Swapping Speedo gears in a Model 18 transfer case?

Hi folks,

Since changing the tires to 33's on my '71, my speedo is incorrect by about 3MPH.
By doing the math, if I were to use the 3.73 speedo gears from my '68 V6 parts Jeep, I'd be VERY close to perfect.
I have speedo gears for 4.27's right now.

My question is:
What is involved to change the speedo gear?
Do you only have to change the DRIVEN gear or both the DRIVEN and DRIVE when changing from 4.27 to 3.73 speedo gears?
I've only had experience with GM and Ford Automatics where you only have to change the DRIVEN gear to correct the speedo.

Thanks for any and all help.

 

Speedos are not incorrect by a set amount but by a percentage. If you are off say 5% the error will be greater the faster you go. To be accurate compare the tire diameters (old versus new) and figure the percentage of change you made then use this to change and correct your speedo.
Heres an example, 30" tire compared to a 33" tire.
The circumference of the 30' = 94.27"
The circumference of the 33" = 103.67"
That is roughly 10% difference so at a registered speed of:
5mph = 5.5
10mph = 11
20mph = 22
30mph = 33
50mph = 55
70mph = 77
And so on.

 

Thanks for the breakdown.
The only speed I'm 100% sure of is at 45 MPH, I am at 42 on the speedo.

The math I was speaking of was in relation to the original tire height vs. my 33's.
That came up to around 3.68-close enough to 3.73 for me!

Thanks for the explanation.

Anyone know if the 3.73 and 4.27 driven speedo gears are swappable or do I need to change the drive as well?

 

nope. need to swap out the whole deal.


wish my speedo was that close. I read 10mph fast.