Sorry to resurrect an old thread, but I'd like to make a point I didn't see made in the preceding replies.
Given two cars identical accept for the weight, the heavier car will have a greater ratio of weight to aerodynamic drag. When coasting down hills, the propelling force is a proportion of the weight of the vehicle. Where aerodynamic drag is the predominate slowing force, a heavier car could maintain a faster speed down a given slope. Why is this important? Well, because if one car maintains 52 mph EOC'ing down a given slope of the freeway, and the heavier car maintains 60 mph down the same slope, if the driver wants to maintain the minimum 60 mph in order to avoid causing a traffic disruption, guess which car burns less fuel? You guessed it, the heavier car, because the light car will require the engine on and at light throttle to maintain minimum speed (inefficient area of the poweband), while the heavy car will have the engine off.
How about a sweet spot in terms of engine efficiency? If the car is super light, it may be easy to accelerate, but maybe you are not in the sweet spot of the engine's operating efficiency. But, you would have a shorter glide.
If you added weight until you reached the "sweet spot" of engine efficiency, the longer glide may result in a higher "pay back" and thereby justify the extra weight.
Segway/Hijack : Ideal Commute Scenario
Let's say your main commute has an elevation difference. On the way down, there is always "stuff" you can put in your car to add weight and therefore give you longer glides. That "stuff" would not be in your car on the way back (delivering it?), so you would be lighter going uphill.
Example : Offer to drive someone to work, but drop them off "at the train" for the ride home, or visa versa.
Crazier example : Drop someone off at the train on the "up" run. Pick them up at the top. Drive them home without the train on the "down" run.
Just put a huge water tank in your car at work and fill it up there, then empty it when you get home.
if the drag & the engine are the same i don't see how making the car heavier will work unless it's a 1 way trip downhill.
The heavier car will gain more energy going down the hill, and will lose more going up the hill. The lighter car will gain less energy going down the hill, and lose less energy going up the hill.
Also wouldn't more weight equal more friction with the road?
Ya, baddog, the kind of hills where a little more weight might theoretically help would be hills where you would never hit the brakes on the way down. More weight is a sure loser if you are having to press the brakes going down hills.
In my experience the kind of hills where weight would help and you can pick up some good momentum to make it up the other side... are the ones cops are waiting at the bottom of.... I figure on just engine braking enough to keep it in injector cutoff.
These situations argue well for a compressed air or regen braking electric hybrid system though, tempting to rig a lawnmower motor as a compressor/motor to each rear wheel with a tank or two under the car... have an extra push/pull hand throttle, for moderate braking or acceleration when you need it.
edit: another thought. I wonder if a full size spare tire spun on a good bearing up to 2000 rpm (I figure it would hold up to that) weighing about 30lb, would hold enough momentum to shove a 3000lb car up to 10mph and let you pop the clutch in 2nd to bump start, for a 5 minute hold at a red light. Might be doable with an AWD back end transmitting power on braking to the spare on a bearing in the wheel well... needs clutches...
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
I'm thinking of weight as a battery. Instead of storing energy in a chemical battery, you're storing it in a "kinetic battery", like a flywheel. In so far as that lets you optimize your engine usage, that helps fuel economy.
But that's all that more weight does for you. Everything else is negative. Rolling resistance is higher, both from the bearings and tires. It takes even more energy to climb hills and accelerate. If your engine actually operates closer to its optimum thanks to that, then the solution is not to add weight but to tune the power train. Put in a smaller engine, or taller gearing, or both then you can better reap the benefits of less weight.
This thought experiment isn't all that good. Very good from the scientific point of view of holding all other variables steady while changing just one. But you can't just toss 500 pounds and expect to get the full benefit of that weight loss if your engine is still turning at the same rpms it was before the weight reduction, and still sucking down the same displacement as before. You'll get some benefit, but not as much as you should. This fact will skew where that sweet spot is.
This thread being about FE as far as weight is concerned, I do believe lighter is better.
There are some issues concerning handling, specifically the sprung vs. unsprung weight, as it is possible to negatively affect your vehicle's handling, possible I said.
Originally Posted by ma4t
When I had my BMW 540i, rear wheel drive, 280hp V-8, with a custom automatic racing tranny
I would think so long necessity is not the reason for engine size, fuel economy is no concern.
A FE gauge should be standard equipment in every vehicle.