Looking for dana 20 speedo gear info

Does anyone know what tire size was offered as "stock" for a 1975 cj5 with 3.73 axle gears?

Trying to figure out what to do to make my speedo read close to correct. I will be looking into getting a speedo cable soon. Most likely I'll hook the cable up and drive it to see how far it is off. However I did some math and think I figured out the ratio the speedo cable needs to turn vs the tcase shaft. Does anyone know if there is a speedo gear combo that produces a 2.09 ratio?

My speedo is calibrated to read 60mph at 1000rpm of speedo cable.
Thats a mile a minute at 1000rpm of speedo cable.
With 5280ft in a mile my 36" tire rotates 560.27 times.
With 3.73 gears that means my driveshaft rotates 2089.81 times or it spins at 2089.81rpm at 60 which seems about right.
So to get a 1000 rpm to the speedo at 60 I need approximately a 2.09 reduction in rpm.

What other gear ratios were available in 75?

Looks like I am going to need one of those speedo gear gearboxes.

Kevo

Sorry for the math.

 

had to wait for my head to stop spinning from all that math
most likely H78-15 Suburbanites although there was a 6:00-16 option
axle ratios were either 3.73 std or 4.27 option
H78-3.73 ratio would have had a 16 tooth driven gear
H78-4.27 ratio would have had a 18 tooth driven gear
other tooth counts available were 17 and 20 tooth.
The driven gears for D18 and D20 are vey hard to find; the small gearbox might be the answer

 

dont know if this helps,but with 4:10 and 33" tires my speedo was bang on. i had a cop radar me one day and said i was perfect