Originally Posted by dow
Hi everyone, i just bought my first jeep and the speedometer is way off. Its an 03 sport with I6, auto tranny, and 33's. The guy i bought it from said it had 4.10 gears (which i assume to be 4.11 for auto), so i ordered the 36 tooth speedo gear. At 65 mph (gps) with the stock 34 tooth, the speedo was reading around 80 mph. With the 36 tooth at 65 mph, the speedo is reading around 75 mph. Rpm's at 65 mph (gps) are around 2250 to 2300. Does this sound like 4.56 (or 4.88) to anyone? It's not driving so well because it shifts into OD too early. If this setup actually is 4.56 and 33, the chart calls for 39 tooth gear, but would using a 40 tooth gear prolong OD from kicking in for a bit? Any help is appreciated!
If it does have 4.10, it has both 4.10 and 4.11. The front and rear axle ratios are always off from each other by .01 due to the front and rear ring gear diameters being different from having two different axles (Dana 30 front, Dana 35c or Dana 44 in the rear). Only the Rubicon, because its front and rear axles are both Dana 44 with the same diamter ring gears, have identical front and rear ratios. Same with 3.73 which is a mixture of 3.73 and 3.74, etc.
But to know exactly what speedometer gear to use, if the 36 tooth is producing inaccurate results, you're going to have to verify that it has the 4.10/4.11 ratio.
Based on my calculations and that you say you are getting approx. 2250 rpms at a GPS indicated 65 mph when in Overdrive, you have a 4.88 ratio, not 4.10. 4.10 would produce more like 1872 rpms at 65 mph with a 33" tire diameter when your transmission is in Overdrive. Calculations for 4.88 exactly produce 2250 rpms at 65 mph when the transmission is in Overdrive with 33" tires.