Old Britts, Calculate Miles Per Hour for a Commando gearbox

Calculate Miles Per Hour for a Commando gearbox

This article will assist you in determining your speed (MPH) for a given RPM 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 your 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).
CS -- The clutch sprocket in teeth (normally part # 06-2482).
GS -- The gearbox sprocket in teeth (countershaft sprocket).
RS -- The rear wheel sprocket in teeth.
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 = 32 and CS = 86 or 32/86 = .4706
GS = 19 and RS = 42 or 19/42 = .4524
WD = 25.5” or (3.1416 x 25.5 x 60)/ 63360 =.0759
.Therefore at 7000 rpm .4706 x .4524 x .0759 x 7000 = 113.11mph

Leaving the RPM constant at 6500 and WD at 25.5” you can calculate a constant of 493.11:

Then our formula is really simplified to:
MPH = ES/CS x GS/RS x 493.11

Changing ES to 34 and GS to 20 and RS to 38 you get:
34/68 x 20/38 x 493.11 or 129.76mph

If you want to redline at 7000 the constant becomes 531.04
Leaving the ES at 34 and GS at 20 and RS at 38 you get:
34/68 x 20/38 x 531.04 or 139.75mph

You can calculate the speeds at each gear by dividing the above answers by the gear ratio.

Taking the first answer of 113.11 at 7000rpm, that gearing would produce a top speed in 1st gear at 7000 (113.11 / 2.56) of 44.18mph.

The gearing in the second example or 129.76mph at 6500rpm will give at 6500rpm the following:
1st -- 50.69mph
2nd -- 76.33mph
3rd -- 107.23mph
4th -- 129.76mph

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