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Originally Posted by BeEfCaKe
When driving at WOT though, wouldn't engine revs be higher? And so use more fuel? I understand that by using WOT, you would be able to get to higher gears faster so you save gas, but would you not also waste more through the high revs?
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Load is a separate issue from engine speed, wide open throttle is 100% load.
Specific fuel consumption plotted against rpm and load for an Opel engine.
Note that engine speed doesn't affect specific fuel consumption much but that load do. The lowest specific fuel consumption is achieved at close to full throttle, especially in the usuable power range of about 2000 to 4000 rpm but that doesn't affect that much.
So when running the car energy is needed, we will need the same energy even if we accelerate fast or slow and at constant speed the same amount of power will be needed independant on the engine speed, so this means that the mean power consumed is the same (as long we don't compare driving at different speeds).
Of these reasons it's best to use the engine with a high load, that means to acclelerate fast with close to full throttle and at constant speed use the highest gear so the engine speed is low and load is high (compared with higher engine speed and lower load).
At higher load less fuel is consumed per power delivered by the engine.
Note that a turbocharged engine for road use is usually designed to give it's lowest fuel consumption with a little less load, this has to do with the high compression ratios while boosted and the smaller displacement for the same power.