got a freind who is buying brand new stock SP 500 tires from a friend who put different tires on it as soon as he got it. 100 bucks! good deal to the guy is actually going to mount them to. How will this effect his 400? will it slow it down? should he change the gear ratio to compensate?

 

Take the diameter of the tire 25" multiply it by 3.14 and come up with 78.5. (A)

Take the diameter of the tire on the Express right now and multiply it by 3.14 get the total. (B)

Subtract the new total from 78.5 and get the difference. (C)

A-B=C

Divide B by C and get the decimal equivalent. (D)

Count the number of teeth on the rear sprocket (E)

Multiply the number of teeth on the rear sprocket by 1.(D) and that is the number of teeth that you will need on your new sprocket.

E x (1+D) = New sprocket size.

This is the initial gearing which does not factor in rotational inertia but is a good starting point.