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      10-17-2010, 01:49 PM   #22
Lieutenant Colonel

Drives: Legacy GT - 13.704@99.39
Join Date: Jul 2007
Location: Manheim, PA

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Originally Posted by tomvcarter View Post
Hi, has anyone worked out what the most fuel efficient speed to be in Cruise control is? When I'm in the car with the family I try to at least try to be green...
The straight answer is to go as slowly as top gear will permit while the engine is still cruising smoothly.

More speed brings more air and rolling resistance, not to mention that more speed takes more power even without that. More power being used means more fuel is used. Always.

As it's certainly not practical to cruise at, say, 40 mph in top gear out on the highway, just remember that more speed takes more power, and more power takes more fuel.

55 will return better mpg than 70, 100% of the time.

Even 55 is dangerous, though. If you drive down in the 5th percentile, you're just as dangerous as if you were cruising at the 95th percentile, accident-wise. Speed doesn't kill, but speed differential certainly does - meaning the old guy trundling down the road at ten under the limit is just as dangerous as the punk at 25 over.