Sprocket change calculations
Changing only one or two teeth on the rear sprocket makes very small changes.
Here are some numbers for a 1987 YZ125, assuming 10,000 rpm, and a 24" rear tire diameter:
Sprockets = MPH in 1st, 2nd, 3rd, 4th, 5th, 6th gears
50/11 = 18 mph, 24 mph, 29 mph, 35 mph, 40 mph, 44 mph
50/12 = 20 mph, 26 mph, 32 mph, 39 mph, 44 mph, 48 mph
50/13 = 22 mph, 28 mph, 35 mph, 42 mph, 47 mph, 52 mph
49/13 = 22 mph, 28 mph, 36 mph, 43 mph, 48 mph, 53 mph
48/13 = 22 mph, 29 mph, 36 mph, 44 mph, 49 mph, 54 mph
47/13 = 23 mph, 30 mph, 37 mph, 44 mph, 50 mph, 56 mph
50/15 = 25 mph, 32 mph, 40 mph, 48 mph, 54 mph, 60 mph
47/15 = 26 mph, 34 mph, 43 mph, 51 mph, 58 mph, 64 mph
As you can see, countershaft sprockets changes are much more significant.
FORMULA FOR CALCULATING MPH @ ENGINE RPM
mph = ((rpm * tire_diam) / (primary_ratio * gear_ratio * drive_ratio * 336.1352398))
Note: Rounding off the constant to "336" makes things easier and does not significantly change the output.
Example:
Engine RPM = 10,000
Tire diameter = 24 inches
Primary transmission ratio = 3.333:1
Transmission gear ratio = 2.583:1
Drive ratio (50 tooth rear sprocket / 13 tooth front sprocket) = 3.5:1
((10000 * 24)/(3.333 * 2.583 * 3.846 * 336)) = 21.6 mph
I have this already plugged into a spreadsheet. Just enter your bikes gear ratios (should be in your manual), sprockets, and tire size.
You can download it from <
http://home.pacbell.net/pantaz/motorcycles/index.html>