1997-2006 Jeep Wrangler (TJ) Frequently Asked Questions

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6. How do I improve off-road performance?

Note: A lot of the information in this section is found in greater detail in the book Jeep 4x4 Performance Handbook by Jim Allen. If you are thinking about any of these modifications, buy the book -- it's a small investment and it'll provide a lot of expertise.

6.1. What are the different types of lift kits?

6.1.1. Body lifts

A body lift is a spacer that goes between the frame and the body of the Jeep. Generally, a body lift is used to obtain one or two inches of clearance between the body and frame. Anything more than two inches is generally frowned upon, because of the increased stress on the mounting bolts between the body and the frame. A body lift can be used to obtain a little more clearance for slightly larger tires or as the final bit of clearance when using a suspension lift. This FAQ at the 4wd.com forum suggests that most people use a body lift in conjunction with a suspension lift, not as the primary lift.

6.1.2. Spring spacers

A spring spacer provides a small amount of extra lift by placing a spacer between the top of the coil springs and the frame. Typically, this ranges between 1/2 and 2 inches. This can be an inexpensive way to provide more lift, but not a lot more. If you combine a spring spacer lift with a body lift, you can get a moderate increase in tire size, without some of the installation difficulties of a larger lift kit. See also this FAQ at the 4wd.com forum.

6.1.3. Suspension lifts

A suspension lift provides extra clearance by replacing suspension components. A suspension lift includes at least springs and shocks. Larger lifts also include a dropped Pitman arm (for the steering system), a transfer case lowering kit, and/or replacement control arms. The larger lifts, with more components, also require more installation know-how. This is partly because a larger lift changes the steering alignment, such as the caster and pinion angles. Another difficulty is that you might need longer brake lines and you may need to relocate the Panhard (or track) rod, which is supposed to be parallel to the axle.

The Jeep 4x4 Performance Handbook has a complete section about the pros and cons of different lift sizes. The book advises, "The more lift you buy, the more money you spend. Don't buy lift for looks -- buy it for need, and buy no more than you need. The drivability and safety tradeoffs increase with more lift. Giving away your safety for looks is not smart!"

6.2. How big of a lift should I get?

(copied from old FAQ section 4.4)
Many people ask, "How big of a lift kit do I need to fit a tire of a certain size?" I checked some manufacturers' sites and got the basic information on various lift kits.


S4T kit: 4 inches of lift, can accomodate up to 35" tires
S3T kit: 3 inches of lift, up to 33" tires
3 inch TJ lift: no tire size specified
2 inch TJ lift: no tire size specified

Rubicon Express

TJ 4.5" lift plus 1" body lift: up to 35" tires
TJ 3" standard or Superflex kit: up to 33"x12.5" tires
TJ 2" kit: 31x10.5" tires


Tire sizes not listed

ARB/Old Man Emu

Suspension systems are not advertised as a lift kit, though they provide one or two inches of lift


TJ Rockcrawler 2.5 inch lift: up to 33 inch tires
TJ 2 1/2 inch suspension kit: up to 32 inch tires


4" lift system: Tire sizes not listed, but the page shows a vehicle with 33" tires; another catalog I have shows this kit with a 2" body lift and 35" tires
1.75" lift: Tire sizes not listed

Trail Master

4" lift kit: Up to 32 inch tires
2" lift kit: Up to 33 inch tires (note: this looks wrong; it seems obvious to me that a 4" lift would take bigger tires than a 2" lift)

Note that your choice of a lift kit depends on several factors: the amount of on-road versus off-road driving you do, the type of off-road driving you do, the ease of installation, and the reliability and reputation of the manufacturer. If you're considering a lift kit, talk to some different manufacturers and get some technical information from them. Also, see if you can find someone who has the lift kit you're considering.

6.3. What's the difference between different manufacturers?

Note: I need to do more research here. In the meantime, I'll just link to manufacturers' sites:
Rubicon Express

6.4. How do I choose bigger tires?

6.4.1. What's the biggest tire that I can install without a lift?

(copied from section 4.3, old FAQ)
You can fit tires up to 31x10.5 inches on a stock TJ without buying a lift kit. The stock YJ can only fit tires up to 30x9.5 inches. When you put larger tires than stock on a vehicle, you'll be slowing the engine down and losing some power. This may be a concern if you are thinking about putting 31" tires on a 4 cylinder TJ, but I have a friend who has done this and doesn't worry about it too much.

6.4.2. What's the deal with steering bump stops?

If you put 31" tires on the stock 7 inch wheels, you may experience rubbing on the control arms when you turn the steering wheel all the way in one direction. You may need to adjust the steering bump stops to compensate for this. Dave Willams wrote in the newsgroup:

Todd Murray <elkman at spamcop.net> wrote in article <341B366A.84F4414C@nospam.visi.com>...

Byron wrote:

4: what is the biggest size tire that I can put on my Jeep?

It's 31x10.5R15, but you'll have to adjust the steering bump stops to avoid rubbing the tires at full suspension articulation. I don't know how to do this, but other people in this newsgroup can tell you how.

Did you lose my e-mail, Todd? (I probably did.)

The steering stops are located on the brake backing plates (in front of the axle tubes). The easiest way to adjust them is to raise the front of your Jeep off the ground and (ignition off and steering wheel unlocked) turn the tire (using the tire, not the steering wheel) all the way to the left or right and see how much space there is between the head of the stop bolt and the flat spot where it is supposed to hit. Remove the stop bolt/nut combo (they're tack welded together) and put enough washers under the nut to bring the bolt up enough to eliminate the rubbing. Some trial and error is usually required, but 1 or 2 washers will usually work for 31x10.50s on a TJ, or 30x9.50s on a YJ OR for whatever size tires I had on my CJ7. :-) Repeat for the other side.

Here is a page, complete with photos, diagramming how to adjust the steering bump stops.

6.4.3. What size wheel do I need to install a specific tire size?

(copied from the old FAQ section 4.3)

  1. If you have the stock 6 inch wheels, you're limited in what you can mount. You can mount up to a P235/75R15 tire on these wheels, but this might be pushing it. Try to get the 7 inch wheels, if at all possible. Or, drive directly to a tire dealer after buying your Jeep and trade in the stock tires and wheels on something more off-road worthy.
  2. If you get aftermarket wheels, make sure they have the correct offset. Wheels that stick too far beyond the sides of the vehicle, while they may look cool to some people, can put additional stress on your wheel bearings. Ask your Jeep dealer's parts department about this.
Note: I should do more research on this and find out what the correct offsets really are.

Tire manufacturers each have their own specifications for how large a tire can be mounted on a specific rim width. Some manufacturers' tables include:

BF Goodrich AT chart
Bridgestone Dueler AT chart
Goodyear Wrangler RT/S chart

To make things simpler, though, I'll just write out a table for common Wrangler tire sizes (with and without lifts):

Tire Size Rim Width Average Diameter Lift needed
LT225/75R15 6.0 to 7.5 28.3 inches None
LT235/75R15 6.0 to 8.0 (6.0 to 7.0 for some manufacturers) 29.0 inches None
30x9.5R15LT 6.5 to 8.5 29.6 inches None
31x10.5R15LT 7.0 to 9.0 30.7 inches None, but may need to adjust steering bump stops
32x11.5R15LT 8.0 to 10.0 31.7 inches Yes: 2 inch suspension lift (Teraflex)
33x12.5R15LT 8.5 to 11.0 32.6 inches Yes: 4 inch suspension lift
33x9.5R15LT 6.5 to 8.5 32.7 inches Yes: 4 inch suspension lift
35x12.5R15LT 8.5 to 11.0 34.7 inches Yes: 4 inch suspension lift plus 1 inch body lift

Note that these tire sizes are just an average, compiled from the charts I've reviewed. The actual tires you buy might have different requirements, so check carefully.

6.4.4. If I install bigger tires, will I lose power?

Technically, your engine won't lose power if you install bigger tires, but your overall gear ratio will decrease. What this means is that your engine has to work harder to accelerate from a stop, and you'll have to shift at a higher speed than if you had the original tires. It's sort of like having to start from an intersection in second gear rather than first: it's possible, but it takes more work.

If your Jeep has a lower axle ratio, like 3.07 or 3:55 on the six-cylinder engine, then this effect will be more pronounced. If you have a higher axle ratio, like 3.73 (available as an option in certain years) or 4.11 (available on the Rubicon), this is less of a problem. Of course, if you have a four-cylinder engine, then power is already somewhat limited and bigger tires are even more of a concern.

It's possible to swap differential gears, but this is expensive (at least $1000). You'll need to swap both the front and rear differential gears, or chaos will ensue. Nevertheless, this may be a workable option for those who want really large tires.

The Jeep 4x4 Performance Handbook offers the following formulas for computing effective gear ratio:

"Effective" gear ratio: (old tire diameter) / (new tire diameter) x (original ratio) = effective ratio of new combination
Engine RPM at a given speed = mph x (total gear ratio) x 336 / (tire diameter in inches)
To obtain equivalent ratios: (new tire diameter) / (old tire diameter) x old ratio = new ratio

For example, the stock Wrangler X comes with a 3.07 axle ratio and P225/75R15 tires (28.3 inches in diameter). An upgrade to 33x12.5R15 tires (32.6 inches actual diameter) would give an effective gear ratio of 2.665. To obtain a gear ratio with an equivalent power output before the tire swap, the Wrangler would require a 3.54:1 axle ratio.

So, if you're ever thinking about bigger tires, here's why you want to look for a higher axle ratio when you're buying the Jeep in the first place.

6.4.5. If I install bigger tires, will my speedometer read wrong?

Also, bear in mind that if you change your tire size, your speedometer will not be accurate. To compensate, you can change your speedometer gears. Randall Groh wrote this handy table:

Mark Tener wrote:

Just got my spiffy new factory service manual for my spiffy almost new 97 XJ. Saw in the chapter on the t-case that there is a removable 'speedometer pinion' gear. Called my local Jeep dealer and asked about it and they said that there is quite a selection of number of teeth to be had. Like from 29 up to 41 with a few odd skips in between. My question is: if I put on larger tires then instead of putting in different ring/pinions front and back to get the proper speed and miles...could I just put in a different speedo gear with a different number of teeth? If so, does anyone have any calculations or knowledge on which number of teeth to use for which size of tire? I know what putting larger tires on will do to usable power but I'm more concerned about getting the right speed and miles. TIA Mark

Mark, I own a '97 TJ and ran into a similar question. It turned out the the dealer that I worked with had the speedometer gears in their computer based on tire size and axle ratio. Below is a chart that I created so that I would know the best gear to use if I change to a non-standard tire.

          DS/RPM        TIRE SIZE       REVS/MILE       AXLE
           2,167        30X9.50R15      706             3.07
           2,275        P225/75R15      741             3.07
           2,315        P215/75R15      754             3.07
           2,385        P205/75R15      777             3.07
           2,506        30X9.50R15      706             3.55
           2,631        P225/75R15      741             3.55
           2,677        P215/75R15      754             3.55
           2,758        P205/75R15      777             3.55
           2,902        30X9.50R15      706             4.11
           3,046        P225/75R15      741             4.11
           3,099        P215/75R15      754             4.11
           3,193        P205/75R15      777             4.11

The key columns are the driveshaft speed (assuming a 1:1 ratio), and the tire revolutions per mile. Tire manufacturers generally publish revolutions per mile for the tires. For example: a BFG All-Terrain T/A 33x12.50R15C is rated at 634 revolutions per mile. With a 3.55 axle ration this yields 2,251 driveshaft revolutions per mile. If I were to install this tire on my TJ I would tell the dealer that I need the speedometer gear for the P225/75R15 tire and 3.07 gear ratio. This is 2,275 driveshaft revolutions per mile for an accurate reading. My speedometer would only be off by 1%.

If you check with your dealer you should be able to find out what tire and axle ratio options are available for the '97 Cherokee and build a similar chart.

Rob Chaput (rchaput at frontier.net) mailed me the following, which seems easier to deal with:

When changing wheel size and or axle ratios, the speedometer must be re-calibrated in order to display the correct MPH and odometer readings. On the transfer case there is a speedometer cable which is driven by a gear. By changing to a gear with a different 'tooth' count an adjustment is made which causes the speedometer to function properly. In order to determine the 'new tooth count' the following information will be needed:

    Old tooth count
    Old tire diameter
    Old axle ratio
    New tire diameter
    New axle ratio

The basic relationship is:

New tooth count / Old tooth count = (Old tire diameter / new tire diameter) *

(Old axle ratio / new axle ratio)

The 'old tooth count' can be obtained from a Jeep dealer's parts catalog (Catalog "97-98 TJ" GRP 21P.) The following table will provide some common configurations:

Axle    Tire Size                    Tooth    Part
Ratio                                Count    Number

3.0    30x9.5R15                        28    52067-628
3.0    All P225 R15/R16/R17             29    52067-629
3.0    29x9.5R15, P225/75R15            29    52067-629
3.0    All P215 R15/R16/R17             30    52067-630
3.0    All P205 R15/R16                 31    52067-631

3.5    30x9.5R15                        33    52067-633
3.5    P225/75R15                       34    52067-634
3.5    All P205 R15/R16                 35    52067-635

3.7    30x9.5R15                        34    52067-634
3.7    P225/75R15, P215/75R15           36    52067-636
3.7    P205/75R15                       37    52067-637

4.1    All P225 R15/R16/R17             39    52067-639
"      All P215 R15/R16/R17             40    52067-640
"      All P205 R16/R16                 41    52067-640

Tire diameter for metric tires is determined by the formula:

(((Size in mm./25.4) * aspect ratio) * 2) + rim diameter. For the P225/75R15 the calculation is as follows:

Size in mm. = 225, aspect ratio = 75, rim diameter = 15

D = (((225 / 25.4) * .75) * 2) + 15 = 28.29"

In my case, I wanted to upgrade to a 31x10.5R15 tire. Using the table, I determined that the 'old tooth count' for P225/75R15 tire and a 3.73 axle ratio (Dana 44-3) was 36. Plugging the other numbers into the 'ratio' formula one gets:

new teeth count = 36 * (28.29 / 31) = 32.85 = 33 (rounded)

I therefore need to order P/N 52067-633 from the parts department.

6.5. Limited slip and locking differentials

The standard differential is a completely open differential. When you're going straight on a hard surface, it supplies equal traction to both tires. When you turn, the inside tire slows down. This is good for on-road driving, but if you're stuck, an open differential allows one tire to spin freely while the other is stuck.

A limited slip differential is built to alleviate some of these problems. Like the name implies, it limits the slip by sending some of the torque to the wheel that has the most traction. The Jeep Trac-Lok differential is an example of a limited slip differential. Without getting too much into details, a limited slip differential uses friction between the differential case and a series of clutch plates to achieve this goal. A limited slip differential provides civilized on-road performance and reasonable traction in off-road situations, but it may accelerate tire wear and require special lubricant for the differential.

A locking differential is useful for really complicated off-road driving situations, like rock crawling or lots of mud. An automatically locking differential works using a clutch that is engaged most of the time, but disengages when one wheel is traveling faster than the other (like when turning). This provides the best traction for off-road driving, but can cause squirrely handling for on-road driving, especially in slippery conditions (such as icy or snowy roads). Certain lockers are more forgiving than others, so it pays to check out reviews and ask people's opinions.

Finally, there are on-demand locking differentials. The best examples of these are the ARB Air Locker (an aftermarket item) and the Tru-Lok differential (standard on the Rubicon model). When they are disengaged, they behave like an open differential (or a limited slip, in the case of the Rubicon). The locking is actuated by a pushbutton on the dashboard. Then, the locker is engaged via an air hose or by other means. These are the most reliable lockers for mixing both civilized on-road driving and good traction for off-road driving. Unfortunately, they're also the most expensive.

This article from Jeepforum.com discusses differentials in more detail.

6.6. Winches and other accessories

A winch is a valuable recovery mechanism for getting stuck in off-road situations. (Just look at what happened to Dennis Nedry in the movie Jurassic Park. Dinosaurs never had an easier meal.) Most winches sold these days are electrical winches, though the MileMarker winch is a hydraulic design that gets its power from the power steering. An electric winch takes its power from the Jeep's electrical system, which means that a heavy-duty battery would probably be a good investment. You will need a winch with at least 6000 pounds of pulling power to pull your Jeep out of a stuck situation -- smaller winches just won't handle the load, and they could prove to be dangerous. If you plan to get stuck really seriously, or fairly often, an 8000 pound or higher winch might be a better choice.

If you get a winch, a roller fairlead is a good idea, because it keeps the cable from binding as you are pulling it in. A remote control is also useful, and you will probably also want a pair of leather gloves, a tree strap, and a pulley. Pay attention to safety warnings when winching, because cables could snap. Keep bystanders away in case this happens.