Gear Ratio Chart/Pinion Gear Selection - HPI Blitz With 88T 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 15.253 21 10.895
16 14.300 22 10.400
17 13.459 23 9.948
18 12.711 24 9.533
19 12.042 25 9.152
20 11.440 26 8.800
Efficiency Ratio1
Pinion FDR:RO - Off-Road Pinion FDR:RO - Street Pinion FDR:RO - Foam
15 1.148 15 1.129 15 1.079
16 1.076 16 1.059 16 1.012
17 1.013 17 0.996 17 0.952
18 0.957 18 0.941 18 0.899
19 0.906 19 0.891 19 0.852
20 0.861 20 0.847 20 0.809
21 0.820 21 0.807 21 0.771
22 0.783 22 0.770 22 0.736
23 0.749 23 0.736 23 0.704
24 0.717 24 0.706 24 0.674
25 0.689 25 0.677 25 0.647
26 0.662 26 0.651 26 0.622
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.
3