speedometer gear help please
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Joined: Feb 2015
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From: ND
speedometer gear help please
i switched rims and tires on my 86 f150 4x4 with a 302 and a aod transmission. i went from 235 75 15 to 235 85 16. the 15s are a 29 in tire and the 16s are a 31.5 in tire. i found a calculator on math-prof.com and had final drive figures for transmissions. listed for the aod transmission the final drive was .70:1. based on that info it gave me a rear gear of 3:55 i assume? at 2000 rpm i am doing 75 on my dash and 78 according to my gps. could someone please help me figure out how to buy the correct speedo gear. its racking up the mileage on the dash as well and would like that to stop as well. thanks
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Joined: Jan 2015
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From: Stockdale, Tejas
Ford Truck Enthusiasts Forums - View Single Post - 1973-1979: FAQ's, Tips & Tricks, Projects, and FORUM INFORMATION, -PLEASE READ!
I'm not great with math
but this helped me to correct my daughter's truck. Others will chime in but read this first. And Welcome!
I'm not great with math
but this helped me to correct my daughter's truck. Others will chime in but read this first. And Welcome!
Senior User
Joined: May 2014
Posts: 376
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From: Lake Elsinore
Ummm, if you are doing 78 according to GPS and your speedo reads 75 mph, you aren't racking up mileage, you are actually recording LESS miles that you are traveling. And, 3 mph speedo error at 75 mph isn't out of line for an old truck with mechanical speedo. That's less than 5% error.
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