For those with acewell (or other) digital speedos, this is how you calculate the front wheel circumference when using the ANALOG-DIGITAL cable. If you are using the magnet method, just input the actual circumference in mm. I like the conversion cable because it means not having to add anything to the front end.
Something to keep in mind: SOHC Hondas all have the same speedo drive ratio. 2240:60. This means that the speedo drive turns 2240RPM when the bike is going 60MPH on stock wheels. Converted to metric, that is 2240:96.56ALL OF THESE CALCULATIONS ARE DONE IN METRIC. USE MM and KM. KEEP AN EYE OUT FOR WHERE THE CONVERSIONS ARE MADE.
First, find the circumference of your TIRE as it is on the wheel. measure from the centre of the front axle to the furthest tire edge that touches the ground. Multiply by 2 and then by Pi (3.14159265359). For me, my circumference was 2075mm, or 0.002075km.
Now you need to convert this to
Revolutions per Kilometer. One revolution is equal to the circumference, and this is also the distance travelled, thus 0.002075km/1Rev. Invert this to get revolutions per KM! Use the "x
-1" or "1/x" key on your calculator. You should get a large, positive number. WRITE THIS NUMBER DOWN.
Now you need to find the Rev/km for the speedometer drive. Converting involves changing from RPM to RPH, and cancelling out the minutes and hours. This will give you Rev/Km.
See image below. The result should be 1391.88Rev/km.
NOW...
Divide the Rev/KM of the WHEEL by the Rev/KM of the SPEEDO DRIVE. This will give you the magic ratio of how many revs the drive makes per wheel rev. WRITE THIS NUMBER DOWN.
Multiply your tire circumference by this number. The result will be the number you will input into the digital speedometer as your circumference.
In my case, my distance from axle to tire edge is 330.20mm (AKA 13"). This gave my circumference as 2075mm. My final, resulting, calculated circumference that I put in my speedo is
DRUMROLL PLEASE...
719mm
