"My Rover with a 1.8t engine is doing 2200 rpm at 55mph in 5th gear. Rising to about 3000 rpm at 75mph, and I imagine 3750 rpm at 90mph! "
Lets have a look at this a different way. By dividing the speed by the rpm we should get an idea of efficiency (assuming each engine revolution uses the same amount of petrol).
The results are:
At 2200/55mph each rpm gives .025 mph
At 3000/75mph each rpm gives .025 mph
At 3750/90mph each rpm gives .024 mph
So it appears linear. So why does every one say that efficiency decreases dramatically with speed?