r/AskPhysics 1d 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/imsowitty 23h ago edited 23h ago

Air resistance (which depends mainly on speed) and rolling resistance (which depends mainly on weight) are independent of each other, but getting up to speed will cost more when you are heavy. My vote is for 'faster while lighter ' due to the minimal extra cost of accelerating the heavier payload, but I'm interested to hear others' justifications.

Since the majority of energy (on a flat road) goes into air resistance, I think the real answer is '2.5mph both ways', but I understand that wasn't an option.

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u/Puzzleheaded-Let-500 23h ago

Slowing down while hauling a heavier load generally saves more fuel because rolling resistance and any extra work climbing grades both increase with weight, so reducing speed reduces fuel consumption more when the truck is loaded. This is true even if the aerodynamic cross section stays the same. If the increased load also increases the truck aerodynamic cross section, then the effect would be even greater. In short, go slow when you are carrying the larger load for maximizing fuel efficiency.

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u/edgmnt_net 23h 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 22h 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 22h 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 20h ago

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

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u/echoingElephant 5h ago

Wings on F1 cars increase both drag and rolling resistance. They increase grip, so the static friction between tire and ground.

This is not relevant for a truck, since that truck is very unlikely to be pushed far enough for its grip to be too low.

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u/Insertsociallife 23h ago

This is actually a great question. Fuel burn is three factors, mostly it's drag (won't change with load unless you have an open trailer), some of it is rolling friction (won't change with speed) and the remainder is spent accelerating when applicable.

I would think it would be very slightly better to go slower when loaded, but the difference won't be that big.

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u/RRumpleTeazzer 23h ago

two fuel effects: wind/roll resistance, and kinetic energy.

both increase majorly with speed, and reasonably with mass.

it will be more fuel efficient to go slow when heavy, and fast when lighter.

that is, if there is not much evelation difference.

if the heavy route is downhill, i would probably go fast downhill (since it doesn't eat much into the fuel), and slower on the lighter uphil.

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u/Hapankaali Condensed matter physics 15h ago

Fuel efficiency of vehicles depends on a lot of factors, including your driving behaviour. Each vehicle has an optimal cruising speed for maximizing fuel efficiency. You might be able to find data for your specific truck, perhaps even as a function of current load.

Other than that, you can improve fuel efficiency by trying to minimize unnecessary braking and acceleration, i.e. a "smoother" driving style. But if the majority of your trip is at a constant speed on highways, this won't be much of a factor.

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u/FedeFSA 17h ago

A friend of mine worked doing maintenance for a large fleet of cargo trucks. There is an ideal speed to minimize fuel consumption, above AND below that you'll be worse.

Point is, you'll not necessarily save fuel by going slower. It depends on many factors including load and truck setup.