I agree with what the previous poster stated.
There are two different methods.
One, if you are in the city and must make frequent stops, and
Two, if you need to go long, sustained distances without stops.

Not considering stops, for steady state driving, which is what I think you are after...
The "sweet spot" is 47 or 48 miles per hour.

The internal combustion engine will net about 50 miles per gallon at a touch under 50 miles per hour. For my vehicle, it's easy to remember " 48 MPG at 48 MPH".

In stop and go, the battery comes into play, and up to 70 MPG is possible with the most ideal conditions. This is done with extensive "Pulse & Glide" .... using battery and/or engine to get to 40 miles per hour, and coasting with engine off down to 25 MPH, ( or a stop at an intersection, etc. )

-John