stock rear end pretty sure its the 8.8 not positive and i have 31 goodyear mtrs not very pround of the size but they came with the truck and go anywhere
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Nothing wrong with 31's, bigger ones look neat but they generally give poorer milage because of the extra drag. For gearing, look at the axle code on the safety compliance sticker which is located on the driver side back door post. There will be an axle code on the bottom line which can be decoded. ex. 13 = 4.11, 16 = 3.50, 19 = 3.55.
Taking all that information and your normal highway cruising speed it is possible to calculate where your engine is running and if a gearing change would help. Recommended cruising RPM is between 2000 and 3000 RPM. For best economy, you want a gear ratio that puts your engine closer to the low end. For best power you want a gearing that puts you near the top end, or a comprimise between the two would be in between around 2300 to 2600 RPM. My truck, for example has 31 x 9.5's and 3.08 gears, I cruise at 65 mph, which gives me a RPM of around 2170. If I cruise at 60 my RPM drops to just over 2000.
The formula for RPM is:
RPM = (MPH x Gear ratio / Tire Diameter) x 336 My speedometer is dead on with less than a .1 mile error in 50 miles.
Another thing you should check, your odometer. You can do that by going out on the highway and mark the odometer when you pass a mile marker and then cruise for 10 miles according to the mile markers. Then note the odometer reading at the 10th mile marker, (Using this method you don't have to take your speed into consideration). If your system is dead on it will read 10 miles exactly. If it is off by more than 5% you should see about getting it corrected. On some trucks that is with a new speedo gear and on others it is a computer adjustment.
Good luck,
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