Join Date: Feb 2007
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I would say the slowest speed you can do comfortably in top gear will be most efficient.
The faster you go the higher the wind resistance which means more power needed and therefore more fuel.
Lets say a car has 120BHP and can do 120mph top speed, for that car to do 240MPH the car will need 600BHP, speed has only doubled but power required has more than quadroopled.
So roughly speaking you need 4x more power to do 100mph than 50mph (roughly speaking)
So say the difference in MPG between 50 and 60 is 50=50mpg and 60=48mpg, 70 will probably be 44MPG.
Sorry to rant but probably be easier to explain with a graph.
Currently in tomandjo's garage:
2001 Renault Laguna II 1.9DCi