I used the Advanced Adapters speedo chart and it said 33T for 33" tires and 4.10 gears...

I've seen charts from 33T to 34T to 35T all for 33" and 4.10 gears..

Running new condition duratracs,

Just got the jeep on the road, my numbers are in km/h but I'll convert!!

33T gets me!

60 km/h (37.3 mph) speedo

Actual GPS speed

53 km/h (33mph)

Do you guys figure a 34 or 35 tooth gear ?

 

I think 33*60/53 = 37

 

Morris 4x4 center speedo gear chart and gears if you need to order one.

http://www.jeep4x4center.com/jeep-sp...ears/index.htm

 

Took it for a hwy run, and at 62 mph speedo, my gps is 56 mph, so I'm 6mph out. Can't believe I need 4 teeth for 6mph????

 

Did you measure the Duratracs? I've got 33x12.50x15 Duratracs, and I think they are a little shorter than 33". According to Tire Rack they are actually 32.5".

 

In your 1st run, you measured 13.2% fast. 13.2% of 33 is 4.4 teeth.

In your second run, you measured 10.7% fast. 10.7% of 33 is 3.5 teeth.

What seems wrong?

 

http://www.seriousoffroadproducts.co...edometer-Gears

 

2.5% difference at different speeds makes me wonder if there is also something in the speedo itself not working completely right.

My 85xj has major problems when the weather gets cold. Using a GPS to check it, if it is below freezing when I leave the driveway in the morning I get 60 on the speedo at 50 on the GPS, in summer I get 49 on the speed at 50 on the GPS. I just use the GPS now to be sure.