On 35 miles per hour or 40 MPH 4 lane roads, I do a lot of P&G in my FEH in the 25 to 42 MPH range. If there is light traffic behind me, I do 30-40 MPH. In heavy traffic, or on 2 lane roads, I skip it.

I usually net 10% better ( 44 MPG vs. 40 miles per hour ) compared to steady driving. I account some of it to thermal effects. The fewer number of minutes my ICE is on, the fewer minutes I am dumping heat into the radiator/environment... and by running the ICE water at 140'F with P&G vs. 188'F with constant on, there is slower heat loss to the environment by conduction/radiation... yes?

-John