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I recently got pulled over confirming something ive been suspecting. my speedometer is off by like 6-10mph. I was wondering on how to get that corrected. I dont have that issue till i hit 50mph. then people start saying im going faster then what my speedometer says, much faster. its a 1983 F150 300ci engine with 3 speed manual with overdrive. Truck runs smooth. took a trip from virginia to new york towing like a champ so i know the truck is ok. I just want this corrected so i dont have this happen to me again
Why not first verify how much you're off. The mileage markers on the side of the freeway work good. Drive ten miles. See how many miles your odometer changed. Calculate the percentage error.
I have a 1982 F150 300 - 6 with the 3 speed auto and my speedometer is off also, but not in a consistent manner. Because it varies so wildly, I use a cheap GPS as a speedometer. Instead of the map I just press on the button that shows speed and I get a whole dashboard with a big speedometer on it.
It works great and the police leave me alone. Well for speeding anyway... LOL
If the speedo is reading 6-10 MPH too fast, add a tooth to the speedo DRIVEN gear.
If the speedo is reading 6-10 MPH slow, deduct a tooth from the speedo DRIVEN gear.
DRIVEN gears are located at the end of the cable inside the trans. Same 16 thru 21 tooth nylon gears used since 1960, are available from Ford and auto parts stores.
Does your truck have the stock sized tires? If you have larger than stock tires, the speedometer will read slower than the actual speed. The amount "off" is a percentage, not a flat 6-10 mph, so the faster the truck, the greater the error.
Does your truck have the stock sized tires? Probably not!
If you have larger than stock tires, the speedometer will read slower than the actual speed. The amount "off" is a percentage, not a flat 6-10 mph, so the faster the truck, the greater the error.
The original tires were either bias ply or radials, but not P-Metric radials as they weren't available until the 1990's.
Once P-Metrics are installed. the speedo won't read correctly and there's nothing that can be done to correct it.
Since the OP did not list the trucks rear axle ratio, I cannot post the part number of the correct DRIVEN gear and the number of teeth it has.
P-metric tires were available long before the 90's, but most of these trucks have long had the stock tire size forgotten about.
My 80 F100 reg cab long bed came with P195/75R15's, per the door tag and it still had them when I owned it.
My 81 and 82 F100 FlareSide's came with 215's, which neither had when I got them.
While neither is still an F100, the door tags still reflect what they once were.
What I should have said: P-Metrics didn't become standard equipment until the 1990's.
While radial tires were first installed as standard equipment on the 1955 Citroen & 1966 Oldmobile Toronado, Ford didn't offer them for trucks until the mid 1970's, as then only as an option.
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