Axles

[SteveF]

My JKU was built when I bought it. The rear axle shows signs that it could have possibly been opened up. I have 35's on the jeep and my speedometer is accurate. RPM's seem normal. Without opening up the axles, is there any way to tell if the gears have been changed? Since this is my first JKU I don't know if I have an issue of power loss. It seems to move along quite well. The build sheet says 3.73 gears. My last jeep was a 2000 TJ with a 4.0 ltr.

 

[Hooperdog]

373's are not to bad but is it downshifting a lot at highway speeds? Auto/manual? jack up the rear end, mark your driveshaft and then spin a wheel 1 full turn and count how many turns the driveshaft goes around.

 

[SteveF]

373's are not to bad but is it downshifting a lot at highway speeds? Auto/manual? jack up the rear end, mark your driveshaft and then spin a wheel 1 full turn and count how many turns the driveshaft goes around.

Now that you mentioned it, downshifting at hwy speed does happen with a slight increase in grade.

 

[SteveF]

It is also an automatic

 

[SteveF]

373's are not to bad but is it downshifting a lot at highway speeds? Auto/manual? jack up the rear end, mark your driveshaft and then spin a wheel 1 full turn and count how many turns the driveshaft goes around.

What will 4.10 gears do to my engine RPM's at highway speeds. Right now at 65 mph the tach is about 1800 RPM's. I have a 5spd automatic.

 

[Hooperdog]

if you google it there are a few charts out there that show RPM/tire size/gears.. don't have it or I'd post it for ya

 

[MacGuyver]

There are a number of examples on youtube for figuring your gear ratio.

Or the next post by @Anybodyhome explains it.

 

[Anybodyhome]

This chart is based on 65 MPH and a gear ratio of 1:1, on a manual transmission in 4th gear. Please NOTE: If you have an automatic transmission your RPM will higher due to slippage in the transmission and the torque converter. With an overdrive transmission your RPM in overdrive will be 15% to 30% less than indicated.

This is pretty accurate. My 2014 3.6 5-speed auto w/3.73 runs about 2200 or so at 70mph on 35" tires.

While you may think you know your Jeep's gear ratios based off its year and/or differential type, if the vehicle was purchased used then someone may have changed them at some point in the past. So an easy way to determine your actual gear ratio is to check the tag attached to the differential cover by the cover bolts. On the tag there should be some numbering such as 3.54 or 3.73, either of those numbers will give you the stock axle ratio. To verify, or to see if that ratio has changed, you can jack up the rear axle and support it with jack stands. With the transmission in neutral, rotate the rear driveshaft by hand. Count the number of driveshaft revolutions required for one revolution of the tire.

If the driveshaft turns approximately 3-1/2 times, you have a 3.54 ratio, etc. It is helpful to use a grease pencil or chalk to make a reference point on both the tire and driveshaft while performing this test.

Number of Turns	Approx. Gear Ratio
3	3.05 : 1
3 - 1/2	3.54 : 1
3 - 3/4	3.73 : 1
4	4.10 : 1
4 - 1/2	4.55 : 1