U.S. highway cruising speed

[emax]

The real sweet spot will be somewhere between 55 and 85 km/h (35 - 55 mph).

In general, I expect the sweet spot to be at the highest torque, since this results from the best efficiency due to the optimal cylinder filling.

That would be (out of the head) around 1250 RPM which would be (again out of the head) around 70 km/h in highest gear.

I don't have the RPM chart at hand for the exact value.

But it would fit @bemax's estimate.

 

[emax]

Physics.

I see.

75-80 mph compared to physics yields "NaN".

So no effect on mpg.

 

[FlyingTexan]

My Gren drives very sweetly on cruise control on the motorway at 78mph, mpg drops to about 20 from c23.

That’s an imperial gallon I’m assuming? So 20% off from a U.S. gallon

 

[CountyV8]

The B58 seems to go into 8th at 51 MPH resulting in 25.5MPG (imperial). new engine not run in