Question

It's been a couple years since I've taken a physics class, and I have been wondering about this basically since I bought my car:

I drive a 2010 Honda Fit, which has a street weight of approximately 2489 pounds (1129 kg). One feature of the Fit is that the back seats can sit upright like a normal car, or they can completely fold down (this image from this page shows a folded seat next to an upright seat).

Since I rarely have more than one passenger in my car, I can leave the rear seats folded down almost all the time. Having the seats folded down will result in a lower center of gravity which should increase my fuel efficiency (I think!).

I haven't been able to find an estimate on the weight of the rear seats, other than "pretty substantial" according to this forum post. But I'm not sure how much of a difference it will make overall, anyway.

The first part of my question is, assuming that raising or lowering the center of gravity affects acceleration and deceleration, how does one quantify that?

Then, given that I may be able to move, say, 100 pounds of seat from the "dead center" of my vehicle to, say, 1-2 feet closer to the wheels, how can I estimate how that will affect my fuel economy? Perhaps a more specific question would be, how can I estimate how that will affect my acceleration? (which is of course when I get the worst fuel economy, besides being stopped)

Answer 1

The center of gravity should have no effect on economy, besides possibly some indirect effect via changing the load on front/rear wheel bearings or something like that. I would expect secondary effects like that to be insignificant.

Changing the CG won't have an effect on acceleration either, unless you are accelerating at the limit of your tire's grip. A car shifts more of it's weight to the rear tires during acceleration, thus causing the front wheels (in a front wheel drive car) to slip at a lower acceleration than they otherwise would. Lowering the CG of your car will reduce this weight transfer to some degree. So, if you were in a road race it may be a good idea, but for normal driving I doubt that it matters.

NB: The same weight transfer during acceleration occurs in a rear wheel drive car, but because the wheels with more weight on them are now the driven wheels this actually improves acceleration. This is one reason why most sports cars are rear wheel drive -- they can accelerate much harder when tire friction is the limiting factor. ie., when accelerating from a standstill.