Gear Ratio Chart/Pinion Gear Selection - HPI Blitz With 77T 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 13.347 21 9.533
16 12.513 22 9.100
17 11.776 23 8.704
18 11.122 24 8.342
19 10.537 25 8.008
20 10.010 26 7.700
Efficiency Ratio1
Pinion FDR:RO - Off-Road Pinion FDR:RO - Street Pinion FDR:RO - Foam
15 1.004 15 0.988 15 0.944
16 0.942 16 0.926 16 0.885
17 0.886 17 0.872 17 0.833
18 0.837 18 0.823 18 0.787
19 0.793 19 0.780 19 0.745
20 0.753 20 0.741 20 0.708
21 0.717 21 0.706 21 0.674
22 0.685 22 0.674 22 0.644
23 0.655 23 0.644 23 0.616
24 0.628 24 0.617 24 0.590
25 0.603 25 0.593 25 0.566
26 0.579 26 0.570 26 0.545
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.
5