The question is a difficult one to answer.
Back in 1979, tire sizes from the factory ranged from 9.00 x 15 to H or L78-15.
Axle ratios were 3.07, 3.54, and maybe 4.09.
So as you can see, we are no where close to what you are running for tires and axles ratios today.
The speedometer drive gear is mounted on the
Transfer Case output shaft, behind the rear output yoke and housing. It's necessary to remove all this to access the drive gear. Why ? Because it will be necessary to identify which drive gear is installed and what the Stewart-Warner number is on the gear. The number is usually stamped into the side of the gear, very small print/numbers to read.
The number is important because you must have the correct matching series of numbers for the driven gear; the driven gear is what the end of the speedometer cable installs into to drive the cable. The driven gear is accessed by removing the cable and the cable adapter, the part the cable threads onto. The driven gear can be removed easily and the Stewart-Warner number identified and recorded.
Failure to use matched, series numbered gears will result in gear destruction.
The gears are no longer manufactured new, so the only supply is NOS, used, or from donor transfer cases. Hence the need to not destroy a gear by not doing the research first.
If you have a currently working gear set and speedometer, regardless of the accuracy, in my opinion, you're best to get an in-line reduction unit or something similar purchased at a local speedometer shop; it would simply screw onto the cable adapter at the
Transfer Case , and then your cable would thread onto it. These were commonly used on big trucks ( ie., IH, etc).
To your question regarding tooth counts:
for 1979 CJ models, from the factory:
13, 15, 17, and 18 tooth driven gears were used.
Jeep factory part numbers were typically:
535 1827 17 tooth, SW # 890208, 4.09 axle ratio, 9.00-15 tires
535 1828 18 tooth, SW # 890209, 4.09 axle ratio, H78-15 tires
535 2812 13 tooth, SW # 890237, 3.07 axle ratio, 9.00-15 tires
535 2813 14 tooth, SW # 890238, 3.07 axle ratio, HR78-15 tires
535 2814 15 tooth, SW # 890239, 3.54 axle ratio, 9.00-15 tires (Tracker AT)
Sometimes these appear on Ebay as NOS gears.
The correct drive gear used with these gears would be:
Jeep factory part number:
535 2816, SW # 447710
The numbers mentioned above should most likely be found inside your
Transfer Case , as it left the factory. Of course, different tooth counts could/would be used depending on tire size ordered.
The drive gear mentioned above should be all metal construction.
There were some drive gears constructed of aluminum hubs with a plastic gear; they appeared for a short time, but they were eliminated over time as the plastic gear would heat up and start slipping on the aluminum hub. An indication of this occurring would be speedometer works fine when cold and first driven, but the needle slowly falls/drops while driving after vehicle is warmed up or driven any distance.
So most likely you will have to start with a matched gear set and get a working speedometer first, then adapt it to your tire/axle ratio needs.
With D18 and
Dana 20 transfer cases. the selection of gears sets is
Limited ; you won'y have the availability and selection that are available for
Dana 300 transfer cases.
FWIW, 1 tooth of driven gear equals approx 5-7 mph on the needle.
Be aware that you may find different numbered gears that may also work with your gear from full size Jeep J-10, J-20, and Wagoneer and Cherokee
Dana 20 transfer cases. Avoid the gears used in Q-trac equipped transfer cases and they use different tooth counts.
Also, this list is not complete, factory variations and different combinations do exist, and time/vehicle ownership along with mechanical repairs to transfer cases thru the years, may affect any gear combination that can be found today.
So always ask questions or check available resources before installing and running any of the S-W speedometer gears.
