Recalabrating spedo - Diesel Forum

ClassicFordGuy · 06-22-2007, 12:47 PM

Has anyone racalabrated their spedometer? I tried to follow the directions I had saved a year ago but they require information like rotations per mile at 45MPH that I cannot find for my tire. They also give constants to multiply by based on rim size but they don't give one for a 16.5" rim. I was under the impression there was a way to do it by measuring the circumfrance. I had no idea things like speed and sidewall size and flex effected rotations per mile so much.

The truck is a 94 and I went from 33" BFG mud terrains to 36" goodyear millitary OZ bias ply hummer tires.

Thanks in advance,

-Rob

9ENGEL · 06-22-2007, 02:42 PM

I did mine awhile back with directions I found here. Don't remember anything about 45 mph. Try searching it or maybe someone will post them. I thought I had saved them but apparently not.

I seems to me I made a paint mark on the tire, drove it forward three revolutions, and measured the distance from first mark to last on the driveway, then divided by three to get an accurate rolling circumfrance. Then did the math to get the new calibration number, that part I don't remember.

But it worked, verified with GPS.

On Edit: found the directions I used.
Speedo
Recalibrate Your Speedometer After Changing Tire Size

#1 Measure your tires with 3 rolling stops by marking your tire with a piece of chalk and the ground. Make a mark at each 360 degree rotation until you did 3.

#2 Measure the distance of the 3 lines you made and then divide them by 3.14. (mine was 301 divided by 3.14 = 95.8) or 958 by dropping the decimal.

#3 To set the calibration, ground the single wire connector under the glove box it says PSOM (Programable Speedometer Opdometer Module). Yes there is only one and it has a spade type plug on the end". I used a wire with clips on both ends to ground the plug to a screw under the dash or try a test light and use the clip end to the PSOM and the other jam in a dooe hinge. Now look at your speedometer on the face of your dash while sitting in the drivers seat and there should be a "reset" button and a "select" button. While holding the reset button in on the trip meter, turn the ignition to 'on' while the wire is still grounded. Let go of the reset button. The speedometer display will sweep once and will show a code of some kind then push in again the "reset" button and the existing code will be displayed with RECAL?, mine said 976 RECAL? Now you enter your new code you came up with by dividing the rolling measurement by PI or 3.14 by pushing the "select" button until it gets to the new calibration code number. You may have to push select a whole bunch of times until you get to your code. If you turn off the key at this point the original code will not be changed or any of the 6 lives will not be used. To store your new code press Reset. Turn off the key. Unplug the ground wire. To check the new calibration, use the mile markers on the interstate. Run an indicated 60 mph and it should take exactly 60 seconds to go 1 mile.

Warning: You can only change the calibration 6 times without replacing the module. Your Ford dealer should be able to do this for you too if you aren't comfortable doing it yourself.

Mr_Roboto · 06-22-2007, 07:20 PM

Rim size doesn't matter, you need the revs per mile which is determined by circumference.

You can get close by measuring around the tire with the tape measure, then divide by inches per mile (63,360).

This method will give you a number a touch too low because the tire compresses under load. But speedometers aren't precision devices to start with.

OkieGringo · 06-23-2007, 01:20 AM

9ENGEL, these instructions are now in my "Library" with you listed as the author. You're now "famous" [img]/ubbthreads/images/graemlins/rolleyes.gif[/img]. [img]/ubbthreads/images/graemlins/smokin.gif[/img]OkieGringo

ClassicFordGuy · 06-23-2007, 01:43 AM

Thanks for the procedure, sounds alot better than the other one I was looking at. I don't think tire compression changes anything, the same amount of tire circumfrance is still traveling over the pavement, the circumfrance remains the same.

Thanks,

-Rob

9ENGEL · 06-23-2007, 10:21 AM

[ QUOTE ]
9ENGEL, these instructions are now in my "Library" with you listed as the author. You're now "famous" [img]/ubbthreads/images/graemlins/rolleyes.gif[/img]. [img]/ubbthreads/images/graemlins/smokin.gif[/img]OkieGringo

[/ QUOTE ]

Well, I guess that's my 15 minutes. [img]/ubbthreads/images/graemlins/grin.gif[/img]

I wish I remembered who originaly posted them, so I could give credit where credit is due.

Just glad I could help, as I've been helped so many times on here!