Gear Ratio Chart/Pinion Gear Selection - HPI Blitz With 83T Spur
Internal Gear Ratio 2.6
  Off-Road Tires Street Tires Sand Tires * Off-road calcs = HPI Trepador tires w/ 4.23" diameter
Roll-out 13.289 13.509 14.137 * Street calcs = Street Fighter SC tires w/ 4.3" diameter
* Sand calcs = Sling Shot paddle tires w/ 4.5" diameter
Final Drive Ratios
Pinion FDR (*:1) Pinion FDR (*:1)
15 14.387 21 10.276
16 13.488 22 9.809
17 12.694 23 9.383
18 11.989 24 8.992
19 11.358 25 8.632
20 10.790 26 8.300
Efficiency Ratio1
Pinion FDR:RO - Off-Road Pinion FDR:RO - Street Pinion FDR:RO - Foam
15 1.083 15 1.065 15 1.018
16 1.015 16 0.998 16 0.954
17 0.955 17 0.940 17 0.898
18 0.902 18 0.887 18 0.848
19 0.855 19 0.841 19 0.803
20 0.812 20 0.799 20 0.763
21 0.773 21 0.761 21 0.727
22 0.738 22 0.726 22 0.694
23 0.706 23 0.695 23 0.664
24 0.677 24 0.666 24 0.636
25 0.650 25 0.639 25 0.611
26 0.625 26 0.614 26 0.587
1 Ideal Efficiency Ratio is closest to 1:1.  Depending on the kV of your motor, you may need to gear up or down to find the sweet spot.
4