How much further off? It might still be in tolerance.
. I've not checked speeds between too accurately. I might in the coming days, but not tomorrow.
Correcting a PSOM Offset, not scale
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Tuned
Joined: Jun 2025
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Correcting a PSOM Offset, not scale
I noticed that my PSOM, as compared to my phone's GPS, is fast by about 2 MPH from at least 25-50. After that, it gets further off, but I'm guessing that's due to tire slippage/wheel hop. Next time I have a load over the rear wheels, I may try the higher speeds again.
I know there's a way to correct the scale, aka calibration, but is there a way to change the offset?
I know there's a way to correct the scale, aka calibration, but is there a way to change the offset?
Hotshot
Joined: Jul 2020
Posts: 10,359
Likes: 2,593
From: Near Salt Lake City
Offset?
There is only a tire size adjustment in the PSOM
4mph at 70 is not enough to worry about
Adjusting the tire size to a smaller tire may make you read 76 at 70
If less than a tenth off, we wouldn't mess with it at the dealer
There is only a tire size adjustment in the PSOM
4mph at 70 is not enough to worry about
Adjusting the tire size to a smaller tire may make you read 76 at 70
If less than a tenth off, we wouldn't mess with it at the dealer
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Tuned
Joined: Jun 2025
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I followed the factory service manual and lifted a tire, measured it, and followed the formulas. Checked the program in the PSOM and it matches just fine. I don't think the calibration is off at all. I was just curious is there was a way to adjust the 0 mph point of the PSOM. If not, that's fine. The speedometer is a bit off on my 2011 F250 as well, and we just live with it.
I really should load up some concrete blocks and test it out at higher speeds to see if it gets more accurate. Speaking to my father in law, he said it always felt a little squirrely on him at 60 when he was unloaded, which fits with the speedometer being further off at speed.
I really should load up some concrete blocks and test it out at higher speeds to see if it gets more accurate. Speaking to my father in law, he said it always felt a little squirrely on him at 60 when he was unloaded, which fits with the speedometer being further off at speed.
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Hotshot
Joined: Sep 2001
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What is offset?
Mine is calibrated for 31 10.50 and still reads 3 or 4 mph off on the highway at 60-65. Haven't worried about it since I got this truck a decade ago and consider it a non issue.
But could be the needle was removed at some point and is no longer positioned correctly. It should move up just slightly from the resting peg when key is switched on. Seen some where it moves up a bit more than it should.
Can't imagine its tire slippage or wheel hop etc.
How far is it off on the highway?
Loading the bed shouldn't make any difference. Just go for a test drive.
Feeling squirrelly on the highway wouldn't have anything to do with the speedometer. Most likely do to bad or improperly inflated tires or worn suspension or steering components or bad alignment.
Is this a lifted truck? What size tires are on it? Fairly common for either or both to make it squirrelly at speed also. This can be from bad geometry or alignment. Big tires exaggerate these types of issues.
Mine is calibrated for 31 10.50 and still reads 3 or 4 mph off on the highway at 60-65. Haven't worried about it since I got this truck a decade ago and consider it a non issue.
But could be the needle was removed at some point and is no longer positioned correctly. It should move up just slightly from the resting peg when key is switched on. Seen some where it moves up a bit more than it should.
Can't imagine its tire slippage or wheel hop etc.
How far is it off on the highway?
Loading the bed shouldn't make any difference. Just go for a test drive.
Feeling squirrelly on the highway wouldn't have anything to do with the speedometer. Most likely do to bad or improperly inflated tires or worn suspension or steering components or bad alignment.
Is this a lifted truck? What size tires are on it? Fairly common for either or both to make it squirrelly at speed also. This can be from bad geometry or alignment. Big tires exaggerate these types of issues.
Last edited by TexasGuy001; Nov 28, 2025 at 05:15 PM.
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Tuned
Joined: Jun 2025
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I've not noticed it being squirrely on the highway, at least since I replaced all the suspension bushings, tie rods and ends, and tightened up the steering gear. I just noticed that the speed is further off at higher speeds.
Tires are stock size and truck is at stock height, except maybe sagging in the front due to age. I have had it aligned with where it sits and it runs fine.
Honestly I'm not worried about the speed being off a little, I just want to know if there's any more adjustments than the cal. Oh, and the call number programmed into the PSOM matches the factory service manual for my rear end and tires.
Tires are stock size and truck is at stock height, except maybe sagging in the front due to age. I have had it aligned with where it sits and it runs fine.
Honestly I'm not worried about the speed being off a little, I just want to know if there's any more adjustments than the cal. Oh, and the call number programmed into the PSOM matches the factory service manual for my rear end and tires.
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Tuned
Joined: Jun 2025
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Yes, the needle jumps up slightly off the rest position when I start the truck.
I suppose the speed getting off as I drive faster could be tire deformation. It's not like it's a huge amount off, just noticable.
I suppose the speed getting off as I drive faster could be tire deformation. It's not like it's a huge amount off, just noticable.
Hotshot
Joined: Sep 2001
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Tire deformation? The reason that the speedometer may read increasingly more off at higher speeds is because its exponential.
Last edited by TexasGuy001; Nov 30, 2025 at 03:23 PM.
Cross-Country
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There is practically zero slip when traveling at a steady state down the highway. What you are describing (error increases as speed increases) is not an offset; it's a scaling error due to inaccuracy in wheel size and the inherent limitations of the system. If you're using stock size wheels with the stock calibration, you're getting about the best the truck can do. Your wheel size is not constant due to tire wear. Tire diameter can change by 2% or more as it wears. If your speedometer is good to 100mph, then that 2% turns into 2 mph. You can see how quickly this can stack up when combined with aging dashboard actuators, weak speedometer signals, and so on.
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