Recalibrating Spedo?

[Classicfordguy]

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

 

[Cheaper Jeeper]

Got a GPS? That's the easiest way I know of.

Get on the highway and do the speed limit (according to the GPS) and make note of what your speedo says. Use those two numbers to calculate how far your speedo is off. Then pull your speedo gear and decrease or increase the gear size by that amount. For example

GPS speed = 65
Speedo = 60

Speedo is reading 92% of your actual speed (60 / 65 = .92).
If your speedo is slow you need a smaller speedo gear.
In this case suppose the speedo gear has 31 teeth.
31 x 92% = 28.7 but since a gear can only have a whole number of teeth you have to round up and get one with 29 teeth or round down and get one with 28 teeth.

Of course if your speedo were reading too fast you would apply the same formulas and get a speedo gear with MORE teeth to slow it down.

Hope that helps.....

 

[Classicfordguy]

The speedo on a 94 is driven off a large thin gear next to the ring gear in the rear diff, it goes through a sending unit and sends an electrical signal to the dash. So you can't change the gear like you can in a tailshaft drive type speedo. There is some long complicated way you can do some math and punch a code into the dash via the reset and mode buttons(or whatever they are labeled) such that it will be accurate again with the larger tire size.

-Rob

 

[subway]

The speedo on a 94 is driven off a large thin gear next to the ring gear in the rear diff, it goes through a sending unit and sends an electrical signal to the dash. So you can't change the gear like you can in a tailshaft drive type speedo. There is some long complicated way you can do some math and punch a code into the dash via the reset and mode buttons(or whatever they are labeled) such that it will be accurate again with the larger tire size.

-Rob

i will have to chech that out, that is some good info to know.

i know they just did a little tidbit in car craft this last issue where they spin older style speedo's at 60 mph and use a big magnet to either magnatize or demagnatize the cluster to read right. they were mentioning it is a dying art.

i would think a local speedo shop or dealer would be able to help you out quickly.

 

[hoodshauler]

seams like Like I remember A member of another forum having the instrutions on how to do the buttons thing let me see if I can find that tech artical.I may have sayed it on my putter even.

 

[hoodshauler]

Ok here ya go this what is what i found hope it helps.

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.

 

[Classicfordguy]

That's what I was looking for you guys are great.

Thanks,

-Rob

 

[TLBREWER]

Holy Smokes I had no idea you could do that!

Tom

 

[towcat]

Holy Smokes I had no idea you could do that!

Tom

don't get too worked up Tom.
this is for '92 and up only. If you have the digital ODO, this works. If you have the mechanical ODO....you're SOL......changing speedo gears is the only way for you.

 

[TLBREWER]

don't get too worked up Tom.
this is for '92 and up only. If you have the digital ODO, this works. If you have the mechanical ODO....you're SOL......changing speedo gears is the only way for you.

Oh yea, I know. But I traded a '92 CC for my DMax. At the time, I didn't know there was a difference from '91 to '92. It never dawned on me it would be that way with the digital ODO. Which reminds me, I stll have to figure out why the speddo in my daughter's truck only works intermittently. I do know it's not the drive gear at the tailshaft. Guess it's time to take the dash apart.

Tom

 

[towcat]

Oh yea, I know. But I traded a '92 CC for my DMax. At the time, I didn't know there was a difference from '91 to '92. It never dawned on me it would be that way with the digital ODO. Which reminds me, I stll have to figure out why the speddo in my daughter's truck only works intermittently. I do know it's not the drive gear at the tailshaft. Guess it's time to take the dash apart.

Tom

hey tom-
you sure the gear on the output shaft is OK? I'm talking about the one the gear on the cable meshes to. I've seen those go bad more often than a speed cluster going bad.

 

[TLBREWER]

hey tom-
you sure the gear on the output shaft is OK? I'm talking about the one the gear on the cable meshes to. I've seen those go bad more often than a speed cluster going bad.

Well now that you mention it...When I pulled the cable out of the tail shaft, I stuck my finger in to make sure the bearing was OK as Tonka mentioned. I did feel the gear and it seemed OK, although I didn't look at it with a mirror.
I drove it a couple of days later and the speedo is not completely inop. At about 40-45 it jumps up and searches around a little, then goes back to zero. It seems to do that every time the 40-45 mph threshold is passed. I just hope it's not a speedometer rebuild.

Tom