Speedometer not reading right
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Laughing Gas
Joined: Jan 2023
Posts: 1,172
Likes: 114
Speedometer not reading right
I have a 1988 f150. It does not have the stock 235/75/r15 tires. It has the 31x10.5r15 tires. When I drive my other car going the speed limit in the fast lane people will stay behind me. But if I do that going 10 over according to my trucks speedometer everybody passes me. I downloaded a speedometer app for now. How do I calibrate the speedometer?
ill put the specs below:
31x10.5r15 tires
3.55 rear end
Aod transmission
302 v8
and it has the gauge cluster without the tac
ill put the specs below:
31x10.5r15 tires
3.55 rear end
Aod transmission
302 v8
and it has the gauge cluster without the tac
Hotshot
Joined: Jun 2015
Posts: 12,763
Likes: 2,420
The speedometer is driven by a cable connected to either the rear of the transmission (2wd) or transfer case (4wd). Remove the speed sensor/cable assembly from the trans/transfer case to see what color driven gear is currently in the truck. That will tell you how many teeth.
I looked online at a calculator to figure out what driven gear I would need to correct the speedometer when changing to 4.10 from 3.55 gears. I’m not sure which website I used.
I looked online at a calculator to figure out what driven gear I would need to correct the speedometer when changing to 4.10 from 3.55 gears. I’m not sure which website I used.
Fleet Mechanic
Joined: Aug 2015
Posts: 1,827
Likes: 358
From: Candy Mountain
On the pre 92 trucks there is no PSOM. The speedo runs off a cable from a little gear housing in the transmission or transfer case output housing. There is a gear that needs to be changed to match the new tire height or change the tire height to match the installed gear.
Hotshot
Joined: Jun 2015
Posts: 12,763
Likes: 2,420
How to Calibrate Your Ford PSOM (P'ing OBS Trucks) - YouTube
Sorry, for some reason the link is not working. Go to youtube and punch in "How to calibrate your Ford PSOM"
A few good videos on the procedure. It's not real complicated.
Sorry, for some reason the link is not working. Go to youtube and punch in "How to calibrate your Ford PSOM"
A few good videos on the procedure. It's not real complicated.
Mountain Pass
Joined: Feb 2010
Posts: 140
Likes: 17
From: Brantford Ontario
A today real simple way... go on a tire calculator site & compare your original tire size to the new size...It will tell you how much faster/slower the new size is reading.
Subtract the 2 speeds given that is your difference ...When driving ad/subtract this difference ...That will be your actual speed....Rember a bigger tire will register a slower speedo speed.....so you will need to add the difference each time you look @ the speedo....
Subtract the 2 speeds given that is your difference ...When driving ad/subtract this difference ...That will be your actual speed....Rember a bigger tire will register a slower speedo speed.....so you will need to add the difference each time you look @ the speedo....
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