Here is a good graph explaining where speed and fuel efficiency meet. We see that at 55 mph we reach the maximum efficiency, and that 60 isn't much different, but slightly lower. How ever, the rate quickly diminishes and at 75 mph you are actually getting close to city (low speed) averages.

Estimates for the effect of speed on MPG are based on a study by West, B.H., R.N. McGill, J.W. Hodgson, S.S. Sluder, and D.E. Smith, Development and Verification of Light-Duty Modal Emissions and Fuel Consumption Values for Traffic Models, Oak Ridge National Laboratory, Oak Ridge, Tennessee, March 1999.

Here is a picture depicting the current speed limits in the US. What is not common knowledge today is that speed limits were originally set for both safety and fuel efficiency during the 1970's fuel crisis. Now a day we just want to get someplace fast, regardless of what it is doing to the environment or our pocketbooks.

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