The rate of fuel consumption increases as you exceed an optimum speed. Assuming a flat, straight highway at sea level, what is the maximum speed you can drive a 450h before the mpg starts to decrease? What is your experience, in terms of mpg, between 55 and 70 mph?

 

Note for 350: 50-60

 

In the testing I have done so far, I find that the fuel efficiency slowly decreases as your speed increases above 60-65 mph. There is a small difference between 65 and 70 mph but a larger difference as your speed is greater than 75 mph.

 

I found in the 400h on a flat road holding 55 MPH the gasoline engine is not used at all. A/C off.