I was just wondering, after changing gears from 4.10 to 3.55 if I needed to do anything to the setting for the the speedometer

 

First off Welcome to FTE!

I forget where I found this but here you go (I think it was from here):
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

 

To use this you'd have to pretend you put smaller tires on. I think your regear would be like going from a 35" tire to a 30". But I could be wrong.

 

You dont need to recalibrate the speedometer when you change gear ratios.

 

Justin it's a lot easier if you dont use the figures for a 15" tire and then try to convert it for 16's. Try this:

"First, you'll need to know the actual height of the tires, not the
nominal height of 35" that's printed on the sidewall. To get that,
make a mark on the edge of the tire at the ground, and mark the ground
directly under the mark. Now, roll the truck far enough for the tire
to have made three revolutions. When your mark on the tire is at the
very bottom, near the ground, mark the ground again, and measure the
distance between the two marks on the ground. Divide that distance by
three, since there were three revolutions of the tire, then divide
that number, (which is the circumference of your tire,) by 3.14. The
answer you have is the true height of your tire. Write that number
down, because you'll need it for the next step. Divide 30282 by your
actual height. If your true height is 34", the math goes something
like this: 30282/34=891. That would be your speedometer calibration code."

 

you can change gears all day long with no problem, you only calibrate when you change tires

 

^^^^^^ True for tranny gears. But the diff is "downstream" of the vehicle speed sensor, right. Isn't it safe to say that if you change anything after the VSS (meaning, diff gears or tires), it affects the real road speed relative to the reported speed? Or is there something I'm missing?

 

The Speed sensor is on the rear diff and only cares how many times in a mile the carrier will rotate so the only thing that will change that is tire size.

 

OK duh, I knew that , Doh. I forgot the VSS registers off the ring gear.

 

I know this is an old thread, but I'm curious about the same type of issue. Since the VSS counts rotations of the carrier, what needs to be done if you install a full locker or something that replaces the carrier?

 

Nothing needs to be done for your speedo as long as the tone wheel of your stock carrier gets put onto your locker or other carrier.

 

Did not know that... I probably would've lost my mind if I swapped in a locker and then my speedo didn't work. Good info TJ!

 

Good info! Gotta do this to my '96 F350 since I swapped from the stock 215's to 235's.