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Every car has a speed at which the engine is working at its most fuel efficient. Generally, that speed is somewhere between 40mph and 60mph. I have an 07' Civic and was wondering what that MPH sweet spot is?

 
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i got the same car. honestly, before i put my intake in, i was drivin to work everyday @ 65 and i was running good the whole way. but now that i have a intake, i put that cruise control at like 67 and i think its runnin good. OR if u got a MT, as soon as u get in 5th, keep the tach meter around 4000RPM.

 
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The lowest aerodynamic drag is a walking speed; it's not proportional to the speed but the square of the speed. But you want the transmission to be in its top ratio to minimize the number of engine revolutions per distance traveled. So the speed with the least fuel consumption is the lowest speed you can go in top gear. That's around 40 mph in most cars (including the Civic). But on the highway where everyone's going 60 or 70 mph, going 40 is a safety hazard. And around town 40 is too fast for most city streets. This leaves the little country lanes with no traffic as the only place where you can practice your miserly driving technique.