
get smaller, more aero mirrors
get under-drive pulleys for engine
tune up motor
get a truck cap or tonneau cover
upgrade to HID headlights and LED bulbs
Let vehicles warm up before driving and/or use block heater

More expensive
swap motor for a duramax w/the 6-speed allison trans and put in a 2.42 or 2.72 gear (should easily get 28mpg at 75mph and will run on bio)

reduce frontal area- narrow tires, small mirrors
reduce weight-swap spare tire for plugs and fix-a-flat, unused trailer hitch
increase gear-taller tires, change ring and pinion
driving style-use cruise control, drive speed limit or slower, try to maintain momentum, don't idle more than 45 seconds, combine errands and plan ahead (no left turns)
extras- turn off headlights in daylight, turn off any extra electronics, don't use A/C

Headlights can reduce your mileage by up to 5%

DonE has left us a few questions, but has given us some important info:

You're nowhere close to a paved road: vehicle failure in the middle of your driveway leaves you walking either 1.5m to the road or 1.5m back to the house. In very inclement weather, this could be fatal.

We don't know if you ever tow anything. Referring again to the miles of unimproved road, I couldn't in good conscience recommend narrow tires.

Is yours a belt-driven fan? Swap out to an electric. If nothing else, the engine warms up faster and it actually frees up a bit of space in the engine bay.

Looking at Smoky's suggestions above, I second all of them except the tires and the final drive changes - hold off on that until the weather is nice and you've got plenty of warm weather in which to shake it down - let any stranding failures happen when you're not likely to freeze to death walking home.

Regardless of Smoky's opinion, consider adding a tonneau or a smoothie bed cap. Mine makes a big difference, and it's possible that those differences experienced are truck-dependent, so one particular conformation of vehicle may get benefits while another doesn't. Aerodynamics can be subtle.

As for the Cruise control.. I get worse mileage with it because I can keep a constant throttle with my foot better than I can with cruise. Less Electrical load. But if your the type that can't seem to keep a steady highway speed, cruise would benefit. Cruise control seems to be more of a speed control, not a throttle control. Hills and cruise=bad mileage if your an ecodriver, if your a regular driver, it depends (regular as in gets on the gas up big hills to keep going 60).

Very true I was in vermont going up a decent size mountin with cruise on. And all of a sudden I knocked it down a gear or two and threw it to the floor. Cruise control is speed control not throtle control. It dont care what kind of rpms your running or how it fast it gets there it just wants to get to the disired speed and stay there. I've had it A couple of times where I was going up hill and had it on and It hit the throttle so fast I started to slide. Fun times. Just for the record never use it in the rain.

Thought you were talking about my Valkyrie. That critter is so draggy that when you roll off the throttle you think you've driven into a bridge abutment.

My big honkin' truck will take a half mile to coast down. I can't coast that bike 200 yards.

Back on topic.

Did I miss something? I think Don indicated he lives in Michigan. Useless he's a yooper, he drives on roads flat as a pool table for the most part. Cruise control is A-OK in flat country.

Don, like most folks in country dominated by tough winters need two different set ups. A set of stock wheels with the snow tires of choice. My own snow strategy has always been "pizza-cutters." Tall skinny tires with aggressive tread that reach down to find something sorta solid. And another set for July and August.

Despite what Ohio State fans tell you, there are periods of dry weather in Michigan. Those dry periods off Don some relief from high fuel consumption. I'll speak to the Chevy pickup, because I know Chevvies better than Jeeps.

That truck probably has GM 12 bolt axles with a 3.73:1 gear ratio. This is a common and flexible setup that lets his 350 rev up and make a lot of power and burn a lot of gas. IIRC, you can get at least 3.08s and maybe 2.73s for the front and rear twelve bolts. If he has a ten bolt he could even go on down to a 2.56 ratio. A lower fear ratio will reduce his fuel burn at any road speed by reducing the engine frictional HP and forcing the engine to operate at a more open throttle setting. The more the throttle opens, the more efficent his engine becomes. R&P sets can be bought and jeep shops do this gear change thing all the time. a numerically lower ratio will help in winter as it will slow down the small-block's tendency to rev up and bury the truck from wheel spin due to the small-blocks huge torque rise.

An aero treatment - fastback or tonneau and air dam - is worthwhile if Don drives a lot over 40 MPH. If he spends most of his nice weather bouncing down country roads at 30 MPH, then weight reduction is the way to go.

Deadma1474,

Are you really getting 28 mpg on the freeway or am I reading this wrong?

Regards,

I've noticed this to happen quite often I think a lot of it has to do witht he transmissions. To get the same power output, the smaller engine has to be at higher rpm than the large one, so they usually stick a higher final drive ratio on them.

As far as underpowered vehicles go, my newest aquisition has a whopping 90hp. I was a little concerned about fuel economy since it only has a 3sp automatic, and not much for aerodynamics. My first trip with it was 800 miles of interstate at 70-90 mph. The pedal was nearly to the floor the whole time, and to my amazement, it retured 26 mpg!! A vast improvement to the Merc. with twice the H.P. and roughly equivelant Cda.