Click on link-- then in the drop down menu box. Select 98 Fbody M6.
It should fill in your tire/ wheel size/ etc..
Click calculate.
Then go to the right side and scroll down, and the second set of numbers should be filled in, giving you the top speed in each gear (based solely off of gear ratio- not actual. It has my car doing high 200s in 6th, but we all know that it won't do that, but it would if it could, with the gears it has in it.