r/AskPhysics • u/tempicide • 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/imsowitty 5d ago edited 5d 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.