Well, I am sure there is a scientific way to do it, but I drove my TR7 at various speeds on long distant runs and recorded the gas miliage. Always does best at 70 as long as it is tuned up. I have an old VW bug that does best at about 65, altho truth to tell it doesn’t vary much at higher speeds either.
Have you noticed if the grade of gas makes a difference?. Just started recording gas mileage and I think that the grade of gas (octane) is making a difference.