Speed in Gears

A vehicle's top speed is limited by drag and/or gearing.  In road racing, a lot of effort is put into making the two limits coincide.  Dirt bikes usually don't need a high top speed so other factors are considered when making gearing choices. 

The nice thing about a single-speed gearbox is that you're never in the wrong gear.  Conversely, you're never in exactly the right gear either.  

Spreadsheet Notes

The spreadsheet below summarizes the overall gear ratios and speeds in each gear for all the bikes examined in detail. 

Overall gear ratios are computed from the product of the primary ratio, secondary ratio, and each gearbox gear's ratio (if present).

The secondary gear ratio is derived from the front and rear sprockets that come fitted as standard.

The rear wheel circumference is user-defined and measured in inches.

Speeds are computed in 1000 rpm steps from 1000 to 10000 rpm.  Rows in red indicate the rpm value roughly equivalent to the motor's capability.  The actual rpm limit may be different.  I have doubts that the GasGas TXT300 revs beyond 10000 rpm - if even that high.

Gearing for the Montesa Cota 242 was estimated from a speed chart presented in a magazine test.  

The Constant 0.00947

The spreadsheet is based on code I wrote in the C programming language long ago.  It uses the constant 0.00947 to simplify repetitive calculations.  I called this constant the WHEEL_FACTOR.  It converts engine RPM into ground speed in MPH given rear wheel circumference in inches and overall gear reduction ratio.

MPH = WHEEL_FACTOR * wheel_circumference * RPM / overall_ratio

The reasoning is as follows:

The rear wheel rotates at a speed dictated by the engine RPM divided by the overall gear ratio. 

For each revolution of the rear wheel, the vehicle moves forward by the circumference of the rear wheel.

1 mile = 5280 feet

there are 63360 inches in 1 mile  (12 * 5280)

miles forward per wheel revolution = wheel circumference in inches /  ( 12 * 5280)

rear wheel revolutions per hour = rear wheel RPM * 60

0.000947 = 60 (minutes per hour) / 63360 (inches per mile)