speedometer fix
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Elder User
Joined: Aug 2005
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speedometer fix
Ok I have a 54 f100 with the y block. The truck orginally came with a 3 speed and 4 something gears( cant remember off the top of my head and the truck is 120 miles away) I am wanting to fix the speedo. I know it is way off. When my dad got the truck it had a 4 t98 speed out of a 600 in it. Then later he put a nine inch with 325 gears in. So How do i even get close to fixing the speedo? Tire size now is 235, 75 15, or 235 70 15( undeiced) So how do i fix it? is it just a gear swap on the tranie? is there a formula to do this? I dont have a gps but there is a tach in the truck. I think when the speedo say i am going 25 it is about 50. I read some old post and some mention changing the gear, Other mention an adaptor. Also where do i find what i need? thanks
Lead Driver
Joined: Apr 2002
Posts: 8,813
Likes: 13
From: Littleton, New Hampshire
I'm not sure if the transmission you have has the nylon gear in it or not. If it does then correcting the speedometer is just a matter of getting a transmission gear with the right number of teeth. Generally, one tooth is worth about 5 mph, so if you think the speedometer is 25 miles an hour too slow then you need a gear with about 5 less teeth than what you currently have to speed it up. It's a trial and error process and it's also possible that there is no speedometer gear with the number of teeth you need for your particular combination of transmission, rear end, and tire size... 
