Eugene55 , firstly , your 3.9 gears will enable you to run 55mph easy cruise , asuming your engine is ok .
Here is the formula you can use to see what alternate gear sets will do for your RPM.
MPH = 0.01136 X RPM X tyre circumference in feet
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Gear ratio X Diff ratio
e.g. 750 x 16 tyre @ 31'' tall = 8.11 ft circum.
MPH = 0.01136 X 2500 X 8.11
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1 x 3.9
= 59.05 mph top gear speed
you change the variables to suit your tyre size and diff ratio options .
