speed-o calibration

[sivlersnake]

I bought a '90 F-250 4X4 a few weeks ago. It has 36" tires. This, of course, throws the speed-o off. Is there and easy way to fix this or is this something that I just need to live with?

 

[Skandocious]

Not sure if the procedure is the same for gen 8's and 9's but here is the one for a gen 9, it's worth a shot! Taken from another site:


HOW TO RECALIBRATE THE SPEEDOMETER ON 92-96 FORD F-SERIES AND BRONCO

By Benjamin Ratterree

1. Check whether the conversion constant in the speedometer is correct with the following procedure:
# Press and hold the RESET button on the front of the speedometer while turning the key to RUN.
# Release the RESET button. The pointer will prove out and some codes will appear on the odometer display. If an "E" is on the left side of the display, the module is programmed for English display mode and should have English graphics (mph). A lower case "o" signifies an overseas (metric) graphics (kph). Verify the graphics and display modes agree. (The number appearing after the type is the microprocessor revision level and is not used in this test.)
# Rress and release the RESET button again. The conversion constant and the word CAL will be shown on the odometer display.
# Check the conversion constant shown on the odometer display with the chart below and verify the correct constant is entered.
# Turn off the key to exit this test.

2. Reprogram speed conversion constant using the following procedure:
# Locate the dealer mode programming enable single-terminal connector. The connector is located at the bottom of the instrument panel below the center of the glove box. On all vehicles, the dealer mode enable wire is Circuit #567 (LB/Y).
# Insert metal clip in service wire panel connector. Clip other end to vehicle ground while the key is in the OFF position.
# Turn key to RUN (not crank) position while holding down the trip odometer RESET button on speedometer. Release RESET button.
# Speedometer will prove out (sweep across dial and back). The English/Metric and revision levels will be displayed. The last number of the display is the dealer mode programming lockout count. This count shows the number of allowable conversion constant changes remaining. When the count is zero (0), no additional changes can be made to this instrument cluster. If a conversion constant change is required, a service instrument cluster must be ordered. Refer to this section for ordering information.
# Press and release RESET button again and PSOM conversion constant (without the decimal point) followed by the word CAL.
# Press and release the odometer SELECT button as many times as necessary to change conversion constant to the correct value.

# NOTE: Pressing and releasing the RESET button to lock in the new Conversion Constant reduces the number of times the PSOM can be reprogrammed by one count. The module can only be reprogrammed six times. If you change your mind and want to go back to the old conversion constant, turn the ignition switch off before you lock in the new constant. When the correct new value is displayed in odometer window, press and release the RESET button to lock in the new conversion constant.
# Turn the key off and remove the ground wire. Verify proper speedometer operation.

# To find conversion constants for tires with 15 inch rims that are not listed in chart, above multiply the revolutions per mile (at 45mph) of your tire times .0135. For 16 inch rims multiply by .015. The revs/mile can be found at your tire manufacturers web site. There are slight variations in revs/mile per tire manufacturers, but using one like the popular BFG A/T (see below) will get you very close.

Example: BFG AT 33x12.50R15LT/C
Revs/mile= 636
636 rev/mile X .0135 = 8.59
8.59 would be your conversion constant

Here is a listing of popular oversize BFG All-Terrain tires with conversion constants:

Tire Size Revs/mile (at 45mph) Conversion Constant
30x9.50R15LT/C 705 9.52
31x10.50R15LT/C 677 9.14
32x11.50R15LT/C 656 8.86
33x12.50R15LT/C 636 8.59
35x12.50R15LT/C 599 8.09

# Questions? Email [email protected]

 

[sivlersnake]

I will try it as soon as I get a chance!!
Thanks!!

 

[89frankenford]

i dont believe that will work at all on the Gen 8 trucks. actually im 100% positive it wont. the reason is because the speedometer in the 87-91 trucks is not electrical at all. its simply a gear on the tail shaft or either the transfer case or the transmission (depending if it is 2wd or 4x4) and it runs a cable which is directly connected to the speedometer. the way you can adjust the speedo is get a new gear for the tail shaft. i believe you can either buy it at Napa or you can go to Summit Racing or something like that,

 

[Lost]

Yes the speedos r cable least my 89 was . Napa or other parts house should have what u need.

 

[sivlersnake]

Thanks!!
How hard to install??

 

[Skandocious]

i dont believe that will work at all on the Gen 8 trucks. actually im 100% positive it wont. the reason is because the speedometer in the 87-91 trucks is not electrical at all. its simply a gear on the tail shaft or either the transfer case or the transmission (depending if it is 2wd or 4x4) and it runs a cable which is directly connected to the speedometer. the way you can adjust the speedo is get a new gear for the tail shaft. i believe you can either buy it at Napa or you can go to Summit Racing or something like that,

Ahhhh crap how could I forget. Yeah I forgot that your gen 8 trucks are from the DINOSAUR ERA!

 

[Truckin4life]

Thanks!!
How hard to install??

I want to say there is one small bolt that holds the gear in place.
Pretty easy remove and replace...

 

[Truckin4life]

What engine, trans, and gear ratio do you have.
Also are the gears factory?

Let me know i can tell you how to do the install and which gear you will need.

 

[95F350XL]

Real easy to change, SKAN how could you forget that we have a digital and the older has a manual one, tisk tisk tisk. Ha just bustin chops. So on my 92 and 95 I can change the speedo to read right, damn I didnt know that.

 

[sivlersnake]

I have a F-250 4X4 with the 5.8L, 5 speed. not sure about the gear ratio.

 

[sivlersnake]

Ok folks, how do I find out the gear ratio?
.....short of opening up the rear end. lol

 

[Skandocious]

Couple options--

1) Look on the rear diff for a tag about the size of a stick of gum. There will be a number on there with a format similar to "3 55" (which is 3.55 open diff), "3L55" (3.55 limited slip), "3 73" (3.73 open diff), "3L73" (3.73 limited slip), etc etc.

2) If the tag is missing then look on the driver side door jam for the door sticker. There will be a code at the bottom of the sticker for your gear ratio. Then go here and decode it

If the sticker is also missing then there's another option that is a little more involved and sometimes hard to be accurate. Jack up the rear end, mark the pinion yoke with chalk, then stand next to the truck and spin the tire slowly by hand while watching the yolk; count the tire spins that it takes to make the driveshaft do 1 full 360* rotation

 

[sivlersnake]

 

[Old_Paint]

Couple options--
If the sticker is also missing then there's another option that is a little more involved and sometimes hard to be accurate. Jack up the rear end, mark the pinion yoke with chalk, then stand next to the truck and spin the tire slowly by hand while watching the yolk; count the tire spins that it takes to make the driveshaft do 1 full 360* rotation

Might wanna count the number of times the driveshaft rotates for each full rotation of the wheel.

If you have a 3.55:1, and you turn the BOTH wheels 10 full revolutions in the same direction, you should get 35.5 revolutions of the driveshaft. If you have a locker, you'll have to lift both rear wheels to do this. If you do not have a locker, then you will probably see a lot less turns of the driveshaft because the ratio changes drastically when one axle is stopped and the other is rotating. The spiders will double the ratio to the spinning axle when the other is locked. The ratio listed is the ratio of the pinion to ring gear. You could pull the rear cover and count teeth on the pinion and the ring, but I take it this is exactly what you're trying to avoid.

Take a big cable tie, and put it on the drive shaft, and then trim it where it will hit in only one place so you can hear it tick. On a flat/level road, start some place and put a mark on a tire so you can see it make a full revolution. The more revolutions you make, the better your math accuracy will be. Move slowly so you can count the ticks of the "cat whisker" you put on the shaft, and someone else can walk beside and count the revolutions of the tire, and tell you when to stop. THEN, you will get an accurate ratio of the diff, AND you can also measure the rolling distance of the tire to use to program your PSOM. If you have a 3.55 diff, you'll hear 35 clicks from the cat whisker if you move 10 revolutions of the tire in a straight line. There's enough difference in the various ratios you'll get it close enough if you keep count. If you don't get something close to what sounds like a good ratio, then try again, and make sure the cat whisker isn't hitting two objects.

Oh, when you're done with the cat whisker, just take off down the road at about 60 MPH. It'll come off. Trust me. An electrical contractor I use at work thinks its REALLY funny when they put about 30 cable ties on my driveshaft, especially when I drive our long van and it's easy to get under. They're not doing it to check my diff ratio, though. I can tell.