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.

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).