Speedometer Calibration
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Speedometer Calibration
Hi all, I'm new here, recently bought an 89 f-150 with the 4.9l and a 5 speed trans, and 168,000 miles. I've been reading everything I could on the board for a few weeks and it has been very helpful...Thanks!<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com
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I haven’t been able to find an answer for my current problem; the speedometer seems to constantly read about 20% high. I wasn’t sure what was going on at first and then I compared it against a friend’s GPS, when the GPS says 50 the speedometer reads 60. When the GPS says 30 the speedometer reads 35ish. I also checked the odometer and it also says the truck goes about 20% further then it does on the GPS. Just wondering if there was any easy way to calibrate these. <o></o>
The tires are the same size listed on the doorjamb, I checked that. Thanks!<o></o>
<o></o>
--Sean<o></o>
fficeffice" /><o></o>I haven’t been able to find an answer for my current problem; the speedometer seems to constantly read about 20% high. I wasn’t sure what was going on at first and then I compared it against a friend’s GPS, when the GPS says 50 the speedometer reads 60. When the GPS says 30 the speedometer reads 35ish. I also checked the odometer and it also says the truck goes about 20% further then it does on the GPS. Just wondering if there was any easy way to calibrate these. <o
></o>The tires are the same size listed on the doorjamb, I checked that. Thanks!<o
></o><o
></o>--Sean<o
></o>
Postmaster
Joined: Oct 2006
Posts: 4,816
Likes: 1
From: Davis & Newcastle, CA
Yes the PSOM (programmable speedometer/odometer module) can be recalibrated, but only a finite number of times. Here's instructions:
http://ford.off-road.com/ford/article/articleDetail.jsp?id=198381
Welcome to FTE
EDIT: Whoops. Didn't realize it was an 89 with the speed gear in the tranny/t-case. So Bill, does that mean he can't reprgram his PSOM like mine?
http://ford.off-road.com/ford/article/articleDetail.jsp?id=198381
Welcome to FTE
EDIT: Whoops. Didn't realize it was an 89 with the speed gear in the tranny/t-case. So Bill, does that mean he can't reprgram his PSOM like mine?
Fleet Owner
Joined: Aug 2005
Posts: 23,667
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From: Easton,Ks
Originally Posted by Skandocious
EDIT: Whoops. Didn't realize it was an 89 with the speed gear in the tranny/t-case. So Bill, does that mean he can't reprgram his PSOM like mine?
Postmaster
Joined: Oct 2006
Posts: 4,816
Likes: 1
From: Davis & Newcastle, CA
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Posting Guru
Joined: Jun 2002
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From: Suffolk, VA USA
If you can provide the rear end gear ratio and current tire size, I should be able to calculate the right speedometer gear size.
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Elder User
Joined: Dec 2006
Posts: 550
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From: Hillsboro, OR
I have the same problem. I have a ’91 F350 (2WD, C6, 351) that the speedo is running about 5 MPH more than what I am running. At 45, it is reading 50. I just went out and looked at the door sticker and tires and here is what I found:
The sticker says I have a C9 axle. The tires should be LT 235/85R16E with 16 X 6K rims.
The actual tires on the truck are LT 265/75R16 load range E. No idea on the rims.
This brings up some questions. Are the tires on the truck bigger or smaller than what should be on it? I never get these numbers straight! Would switching to the sticker tires help my fuel mileage or make it worse? If I switch to the sticker tires, will my speedo go back to normal? The tires on the truck are almost new so how much work is it to switch speedo gears and is it worth it? How do I find out what gear to use and can I use my old gear later if I switch to the sticker tires?
Thank you for your help.
The sticker says I have a C9 axle. The tires should be LT 235/85R16E with 16 X 6K rims.
The actual tires on the truck are LT 265/75R16 load range E. No idea on the rims.
This brings up some questions. Are the tires on the truck bigger or smaller than what should be on it? I never get these numbers straight! Would switching to the sticker tires help my fuel mileage or make it worse? If I switch to the sticker tires, will my speedo go back to normal? The tires on the truck are almost new so how much work is it to switch speedo gears and is it worth it? How do I find out what gear to use and can I use my old gear later if I switch to the sticker tires?
Thank you for your help.
Fleet Mechanic
Joined: Nov 1999
Posts: 1,415
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From: Lower 48
Putting the size tires on it that are indicated on the door jam should bring the speedo back in. As far as fuel mileage, it's anybody's guess (my opinion). The rim size is the "16" part of the tire size. LT (light truck) 235/85R"16"E or LT 265/75R"16" load range E. The "E" (as in 75R16E or Load range E) is the weight rating of the tire - the higher in the alpahbet the heavier the weight rating. Load Range E is the equivalent of a 10 Ply tire, as opposed to a Load Range B found on most passenger car tires which is the equivalent of a 4 ply tire. Hope this helps at least somewhat.
Elder User
Joined: Dec 2006
Posts: 550
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From: Hillsboro, OR
Yes, this helps. It may be a while before I put the right tires on since the ones on it are almost new. I am afraid to see what these things cost!! It is good to know that I have a good load range though, thanks.
Elder User
Joined: Dec 2006
Posts: 550
Likes: 1
From: Hillsboro, OR
I just found a great site to compare tires. Link below:
http://www.1010tires.com/TireSizeCalculator.asp
According to the information below comparing the stock tires to what is on the truck, the difference should not be enough to account for the spedo being off as much as it is. Anybody got any other ideas?
Stock Tire - 235/85R16 >Search Tires Tire 1 - 265/75R16
Section Width: 9.25 in 235 mm Section Width: 10.43 in 265 mm
Rim Diameter: 16 in 406.4 mm Rim Diameter: 16 in 406.4 mm
Rim Width Range: 6 - 7 in Rim Width Range: 7 - 8 in
Overall Diameter: 31.72 in 805.68 mm Overall Diameter: 31.64 in 803.65 mm
Sidewall Height: 7.86 in 199.64 mm Sidewall Height: 7.82 in 198.62 mm
Radius: 15.86 in 402.84 mm Radius: 15.82 in 401.82 mm
Circumference: 99.65 in 2531.1 mm Circumference: 99.39 in 2524.5 mm
Revs per Mile: 655.7 Revs per Mile: 657.3
Actual Speed: 60 mph 100 km/h Speedometer1: 60.1 mph 100. km/h
Speedometer Difference: - Speedometer Difference: 0.244% too fast
Diameter Difference: - Diameter Difference: 0.26%
http://www.1010tires.com/TireSizeCalculator.asp
According to the information below comparing the stock tires to what is on the truck, the difference should not be enough to account for the spedo being off as much as it is. Anybody got any other ideas?
Stock Tire - 235/85R16 >Search Tires Tire 1 - 265/75R16
Section Width: 9.25 in 235 mm Section Width: 10.43 in 265 mm
Rim Diameter: 16 in 406.4 mm Rim Diameter: 16 in 406.4 mm
Rim Width Range: 6 - 7 in Rim Width Range: 7 - 8 in
Overall Diameter: 31.72 in 805.68 mm Overall Diameter: 31.64 in 803.65 mm
Sidewall Height: 7.86 in 199.64 mm Sidewall Height: 7.82 in 198.62 mm
Radius: 15.86 in 402.84 mm Radius: 15.82 in 401.82 mm
Circumference: 99.65 in 2531.1 mm Circumference: 99.39 in 2524.5 mm
Revs per Mile: 655.7 Revs per Mile: 657.3
Actual Speed: 60 mph 100 km/h Speedometer1: 60.1 mph 100. km/h
Speedometer Difference: - Speedometer Difference: 0.244% too fast
Diameter Difference: - Diameter Difference: 0.26%
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