I have a 74 shovel with a ratchet top 4 speed. The old speedo was a 1:1 but it had the stock 16" tires. I am putting a 21" on the front and 18" on the back anyone have an idea of what ratio I need?

Thanks

 

If I understand you correctly, you are diving the speedo off the trans. Only the back tire will affect it...and there is very little difference in the diameter of a 16 and 18 because the 18 is low profile...so it will be fine.

http://www.steelthundercc.com/tiresizes.html

 

Leave it alone you'll be fine as is stock speedo's on the old bikes are off 5mph or more anyway , changing it involves a tranny tear down .

 

I've always beem told the front speedo drive was a 2:1 and the tranny drive was a 1:1. Generally, the 16, 18 and 19 inch tires are all the same diameter, +/-. The difference is not enough to affect the speedo, as Twisted said, they are off 5 mph or so anyway.

 

Al they had both ratio's in trannies , you could tell by the tooth count on the drive one had big teeth, 11 count =1:1 and the other was fine toothed 23 count =2:1 and for the life of me I can't remember which was which for what years they mixed & matched all the way to 1980 if I remember right .

The front wheel system has 3 ratios available just to screw with things a bit more you have 1:1 , 2:1 and they added a goofy 2260:60 drive for the evo models & up .

 

At my age, I get confused about all of that. I didn't run a speedo for years because everytime I hooked it up, I got a damn ticket. Though I got a ticket in Oklahoma once cause the speedo wasn't hooked up. The Highway Patrol drove around me and didn't see a cable going to the front wheel. He backed off, pulled me over and I didn't have one going to the tranny. He knew his Harleys. In Oklahoma, motorcycles have to have a speedo. Whoda thunk..........

 

2 guesses how I got the education on all that crap ................

 

Twizted...the 2260 speedo is also used on my 84. Not sure what year, how far back it goes though, but it was a pain to replace. No one fixes them and trying to keep OEM/NOS took some time.

Ran with Tawain Ted $hit (went through two iin less than a year and 6K because they kept breaking) and finally found an OEM outta a HD shop in New York

 

Thanks for the info I didn't know they where in use that far back . You know the only reason they went that way was it's all imported now and they are dancing to make MPH speedo's work with KPH hardware .