[Sven7]

I recall Phil saying the half rule is based on 55mph steady state driving. Thus, basjoos's 65mph figure would be on a different part of the graph, so to speak. Also, drag does not increase linearly, as UltArc pointed out. For example, when you go 10% faster there may be 12% or 15% more drag.

So, if you want to calculate 55mph cruising mpg of a future mod you can use the half rule and get it close enough. If you're looking at other speeds it will not work correctly.