what would a 4.10 do to my M20?

[digger]

Tire Size also needs to be taken into consideration, you could run a 4:10 with a taller tire and have your cruising rpm the same as a 3.73 running a shorter tire.

what would be the point of that though? and you would have difficulty changing the the wheel radius 10% (more than 2.5" bigger diameter) without creating other problems. usually when people use non standard size tyres its a few% difference in rolling radius.