Gearing

GEARING

Here is some helpful formulas to help take some of the guesswork out of
gearing. The figures that you get from these formulas are only as accurate as the figures that you put into them so measure carfully. In the formula RPM refers to
transmission input RPM, not engine RPM. It is best to know the exact ratios of the transmission by counting teeth and doing the math, but a good guesstimation will get you close. Tire size is calculated by outside diameter. Measure them. Don't just read the sidewall. Once you figure input RPM with the formula, divide it by the desired engine RPM you wish to be running at that speed. This will give you the
pulley ratio you would need.

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                      mph x gear ratio x 336
RPM=     -------------------------------------
                             tire diameter

EXAMPLE;
     Say you're running a 18" tire and you know high gear is a 12 to 1 ratio and you want to know the RPM's for 40mph

            40 x 12 x 336 divided by 18 = 8960

8960 being transmission input rpm. And lets say you wish to do this at 4000 rpm, engine speed, simply divide into it.

    8960 divided by 4000 = 2.24

   So by the chart below you would use a 6" engine pulley and a 2.5" pulley on the trans. I would avoid using 2"- 3" pulleys on bigger engine machines ( sp, bp, fx )

This is a chart for using 5", 5-1/2", and 6" engine pulleys. I'd use even larger ones if they are available to you so you may use a larger pulley on the trans because the 2"-3" pulleys do not provide much belt contact and tend to slip under load, even with stronger spring tension on the clutch idler.

RATIO       ENGINE PULLEY    TRANS PULLEY
------------------------------------------------------------------
3.00                     6"                                2"

2.75                    5.5"                               2"

2.5                       5"                                 2"

2.4                       6"                                2.5"

2.2                     5.5"                               2.5"

2.00                    6"                                  3"

1.83                  5.5"                                 3"

1.71                   6"                                 3.5"

1.66                   5"                                  3"

1.57                 5.5"                                3.5"

1.42                  5"                                  3.5"

These formulas are the same formula transposed to figure tire size, mph, or to figure what gear ratio you would need, but remember, RPM is trans RPM .

mph x gear ratio x 336
Tire Diameter = -------------------------------------
rpm

rpm x tire diameter
MPH = ---------------------------------
gear ratio x 336

rpm x tire diameter
Gear Ratio = ---------------------------------
mph x 336

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