Speedometer corrections:

Changing tire size or gears will throw off your speedometer.  This is annoying, but easy to fix by changing the speedometer gear in the transfer case.

Step 1:  Find out how far off your speedo is.
Drive on the highway for 10 miles (according to mile markers) and note how far your odometer advances.  Divide the odo reading by 10 and that will tell you what percentage your speedo is off.

Example:  If you drive 10 miles and your odometer advances 8.75 miles,  8.75 / 10 = .875.  So your speedometer is reading 87.5% slow.

Step 2:  Find out what speedo gear you need.
Remove the speedometer driven gear from the transfer case.  Do this by following the speedometer cable down to the transfer case.  There will be 1 bolt to remove, and the gear should come out with the cable.  Count the number of teeth on the gear.  Multiply the number of teeth by how far off your speedo is and this will give you the gear you need.

Example:  If you find that you have an 18 tooth gear and your speedometer is 87.5% off,  18 * .875 = 15.75.  So you need a 16 tooth gear which is the closest to the result.

Step 3:  Locate the correct gear and install it.
Try your local dealer, and if they don't have them, you may need to go to a company that specializes in obsolete parts.  Pull the metal clip off the gear and the gear should come right off the cable.  Install the new one and insert it back into the transfer case and you're done.

Step 4:  Test it!
I tried this fix, the example numbers are what I came up with.  When I drove the same 10 miles on the highway my odometer advanced 9.9 miles!  I have also tested it by driving next to cars with digital speedometers and compared to my GPS unit, and it is almost dead on!

Please note that most highway markers are not perfect, but over 10 miles it should be pretty close.  You can use a longer distance for more accuracy.

Speedometer Driven Gear Part Numbers
Part Number# of teethGear Color
C0DZ-17271-A16 teethwine
C3DZ-17271-C17 teethwhite
C0DD-17271-B18 teethgold or yellow
C0DZ-17271-B19 teethnatural color
C1DZ-17271-A20 teethblack
C4OZ-17271-A21 teethred

I got my gear from Green Sales Company.


Ramp Calculations:

Finding RTI score from tire height 

If you do not have a 20 degree ramp, but have some other way to get a wheel up in the air (such as a forklift).

(1/sin(20) * Wheel height) / Wheelbase * 1000

for example, If I get a wheel 23 inches high:

1/sin(20) = 2.924

(2.924 * 23) = 67.247

67.247 / 92in * 1000 = 730 RTI Score

Converting a score from a 20 degree ramp
to a 30 degree RTI score

20deg score * sin(20) / sin(30)


sin(20) / sin(30) = 0.781


20 deg score * 0.684 = 30 deg score

For Example, If my 20 deg RTI score is 1000

1000 * 0.684 = 684 RTI score on a 30 deg ramp

Converting a score from a 30 degree ramp
to a 20 degree RTI score

20deg score * sin(30) / sin(20)


sin(30) / sin(20) = 1.462


30 deg score * 1.462 = 30 deg score

For Example, If my 30 deg RTI score is 1000

1000 * 1.462 = 1462 RTI score on a 20 deg ramp



Page by Marc Reiter - last modified 01/05/00
©2002 copyright BroncOhio Early Bronco Club.