will a heavier car coast further?
 

i know it takes more energy to accelerate a heavier car to the same speed as a light car, but what if they're both up to speed , wich will coast furter?

assuming they both have more or less the same tires and frontal area and Cd (in the example i'm thinking of the bigger car actually has a better Cd)
the weight difference is about 300 kg - 660lbs.

will weight effect rolling resistance, or won't this be a factor with the right tire pressure?

It'll effect rolling resistance, yes, but the heavier car will most definitely coast further, all other factors being equal. My old t-bird coasted a mile on level ground, from 60, and it was still going 30mph. My current car can barely make it a half mile. There's a lot more differences, but I think the big one is my tbird was 1000 pounds heavier.

From an FE standpoint the heavy car loses out.
First in acceleration the coast gains don't make up for it, then maintaining a cruising speed consumes more, even with the same size engine heavier isn't the answer for better mpg.

But to answer the question, yes, the heavier car coasts further.

I would answer no. Nothing I've driven coast as good as the saturn. My brothers altima at 400lbs heavier with equal tire pressure comes well short. None of the pickups, vans, etc are any different. The only time a heavier car will coast better will be on long steep hills. This is more evident when heavily loaded because the weight vs frictional losses ratio is higher than on a lighter car. But its still more efficient to use the lighter car in every other circumstance. Of course this is all just my opinion.

My VX doesn't coast worth an expletive. But when its laden with people or stuff... it coasts better down hills or on flats. I believe and please correct me if I'm wrong, if you have a car like my VX with a Cd of .31 that weighs 2100 pounds, and you put 300 pounds inside the car, the car's Cd has suddenly dropped from .31 to some smaller number. But just because its Cd has dropped, doesn't mean the car is now getting better gas mileage! Of course it isn't, since the engine has to haul around more mass. Conversely, you tear out the passenger and back seats and now the car weighs less. It's cD is going to go up and it's going to coast down hills even worse. But because the engine is hauling around less mass, it's going to get better gas mileage, not worse, despite the worse cD. Well, that's how I understand it anyway. Maybe someone can add to it.

Nope, added weight doesn't change Cd. However, it does change the ratio of gravity force to aerodynamic resistance, meaning you have a slight increase in rolling resistance, no increase in aerodynamic drag, but an increase in the gravity vector with a weight increase. This will make it coast better downhill. On the flat you have again ONLY a slight increase in rolling resistance but a significant increase in momentum. . .i.e. the heavy car will decelerate at a slower rate thus coasting further. . .

Try coasting by yourself and then again with 3 passengers ~450# total~you will coast further.

that's what i'd expected, my main consern was that rolling resistance might be higher, although that would mainly have to be a tire thing.

my next vehicle is likely going to weight a bit heavyer (what i have now was build by 80's standards... so it's lot like i'm shopping for an suv)

but the extra weight will take extra energy so i'd like to figure out the best strategy to minimise the loss.

the heavier car has more inertia to carry it further, so it will go the farthest. a nice aerodynamic [looking] heavy can I can think of is the olds aurora. that would coast pretty far, but still, the losses are when you're accelerating. if you're on the highway/country road and not stopping, you could do more effective P&G with a heavy one.

Does downforce equal addend weight and thus more mass?

Is it the same way for cars that have more downforce via wings and spoilers and such...thinking of professional racing. I always thought the opposite, that it drags you down thus you'll need power to keep going.