If you have 20" wheels and the tires are say 29.7"
Engine @ 6000RPM
Your axle ratio is 2.77
1st gear 4.484:1 = 42.7 MPH
2nd gear 2.872:1 = 66.6 MPH
3rd gear 1.842:1 = 103.9 MPH
4th gear 1.414:1 = 135.3 MPH
5th gear 1:1 = 191.4 MPH
6th gear 0.742:1 = 258 MPH
If only air didn't put up a fight.