Every car has an "optimal" speed; essentially the speed at which your car gets the best mpg/m. This is based on gearing, etc. The only way to really determine this would be to take the exact same trip under ideal circumstances at different set speeds and compare gas comsumption.
or you could get one of these and find out what your optimal mpg is.
http://www.porcine.com/gps/mpg/mpg.html