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Discussion Starter · #1 ·
So I bought a 2:1 ratio speedo for my 1960 ironhead without understanding what the ratio is. I want to run one mainly to keep track of mileage on my top end rebuild.

After pulling the tranny out it makes a little more sense, but what exactly does the ratio mean? For every 2 turns of the speedo gear, the rear wheel turns 1 time?

Stupid me didn't consider the fact that my rear wheel isn't stock and that probably affects it. My rear rim is 16" & the overall tire diameter is 27".

I set up a bicycle magnet type speedo on the front wheel and there's quite a difference. If I remember correctly, the digital was showing about 48mph while the mechanical showed around 40mph. I didn't have time to check them both with a gps before tearing it apart.

Is there any way to modify a mechanical speedo ratio? different speedo drive gear? Idk, maybe my digital one is further off. A 1960 would have come with an 18" rear rim right? Anyone know the overall factory tire diameter?

Now I'm rambling... thoughts?
 

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The diameter of an 18 related to a 16 is almost the same, less than 1/2 inch difference overall. Tire profiles can change that slightly.
There are no speedo drive ratios available for Sporty trannys except the stock one.
Did you calibrate the bycicle speedo? They need to be set up for the specific size of wheel they are on!
On an ironhead sprockets also affect the drive speed. Is your countershaft sprocket stock or oversize? Rear wheel stock or smaller? Those sizes make a big difference.
Robbie
 

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Discussion Starter · #4 ·
Yes I calibrated the bicycle one. I didn't think of the sprockets though, doh. Thanks for the info, I might look into calibrating or using a multiplier... IF I don't weld it shut.

Lemme know if you find that formula
 

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One of my customers tapes a phone to his tank, and uses his speedometer app. Precalibrated too.
 
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