Speedometer jumping around so I removed the cable going into the transfer case and behold, a chewed up drive gear. Being 30 years old, it did it's job, but time to replace.
So I figure, since I'm running larger than normal tires I'll recalibrate. Simple as I found a couple of formula's and put things in place and came up with a lessor tooth (17) from what I was running before (21). Now I talked to a couple of guys in parts stores asking if I need to change out the gear inside my transfer case.... I got so many different answers.....! !
Now I'm sure there is someone out there who has been thru this predictament and sure would like to hear what you have to say...
Truck is a 73 F250 with a New Process 203 Transfer case.
Sooooo....the question is, do I need to change both gears or will just changing the drive gear on the end of the cable suffice.....?

 

Change the drive gear, but was your speedo that far off??? If in doubt talk to the guys in a driveline or speedometer shop.

 

My thoughts exactly, as they make many drive gears but only a couple of gears that goes into the transfer case. Trial and error I guess. What's the worse that can happen.....! A bunch of little nylon chunks floating around aren't gonna harm metal parts too much.
About 6 mph off, and since I switch to kilometers (now Canadian), I'm a bit off for comfort.
Thanks for the info, I'll watch and see what else someone may say. Don't have easy access to a tech shop around here that specializes in that sort.

 

You are taliking about a 20% difference in your gears. I assume you are saying that you are 6 mph off at 60mph which is only 10%. You might want to go check your figures.

 

Ummmmm.....rechecked. I probably would be closer at my calculations if I knew my rear end, but based on what I computed on this:
# of teeth X speedometer reading
------------------------------------------- = # new teeth
actual speed

21 X 55.9
------------ = 17.17
68.35

Of course all this was done on a KPH and converted over to MPH on paper. Also taking into the variables of jumping speedo, and other un-natural guess work. I can see if I was 6 MPH off in the beginning, but that was something I always assumed. Never checked really how far off I was. But, according to the conversion, I was a good 12 MPH or so off. Of course I should try doing this with a good gear in place. With a handful of gears coming in, I can always switch them out until one hits close.
The main question was, would I have to switch out both gears or just the one at the end of the cable.....? Thanks for the help and I'm always open to ideas, even a kick in the ??? from time to time.....

 

I had such a hard time with this it was close to impossable just getting the gears. I did find that drive gear was no good and caused the speedo to read slower. The parts guy at the dealer told me me that driven gear you can adjust the speed by useing gears that had more or less teeth. One less tooth ups your speedo 5 mph. Sense then I bought an old dealers parts books looking at it. It seems that they changed the drive gears for different tire sizes and the driven gear stayed the same maybe it was easer to build this way or so it can be adjusted. So I beleave that you can change the driven gear to change your speedo speed. But I would start with a stock drive gear.