You can use this formula EX: if the engine has enough HP to run 120 mph in the qtr.
The gear ratio would be a 5.00 gear. But i'm thinking 4.88's or 4.86 gear would work best. Ofcourse if the HP is higher you'll need to go down on the gear.
Zip.
http://www.wallaceracing.com/calcrgr.php