Here is a calculator that will help you solve for MPH, Gear Ratio, Tire Size, or RPM from Strange.
https://www.strangeengineering.net/g...io-calculator/
IMO you will feel the sweet spot that your car and the driver are humming along at which I think varies with road condition. We are fortunate to have access to many country roads. While my flatheads are quite capable of cruising at highway speeds the sweet spot for me is at a slower speed, even with my Columbia.