Easiest way to figure out what gears I have

Hi guys,

I've been lurking a while, but time for my first post. Donning my asbestos underwear.

I just bought a used 2001 XJ that has a 4.5 inch lift 33 Goodyear MTR's and has had gears installed. What is the easiest way to figure out what gears are in the axles? Speedo seems to be off by about 15-20% reading higher than actuals off the GPS.

I want to correct the speedo gear but nots sure how best to deterimine what I have. I did actually do a search and read the gearing faq, but didn't see this addressed.

My truck is a 4.0 automatic with the part time transfer case and stock axles. (yes I realize the D30 is weak).

Thank you,

I'm guessing its 4.56 or 4.88 but want to know for sure.

put her in 2wd, neutral, block front tires. jack up rear end take off tires. mark the top of the pinion flange with a marker and mark the top of one brake assy. or lug stud. spin the brake assy 10 times and count how many times the pinion flange makes a revolution. if it goes around 35 times you have a 3.55....45 times you have a 4.56 and so on.

If it's an open diff, chock the front wheels on level ground, put the transmission in neutral, jack up one rear wheel and mark the driveshaft. Spin the rear wheel TWO revolutions and count the revolutions of the driveshaft-this will equal your gear ratio (approximately).

If it's a locked or limited slip, jack up both rear wheels, mark the driveshaft, and spin the rear wheels ONE revolution and count the revolutions of the driveshaft. This will equal your gear ratio (approximately).

bronc571 said:

put her in 2wd, neutral, block front tires. jack up rear end take off tires. mark the top of the pinion flange with a marker and mark the top of one brake assy. or lug stud. spin the brake assy 10 times and count how many times the pinion flange makes a revolution. if it goes around 35 times you have a 3.55....45 times you have a 4.56 and so on.

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Not to be stupid, but why do I need to remove the tires to do this?

Georgia Mike said:

If it's an open diff, chock the front wheels on level ground, put the transmission in neutral, jack up one rear wheel and mark the driveshaft. Spin the rear wheel TWO revolutions and count the revolutions of the driveshaft-this will equal your gear ratio (approximately).

If it's a locked or limited slip, jack up both rear wheels, mark the driveshaft, and spin the rear wheels ONE revolution and count the revolutions of the driveshaft. This will equal your gear ratio (approximately).

Click to expand...

Thanks for the quick responses guys. This forum is a great place for info.