A quick way to figure what RPM change will result from a rear gear ratio change is this---a 4:1 rear in place of a 3:1, the engine revs 4000 at the same speed where it turned 3000 before. 3.08 to 4.11, means 4110 rpm instead of 3080 rpm. So you can half the numbers, in the first case, 2000 where it formerly turned 1500.