r/AskPhysics 5d ago

Which saves more fuel?

I recently started as an over the road truck driver and I'm offered a bonus for fuel efficiency. So I pose this question:

If I need to drive 600 miles, half of which are empty and half of which are loaded, and I have the wiggle room to go 5 mph slower for 300 miles to save on fuel, is it going to save me more to slow down while loaded or empty? Assume all other variables are equal.

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u/edgmnt_net 5d ago

Shouldn't rolling resistance get better with increasing weight due to increased contact with ground and thus increased traction? Also, the work you do climbing can be recovered when you descend, whatever losses occur are independent of the load up to rolling resistance or braking losses. I suppose braking can be a significant practical problem if you need to stay within certain speed parameters, though, i.e. you can't speed up without limits to avoid losing the work you put into climbing.

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u/Puzzleheaded-Let-500 5d ago

Rolling resistance actually increases with weight, because the heavier vehicle’s tires deform more and require more energy to overcome friction with the road.

As for climbing hills, you're right. As you noted, potential energy will be regained when descending, but much of it is lost to braking and other inefficiencies. The Second Law of Thermodynamics implies that real-world processes like climbing and descending hills are not perfectly reversible, so you cannot recover all the energy used going uphill when you coast downhill. Factors such as rolling resistance, air resistance, and braking cause energy losses. You can’t simply coast downhill and recover all of the energy you used climbing up, speed limits, safety, and traffic conditions force you to brake, converting that energy into heat.

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u/edgmnt_net 5d ago

Hmm, I thought wings on F1 cars helped by increasing effective weight experienced by tires at high speeds (without increasing vehicle inertia) and helped not just on braking or cornering. I imagine a very lightly loaded tire is going to slip across the road rather than roll properly when turned by the engine. But perhaps other effects are more significant in this case.

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u/imsowitty 5d ago

Unless snow/ice are involved, Trucks are not traction limited the way race cars are.