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P: 43
 Quote by Pengwuino The friction between the tires and the road is directly proportional to the weight of the vehicle. This is certainly not the only cause of friction, but it is a significant, first order, factor. Your estimate is simply not correct. This actually gives me an idea for a lab for intro physics students... If you ever looked at the shape of a train's wheels, the composition of the wheels, and the shape of the track, you might be able to see why. The wheels are smooth and made of metal as are the train tracks. The surface area between the track and train wheel is very small as well. This combines to create a much smaller coefficient of friction. With a car, you have a very large contact area between the wheels and road, neither of which are smooth.
I hear ya. I understand that a train has virtually eliminated the wheel to track friction element...at least per unit of weight hauled. And, for road vehicles, I understand that the friction of tires to road is proportional to weight, but I just can't see it being in the top 3 in terms of variables causing an SUVs highway mileage to suffer vs a "passenger car". Accelerating the extra mass on flat; accelerating mass against gravity and wind drag due to the larger vehicle surely are #1 #2 and #3..??...and probably (just guessing...but someone would have to really do some convincing for me to think otherwise) account for 80+% of the observed reduced mileage per gallon of fuel with a SUV vs passenger car.

I have an "over the road" trucker friend (interesting business by the way). He says he gets, on flat ground, 11-12 mpg full (50,000 lb load). He gets 12-14 empty. Now, that's only a ~15-25% difference....and of that, how much is NOT due to accelerating the heavy load (thus is caused by the increased road-tire friction)? I don't know...but it seems like most of it is due to the acceleration of mass. Tires matter....but if you're keeping them at a good pressure (I go 40-42 psi on trips), just can't imagine that's where the weight, by itself, gets ya. Maybe I'm off on that..... I'm open to evidence.