I have a fat bob tank and dash for my 1988 sportster. Of course.. .my stock speedo was a 2:1 with a 12mm nut. My new speedo came as a 2240:60 with a 12mm nut. Because of this.. when I am doing 35mph my speedo is reading around 45+. I need to replace my speedo with the correct 2:1 ratio speedometer but all the ones I see online have a 5/8-18 thread.
Any help is greatly appreciated.

 

Spend the $$ for the correct cable. I put a Drag Specialists speedo on my fat bob tank and just bought the cable with it. The nut sizes were different, but it was a very easy switch. just remember you don't need a long cable if you are moving the speedo to the tank. Made that mistake myself.

 

I've got the same problem on a bike I just picked-up: speedo reads 10-20+ mph over actual. It's a custom with fat bob tank/dash, 2240:60 speedo, and a 21" front wheel. I'm thinking it's wearing the wrong gear drive for the 21" wheel. Any ideas how to IDwhich one is on there? A "TM", with the bar of the T extending over the M, in the casting is the only stamp I can find on it. Figured on pulling the cable, turning wheel by hand, and counting the gear revolutions.Anyone know the right number of turns for a 21"? 18"?