Re: How do you determine the most efficient speed to drive your car at on a motorway.
Yes, drag - and, rolling resistance Vs speed, internal engine resistance Vs RPM, aspiration efficiency Vs engine temp or Vs RPM or Vs load, fuel quality from one month to the next, time since last full service. . . .
I suggest a big box of sandwiches, drive 1000km at constant speed (motorway only, obviously), fill up noting fuel added and repeat several times at different steady speeds - till sandwiches run out or cash runs out. Let us all know the result!