Background, 1995 Road King:
At 60mph the engine revs high enough to make me check if I am in 5th gear. Oil was leaking from transmission and inner primary. While fixing the leaks I decided to change pulley sizes to increase my top end while lowering the engine rpm. I made the following changes from stock:
Transmission drive pulley was changed from 32 teeth to 34
Rear drive pulley was changed from 70 teeth to 65

Problem:
While making these changes had the exact effect I needed. It also affected my speedometer and is now reading slower than actual speed. I have looked at a few products but have settled on Thunder Heart ASM 4110. This is a programmable speed sensor for Harley.

Both of the changes I made were negative in relationship to my speedometer so here is the formula to determine how much a tire or pulley change will make.

((oldpulley/newpulley)-1)*100 = Neg%

((32/34) -1) *100 = 5.88235% (transmission pulley)
((70/65) -1) *100 = 7.69230% (rear pulley)

5.88 + 7.69 = 13.57 % Round up to 14%. (amount my speedometer is slower than my actual speed)

increase the size of the front drive sprocket lowers engine rpm
decrease the size of the rear drive sprocket also lowers the engine rpm

Now I am off to order the Thunder Heart sensor to correct my speedometer.

This can also be found at http://www.thunder-heart.com/Tech%20...Fs/EI4108b.pdf

Hopes this helps....

 

There are several plugnplay units that don't require any math. Plug them in and push */- buttons to correct the speedo. There are plenty of calculators that will tell you what speed you should be running at, say, 70mph, all you have to do is match your speedo to your tach with the buttons; no formulas or math required. They will run from $75-$100.

 

If you have a smart phone download an app that tells your speed, tape it to the bike just so you get an idea how off your speedo is then remove.