Calculating Speed and Gear Ratios

Determining the required gear ratio or maximum speed is not always intuitive. Many factors have to be taken into account. This page is particularly useful for electric or motor-driven locomotives.

The ratios and speed required can be determined from the calculator below:-

These calculations are applicable to models with mechanical transmission systems, either petrol engine powered or electric motor powered. Allowance is also made for 2 stages of gear reduction, often required due to space limitations.


Chain & Sprocket
Single stage
Single stage gears
two stage
Two stage gears
Gear ratio/speed Calculator
Enter either English or Metric values
below and press 'Calculate'.

Select English or Metric units for each input parameter
The equivalent converted unit is shown in the other (grey) box

Motor speed (max RPM): rpm
Wheel Dia: ins mm
1st stage reduction: no of teeth [driving]
no of teeth [driven]
2nd stage reduction: no of teeth [driving]
no of teeth [driven]
Intermediate shaft Speed ? rpm
Axle Speed ? rpm
Overall reduction ratio: ? : 1
Resultant Speed: ? km/h
(= ? mph)

Note: Simple formulas using +-*/() may be entered if desired,
not just straight plain numbers.
(for example, 43/16 is entered as 4+(3/16) [=4.1875])
If 2nd stage reduction is not used, use 1 for both no-of-teeth inputs.

Note: The number of teeth does not have to be exact whole numbers. If, for example, a gearbox is used for one of the stages, use 1T as the driving input, and the ratio of the gearbox for the No of teeth (driven) input.

Motor speed is 5000 rpm (max),
Wheel diameter is 125mm.
1st stage reduction uses sprockets of 9T and 29T,
2nd stage reduction uses gears of 10T and 29T.
Resultant maximum speed is 12.6 kmph, overall reduction ratio 9.344:1
If stage one uses 11T and 16T sprockets, and a gearbox with a 2.35:1 ratio is used as stage 2 of the reduction, input 11T and 16T for stage 1, and 1T and 2.35T for stage 2. Overall reduction ratio would be 3.418:1.
Note the accuracy of these calculations is not necessarily exact, and figures
calculated here may vary from actual results.
Results to be used at your own risk. Calculator provided for your personal use only.

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