Rear End Gears

[99ABTJ]

From what I've read, I have 3.83 gear ratio on my '99 4.0L. In the event my 31" tires are contributing to my questionable MPG, would going to 3.72 make a big difference?? Is it $$/difficult/possible??

 

[volaredon]

never heard of a 3.83 ratio. a 3.72 (a little "picky" here I know but never heard of that one probably actually 3.73) but; if you have what you say, that little bit of a swap will not be worth the work and xpense to go with. If I had to guess you probably HAVE 3.73s now. for the street a a good all around ratio for that size tire. (yeah... nobody likes to admit how much "street time" their Jeeps get)

Now in my case I have 3.07s and when I had 31s on my 97, my 5th gear was totally useless. It is still marginally so with the 235s on there now.

:???:You drive a Jeep; and youre worried about MPG?:lol:

 

[jps4jeep]

I don't think Jeeps ever came with 3.83. depending on your motor transmission combo, you might have either 3.07, 3.54, 3.73, 4.10

as for running 31" tires again, depending on your motor transmission combo, you should not be seeing a difference in milage that much. 30" tires were factory on many 4.0 equipt vehicles.

going down in gearing would not make a noticable difference. I would consider adding some type of cold air intake, some better spark plugs, electric cooling fan and a free flowing exhaust as a means of increasing milage. Assuming you can stay out of the accelerator all the time, you should yield better milage, maybe 1 mpg increase can be expected.

one of the hugest issues to your milage will be did you lift the jeep, if so, that will contribute to poor milage (more than a tire size increase). Also remeber, a jeep is about as aerodynamic as a brick.

 

[99ABTJ]

Thank's for the feedback, the 3.83 I found on a web site with jeep specs. I recently gave my brick a tune up plugs, wires, dist. cap, cleaned the K&N, drive it nice etc. I guess it's good as it's going to get. I thought about changing the O2 sensors

 

[Bounty__Hunter]

The most common axle ratios for your 4.0L TJ are 3.73 and 3.07. You have to make a substantial change in gear ratios to make it worth the expense and time involved. If you have 3.07 gears an axle swap from a 2.5L 5spd TJ would be a good choice as they have 4.10 gears.

Check the numbers on the tags bolted to each diff cover to determine your current ratio.

 

[jps4jeep]

unless your getting a code, O2 sensor is fine. I have the original o2's in my GMC.. I have over 200K miles. they have some amount of durability and longe

 

[Sine Deviance]

never heard of a 3.83 ratio.

3.83 ratio was used a lot in GM/Chevy trucks. I had an '86 GMC High Sierra (basically a Silverado) with the 305/3-spd combo and it had 3.83's factory. I don't know what exact years these gears were used factory but I know they were prevalent in at least the late 70's and most of the 80's.

Most light trucks didn't have overdrive back then, and top speeds were expected to be no greater than 85-90mph for said trucks, so those ratios were ideal.

 

[xjmarc]

What do you consider bad mileage?

 

[99ABTJ]

I just had some work done on my jeep, my diff stamp say's I have 3.73 gears, mechanic say's the gears are 4.11 inside. Previous owner must have switched. I'm running 31" tires. According to a gear/RPM chart I should be at 2800 RPM at 65 mph - but I'm at 2100 actually. I'm confused - any ideas for the difference in numbers. My speedo is about 1-2 mph out.

 

[tazsean]

one of the hugest issues to your milage will be did you lift the jeep, if so, that will contribute to poor milage (more than a tire size increase). Also remeber, a jeep is about as aerodynamic as a brick.

what your insane bricks have way better aerodynamics. they have holes in them:lol: