Cd or weight?
 

Imagine that you are buying a new vehicle to modify for fuel economy. You plan on using this vehicle for 50/50 city and highway driving. You have narrowed your choices down to two vehicles which are virtually identical except for the following:

Vehicle A has a 10.0 Cd advantage over Vehicle B.

Vehicle B weighs 1000 lbs less than Vehicle A.

Which one would you choose, and why?

I would be inclined to say that it is easier to reduce the Cd.

1000 lbs is a lot of added weight and you would be making sacrifices to shed a portion of it.

This man has a dog and a mother in law that don't like each other!
How do I know?
Elementary my dear Watson, the dog likes to have it's head out of the window, and the mother in law is a bit on the heavy side.
And he has to choose only one to bring on the vacation trip!

IIRC 10 points is 0.10 Cd.

It's all relative. Beginning Cds and weights are...?

The question is purely hypothetical so actual values do not matter.

However, Vehicle A could have a Cd of 0.25 and weight in at 4000 lbs.
Vehicle B could have a Cd of 0.35 and weigh 3000 lbs.

Yes, but relative values do.

Thanks for describing the size of the ball-park. Power to weight? (not trying to be a troll)

Assume both vehicles have the same engine. An average car can put out around 180 horsepower to the crankshaft.

I going to go ahead and side with JulianEdgar: testing.

I would vote lower cD. Careful driving to minimize brake use can minimalize weight penalty. Inertia isn't the enemy until you convert it to heat.

I would also assume in real life that weight could buy you extra utility.

I can't choose. You have one vote each right now.

Less weight means better 60-foot times but better Cd is going to look better. :confused: Intangibles.

I wouldn't bother making the final decision based on those 2 facts. I want to know reliability of the drivetrain, etc. I'd consider the cd and weight to be byproducts of the vehicle. No reason to try to save gas if you're buying a different car early because of the drivetrain.

That being said, if they truly are the same but with those being the only 2 differences I'd probably go with the heavier, but aero one. Utility reasons mostly. Good technique can trump specs. Looks at funkhoss's caprice.

I've read his build thread. That car really is one of a kind.

There are two vehicles that fit the description of the OP, actually. The drivetrain is very similar between the two. I was just wondering if most people here would prefer aero over weight.

I do 50/50 driving, by distance, every day. The 50 in town takes 5x as long as the 50 on the highway, and averages a dozen stop/starts. There is a long hill on the highway to boot. Lighter weight will win every time.

I'd assume the bigger car would need a bigger engine too, so it would have a hell of a time catching up to the lighter. Would need to be 1:2 ratio or higher to ever make it worth it. And, as point out, aero can be worked on...you'll never be able to take 1000lbs out of the bigger car, and it would be a pain to shrink the engine size.

All of this assuming a plain old gasser. Hybrid or diesel and things would narrow a fair bit. Also gotta assume the heavier car can carry more (people) and be more comfortable. I rather like a comfortable ride...it's worth a few bucks a week in extra fuel.

Similar to what I was about to post.

That city driving time is a real killer.

Will never make up for it on the highway.

I mean my S10 probably gets 10mpg in the city, and 20 mpg on the hwy.

No improvement in Cd is ever going to tilt the scales back.

In the real world changing out my spark plugs has had the greatest affect.

Therefore if given a third choice to vote on would say "engine efficiency" is most important.

Going further I's say drivetrain efficiency overall is most important because putting the S10 into 4WD is like throwing out a boat anchor with large chain attached.

In short, everything is important, and the ratios of this or that are moving targets at best.

My intuition says the lower Cd vehicle will be considerably more efficient from the factory, but I'd still rather own the lighter one.

In the real world I picked "both" of course. ;)

Weight vs CD
 

I vote lighter weight. The CD value hurts economy as the car goes faster, while weight must always be moved by the power source. Around town weight is a bigger concern but CD has little effect at in-town speeds. YES, I understand CD is always a consideration, but slower driving is a large portion of daily use. On a long-haul vehicle CD is much more of a concern...but so is weight. Weight is always a concern on long-haul or in town. An old adage heard at Bonneville is, it takes twice as much power to go from 200 mph to 250 mph as it does to get to 200 mph. Drag becomes a major factor as speed increases. Most of us never go 200 mph. I once went 140 mph in a Ford powered Cobra, and I believe that is my fastest ride.

decision
 

I'm already running around a 900-pound weight liability with the crude aero package on the Toyota pickup, however, the HWY mpg is so significantly improved, it has dragged the average mpg upwards in spite of the inertial penalty. So I voted for aero.
When I increased the weight of my Honda CRX to that of a Ford Taurus, and added a 3rd axle, I only lost 2-mpg on the open road ( 50-mpg vs 52 ).

Take the CD, my 'full size' van weighs 2.2 tons, and gets over 30mpg in all city driving (20mph avg speed).

In previous testing (over two years), I've found the weight of a trailer makes no difference to city fuel economy, it can all be accounted for in the aero drag penalty.

Don't brake and weight won't hurt you, and a heavier vehicle that glides further is actually much nicer to hypermile.

In short you can drive a heavy vehicle appropriately, with a draggy vehicle your only choice is to drive slower.

CD v Weight
 

Drive ANY vehicle at 20 MPH average and your MPG will be great. In Texas 20 MPH speed signs are only seen in school zones. Also, a 2.2 ton vehicle is NOT a good comparison to a daily driver weighing 1500 pounds less. DRAG is not a force to be dealt with until the vehicle goes faster. In fact, the faster one goes the more CD impacts economy. You are correct about stopping and starting. As the science says, A MASS IN MOTION TENDS TO STAY IN MOTION, AND A MASS AT REST TENDS TO STAY AT REST. One must use energy to move a vehicle off a static position, and use more energy to stop it. Stop-and-go in town is probably the single biggest drain on MPG. Your 2.2 ton vehicle seems to be set up perfectly for the task at hand. Congratulations.

Why in the world would you assume that a 20 mph average means 20mph speed limits? 20mph average means lots of stop and go (average 1 traffic light per km). Weight should kill my mpg but it doesn't, I simply drive around the fact that I've got a 1.6l engine out of a hatchback pulling a train.

Drive any (ICE) vehicle at 20mph (3rd gear in my tall gearbox) and you get abysmal fuel economy. I see about 25mpg - same as I get at 70mph in 6th. At 40mph I get 60mpg.

At 40-45mph (the typical 'city' speed limit here) more than half your energy goes into over coming aerodrag, so that's why I'd take lower drag. In theory you could drive slower, but in reality that would mean using a lower gear killing the benefit of slowing down.

When I throw my car in 5th or 6th and idle along at 20mph, I get around 65-70mpg with my 2.4L engine. :turtle:

weight vs drag
 

I guess I misunderstood your post. Looking at a very small engine pulling a heavy vehicle, it looks as if gearing is a major factor, coupled with the number of gears in the tranny, plus the height of the driving tires. Like I said, you must have an almost perfect combination. Overcoming weight is the single biggest factor in stop-and-go city driving as it relates to economy. My work is in building a vehicle for ALL TYPES of driving under normal conditions. If one simply adapts driving habits MPG can be improved, but the vehicle must be built for the task it is assigned. I am an old car builder who puts the overall LOOK of the vehicle near the top of my objectives, with efficiency at the very top, which causes compromises in all areas. A butt-ugly car that gets great MPG probably won't sell, and one that is what we call a Trailer-Queen Show Car, that gets very poor MPG, probably won't sell either. I think my reading of your post was me not understanding your parameters...MY BAD.

Racecar gearing is not a brag on this forum though :rolleyes:

Your engine capacity to weight ratio is about the same as my Jeep (4.0), but even that can't pull 6th until ~40mph.

50% CITY / 50% HWY
 

The premise of the thread contains 50% Hwy driving, which for example, in Texas, today, would include up to 85-mph, legally, between Austin and San Antonio.

I'm getting off into the woods a bit here, but I had, at considerable expense, a custom final drive cut a full 40% taller than what came in the transmission, and installed a 6th gear a further 14% taller, then upsized my tires a bit. I'm turning 650rpm @ 20mph, and 2100 @ 65mph, in a high revving engine (revs out to near 8000).

My fuel economy vs speed curve is approximately:

https://i.imgur.com/xDYwVvS.png


The displacement to weight may be similar to your Jeep's, but the power to weight is closer to that of a base C7 Corvette. It's less efficient than it was with the stock 1L engine, but I'd hardly call it "racecar" gearing.

Interesting list of experience, but all from a single person, a person that is interested in getting maximum fuel mileage.

This means that you probably accelerate with purposeful intent of higher mpg (slow and steady retiree on a Sunday) verses a more typical American that is trying to get there yesterday and their hair is on fire.

Meaning weight penalties for quick acceleration will be far more costly based on driving style.

Acceleration is an investment in kinetic energy, one that would be best made based on results from an engine specific bsfc map. For my engines, those bsfc minima useful for charging my "inertia battery" are at moderate to high load. That coupled with my manual transmission means I have a bit of "jackrabbit starts" which is counterintuitive but works better for me. Others may have vastly different bsfc maps requiring different techniques.

The difference i have found lies in drawing down my "inertia battery" as much as possible via coasting to minimize the conversion of its energy to heat via the brakes. Thus I roll out of the throttle much sooner than most when approaching a stop. I leave additional following distance to have reaction time. Time the redlights. Know the speed you can comfortably take a turn, and the landmark that defines canceling the cruise on approach to that turn. Adjust behaviors based on weather or significant (2000lb+) loads being carried.

Yes there are times you have to slam on the brakes. Deer, other drivers, debris, etcetera. But under normal situations brake use is the enemy of fuel economy in a heavy vehicle.

What amazes me is what I call "straight line" drivers.

Might be the software doing the actual throttle in todays cars, but I notice an acceleration slower than what I do, but where I ease off to do the speed limit they quickly catch up and pass me like I'm standing still. They have to be doing 10-15 over the speed limit.

And an irony, they come to almost a complete stop to turn, like a G-force allergy.

I mean seeing a $75,000 BMW turn at the same speed as a 1970's pickup truck overloaded with scrap metal is mind shattering.

Compared to myself people fly up on stops and brake at last second. No patience for coasting I guess.

Driving style is part of the math here, no doubt.

Also "interesting" is when the straight line drivers tailgate me into a curve, my junk makes the turn no problem while they last second stomp the brakes in their luxury cars ending up out of control and in other lanes... they don't seem to tailgate me after that...

Unless someone is coming the other way and I slow way down for turns so the streight line tailgater doesn't kill them...

I lured a 4WD pickup into a corner once with my Notchback. It went up on three wheels.

OTOH, a red Audi lured me onto a freeway onramp that had a solid sheet of ice one December night. I had to fight to keep the title clear on that one.

In my Porsche while entering the highway on a cloverleaf curve I took the entry ramp extra fast to shake a Fox body Mustang off my tail.

Much to my amazement he stuck to me like glue.

I got a better view of the car and driver as he passed me on the highway.

He had some serious mod's to the car and knew how to drive.

Color me impressed.

Decades earlier a Ford Escort driven by an aggressive young woman (I swear she was going to hit me a couple of times) tried to follow me into a sweeping curve at 15 mile (Maple) and Telegraph, a curve I drove every day into work at "speed" and at my Diesel Golf's limits.

Well, I swept though the turn as she tried to mimic me.

It didn't end well.

She jumped the curb and totally messed up her front end.

I was laughing, school of hard knocks calling.

Back on topic...............seems aircraft have other issues to contend with.

What is the relation between drag and weight?
https://aviation.stackexchange.com/q...rag-and-weight
I suppose hull shape and ship weight will have additional factors to consider as well.

More weight on a road vehicle could mean more tire deformation, more contact area, more rolling resistance.

Ever see an overloaded pickup truck?

Nose up and tail dragging would increase Cd one would think (angle of attack).

Okay, we have a real world example to discuss.


Random TRANSPORTATION pictures - Page 2399 - Pelican Parts Forums
http://forums.pelicanparts.com/uploa...1604348240.jpg

I'm not sure where to start, but the scientist in me says don't guess - test!

I like the look of that. I’d do it, if it weren’t illegal to drive on the road like that around here.

In the early 1980s I drove a used '72 Cadillac hearse. It weighed 6000 lbs and with the 472 cid engine I got 10 mpg city/13 hwy. How does that compare with a pickup truck from the same era?

https://www.classicdriver.com/sites/...ch-cars-01.jpg
https://www.classicdriver.com/sites/...ch-cars-01.jpg
Needs some finish work on the rough edges. I'd do two curved arcs from the A to C-pillars.

Yes...that would look good. And save it from my first thought...folding in half when it hit's it's first big bump!

I think it would be better to go with something like this:

https://ecomodder.com/forum/member-f...1-470-1108.jpg

Pickup MPG
 

In the 70's pickups got horrible MPG, mostly due to rear axle gears and cheap gas. Your Caddy was probably about the same. In 73 the crunch hit and guys started changing rear axles, which dropped pulling capacity but helped MPG. At $1.50 a gallon after .50 cents, it made a difference. I was still in my Hot Rod stage of life but had enough common sense to buy a Ford Falcon Ranchero with a 6-cylinder and automatic for my daily driver. Later I got a 55 Plymouth with a flathead 6 and auto for work commuting. With a major tune-up it got OK MPG at 15 to 17, which for the mid 70's was not bad in traffic. No air conditioning, no cruise, no power anything, but the flathead was bullet proof dependable.

same era pickup
 

My grandfather's '62 Dodge D-100, with the 225-CID slant-six,( a decade early ) with 3-spd 1:1 trans, and 4:56 rear diff was good for 50-mph, and 11-mpg.
A nearly- perfect bolt-in of a 1977 Dodge D-100 4-spd OD, and 3:50 rear axle allowed for 16-mpg at 60-mph.
Some streamlining pushed it to 21.5 mpg at 60 mph, and 91-mph indicated top speed.

Ha! My Clark Cortez got 10mpg with the 225-CID six and a side-draft Carter carburetor.
https://ecomodder.com/forum/member-f...2-p1010018.jpg