Sorry to bring this up again. I need to get my speedo back inline after going up from 255-70-16 to 265-76-16 tires. A week ago a Ford mechanic told me my 97 Expy only needed a speedo gear change to accomplish this because I have a non-digital odometer. This matched what some have told me here. I went to the dealership tonight to get a look at their computer to see where the gear was on the t-case and a mechanic told me that I needed to have the computer reprogrammed and the gear was not replaceable. He said even though I have a non-digital odometer that the PCM needs to be reprogrammed. The parts guy printed up the gear assembly that the wiring harness plugs into and he said it was all 1 piece with electronics built in. He showed me an assembly that the wiring harness plugs into then this assembly gets inserted into the driver's side of the t-case tailshaft. Didn't look like any gear could be swapped out.
1) Is this the right item from the t-case that controls the speedo?
2) Is my t-case different than the guys with the F-150's that are swapping the gear? My t-case has 2WD-AWD-N-4HI-4LO on the selector switch.
3) If I can still swap the gear do I go bigger or smaller?
I've seen 2 different answers after doing a search here.

 

to my knowledge the 97 and newer years. the computer controls the input to the speedo. so a gear change would not fix the problem. it would just put the computer further outta wack. if you go to your local lift shop they can probly answer your questions and update your computer to account for your tire change.

 

I don't think the '97s are controlled thru the computer. The reason I think this is that when I bought and installed my MicroTuner, the speedo-adjust feature was disabled for my yr/PCM. I think either the 98 or 99 trucks were the first that were computer controlled.

So, I think the gear change is the only way to compensate with a 97 turck.

 

If I shut the engine off while the truck is moving the speedo goes to 0. Does this mean its mechanical or electronic? I'm thinking electronic.

 

This site will tell you exactly what it is out with the tire change, if it helps.
http://www.miata.net/garage/tirecalc.html

 

I believe 98 is the last year of the tranny mounted speedo gear. 99 went to computer controlled speedo.,,,,98