I believe it allows you to view 2 at one time.
It didn't used to cost but now it does, too bad.
if you do a search on rpm vs speed vs ratio. my link shows gearing w/ a T5. Tire dia counts because it changes the ratio, taller dia means faster speed.

Jim & Caroline
The highway is for gamblers, better use your good sense."
Gooday-that's my 1¢ answer due to the lousy economy ~ cause I ain't got - no . mo . doe
Every Shaver | Now Can Snore | Six More Minutes | Than Before ... | Half A Pound for Half a Dollar | Spread On Thin | Above the Collar || BURMA-SHAVE