This article will assist you in determining your speed (MPH) for a given RPM or the RPM for a given speed and a selected gear (1 through 4) for the Commando (AMC) gearbox.
The Commando gearbox ratios are :
- 1st gear is 2.56 to 1
- 2nd gear is 1.70 to 1
- 3rd gear is 1.21 to 1
- 4th gear is 1 to 1
It is easiest to start the calculations for fourth gear since it is a 1 to 1 ratio. At the end of this article we will show how to calculate the speed for the other gears. If you are using this article to calculate the speed for a gearbox that does not have a gear ratio of 1 to 1, assume the 1 to 1 ratio and convert to your actual ratios as we do for the other Commando gears.
Nomenclature :
- ES -- The engine sprocket, in teeth (normally part # 06-4681, 26T).
- CS -- The clutch sprocket in teeth (normally part # 06-2482, 57T).
- GS -- The gearbox sprocket in teeth (countershaft sprocket).
- RS -- The rear wheel sprocket in teeth (normally 42T for a stock Commando).
- WD -- The rear wheel diameter in inches (not the rim size, but with tire mounted).
- P -- The value for Pie (3.1416).
- RPM -- Engine Revolutions per Minute.
- MPH -- Miles Per Hour.
- Constants in the first formula -- 12 (to convert inches to feet), 60 (to convert minutes to hours), 5280 (to convert feet to miles).
The formula :
MPH = ES/CS x GS/RS x (P x WD)/12 x (RPM x 60)/5280
Or simplified some :
MPH = ES/CS x GS/RS x (P x WD x 60)/63360 x RPM
Examples :
ES = 26 and CS = 57 or 26/57 = .4561
GS = 19 and RS = 42 or 19/42 = .4524
WD = 25.5” so (3.1416 x 25.5 x 60)/ 63360 =.0759
.Therefore at 7000 rpm .4561 x .4524 x .0759 x 7000 = 109.63mph
If you just chance the GS from 19T to 20T, at 7000rpm your speed would be :
.4561 x 20/42 x .0759 x 7000 = 115.40mph
Setting the RPM at constant 6500 and WD at 25.5” you can calculate a constant for [(P x WD x 60)/63360 x RPM] of 493.35:
On the other hand, if you want find the RPM at 65mph for a particular gearbox sprocket the formula would be:
You can calculate the speeds at each gear by dividing the 4th speed by the gear ratio.
Taking the first answer of 109.63 at 7000rpm, that gearing would produce a top speed in 1st gear at 7000 (109.63 / 2.56) of 42.82mph.
The gearing in the second example or 115.40mph at 7000rpm will give at 7000rpm the following:
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MPH = .4561 x GS/RS x 225.02 or MPH = GS/RS x 225.02
By changing your gearbox or rear sprocket, you can calculate what your speed will be at 6500rpm.
RPM = MPH / (ES/CS x GS/RS x (P x WD x 60)/63360)
Leaving everything the same as in the first example and we want to cruse at 65mph our formula is :
RPM = 65 / (.4561 x .4524 x .0759) = 4,140rpm
a 20T gearbox sprocket drops our RPM to 3,939rpm.
1st -- 2.56 to 1 = 45.08mph
2nd -- 1.70 to 1 = 67.88mph
3rd -- 1.21 to 1 = 95.37mph
4th -- 1.00 to 1 = 115.40mph