At the bottom of this page is a Javascript form you can use to calculate MPH at a selection of reference RPM when you know the motorcycles tire diameter in inches, ratios of the primary drive gears, choose your specific transmission gears and sprocket sizes.

For simplicity's sake we'll be using the wheel diameter in the calculation rather than a much more accurate "roll-out" of the rear wheel (measurement of the distance covered in one rotation).  Using just the diameter doesn't take into account tire pressure, sidewall flex and the load on the bike - all of which affect how far you go in one rotation of the rear wheel (as does the speed you are going).  The next version of the page should have a reversed chart (RPM at specific MPH), and a space for the more accurate "roll-out" numbers.

So far, I'm limiting my efforts to 49-72cc street legal Honda horizontal singles.  I still need a few transmission numbers for USA-type street-legal bikes (CT70, SS50, XL70, ZB50).  If you've got them - send them along.  Links to online service manuals accepted as well.

©2006 Mike Gladu - all rights reserved

1969-1973 C-Bikes


C50
C50M
C50M-K1
C65(M)
C70
C70M
CL70
Primary
3.722
3.722
3.722
3.300
3.722
3.722
3.722
1st Gear
3.364
3.364
3.272
3.364
3.364
3.364
2.692
2nd Gear
1.722
1.722
1.823
1.722
1.722
1.667
1.824
3rd Gear
1.190
1.190
1.190
1.190
1.190
1.190
1.300
4th Gear
n/a
n/a
n/a
n/a
n/a
n/a
0.958
Final Drive
3.000
3.000
2.857
3.154
2.786
2.786
3.615
Sprockets
13/39
13/39
13/37
13/41
14/36
14/39
13/47
Wheel
22
22
22
22
22
22
22.5

CT-Z Bikes


CT70
S50
SS50
S65
SL70
XR70
ZB50
Primary
3.722
3.300
4.312
3.300
3.722
3.722

1st Gear
3.272
1.765
3.000
1.765
2.692
3.272

2nd Gear
1.722
1.300
1.765
1.300
1.823
1.722

3rd Gear
1.190
1.190
1.400
1.190
1.300
1.190

4th Gear
n/a
1.043
1.136
1.043
0.958
n/a

5th Gear
n/a
n/a
0.958
n/a
n/a
n/a

Final Drive
2.333
3.154
3.154
3.308
3.384
2.400
2.600
Sprockets
15/35
13/41
13/41
13/43
13/44
13/44
15/39
Wheel
17
22
22
22
19.5
19.5
15

For example, a 1982 USA-type Honda C70 Passports has a 22" rear wheel, a 4.058 primary
gear reduction, 1.19 ratio in third gear, a 36 tooth rear and a 14 tooth countershaft sprocket.

Calculate MPH for RPM range
Tire Diameter
Primary Ratio
Gear Ratio
  Driven Sprocket
Drive Sprocket

Insert the tire diameter and the gear ratios from the engine, transmission and chain drive.
JavaScript must be enabled in your browser for the calculation to work.
1000 rpm mph 7500 rpm mph
1500 rpm mph 8000 rpm mph
2000 rpm mph 8500 rpm mph
2500 rpm mph 9000 rpm mph
3000 rpm mph 9500 rpm mph
3500 rpm mph 10000 rpm mph
4000 rpm mph 10500 rpm mph
4500 rpm mph 11000 rpm mph
5000 rpm mph 11500 rpm mph
5500 rpm mph 12000 rpm mph
6000 rpm mph 12500 rpm mph
6500 rpm mph 13000 rpm mph
7000 rpm mph 13500 rpm mph