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Ideally, 0 rpm is the best (EOC) in terms of FE. If you are not EOC, depends on what car you are driving, road condition and what speed you are cruising, generally, highest gear gives you the best FE. If you are climbing up in low speed, like 25mph or 30mph, 5th gear may not be able to give you enough power. If you don't shift down, you need to press harder to maintain the speed, which will use more gas.
My car's best cruising speed is 35mph to 40mph @5th gear, which is about 1800rpm to 2000rpm.
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