3.50 gears, 33" tires, Dana 20. What speedo gear tooth count?

[WILDHORSES]

3.50 gears, 33" tires, Dana 20. What speedo gear tooth count?

Can't seem to find my chart. I have 16-21 tooth to pick from anyone have a chart or know the answer? And yes I know we are supposed to be the experts.;D Thanks in advance.

Jim

 

[Viperwolf1]

I was using a 17T with that combo and it was within 2% but it all depends on which drive gear is installed on the rear output shaft. That was with the blue plastic drive gear. I went to the steel drive gear and 4.11 gears and kept the same 17T and it is now within 1%.

 

[Broncobowsher]

Since there are 2 possible drive gear options, it is hard to tell you what you need.
Best thing is to stick what ever gear you have now in there and drive. See how far off the speedo is. You need % error, not MPH off. Then figure the percent error to the number of teeth of that gear.

I remember having 3.50 gears and 31" tires and already down to 16 tooth. Then another changed from 18 to 16 tooth when I had 33" tires with 4.10s but put the 18 back in when I went to 4.88 gears. So there may not be a simple fix for 3.50 gears and 33's. They don't make less then 16 tooth. Hope you have the right driven gear to make it work.

 

[ol gray mare]

I'm running the same set up in mine. With the original drive gear that came in the t case with the 3:50s, I am using the 16 tooth and was radared by the local mounty at 29mph with the speedo showing 30. I tried the 17 tooth and got 31-2 mph on radar with 30 on the speedo. The original driven gear was the 19 tooth. Hope that helps. Jim

 

[mitzel]

I agree with Broncobowsher the change required is based on
percent error. My SWAG is stock tire is 29" and if your new
tire measure 33" then that's 13.79% change. Keeping the
same drive gear you need to reduce the tooth count on the
driven gear by 13.79%. Using ol gray mare data that stock
was 19 tooth then you now need a 16.38 tooth driven gear.
Round that up or down and seems right inline with
Viperwolf1 and ol gray mare's experience.

 

[stock1970]

get a sat nav that tells you how fast you are going..

 

[bingen]

WH i guess any speedo gear will do

I guess that question was for me?....hahaha.
I guess any speedo gear will do as I don't have one for now...

am the guying in the Philippines...

 

[broncnaz]

I used a 16 tooth with my setup low speeds its pretty close but at high speeds it seems like its off more but I've never actually checked it against anything to be sure seems like when I did the math it was about 5% off at 75mph. But its been a long while. I usually just judge/verify speed by RPM's

 

[Viperwolf1]

One tooth difference in the driven gear equals 5-7% change in indicated speed. That means with the correct gear you will have no more than 3.5% error, unless your off the end of the gear scale.