98 TJ Sahara tire size upgrade - JeepForum.com
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post #1 of 44 Old 03-17-2020, 12:59 PM Thread Starter
RedStar1991
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98 TJ Sahara tire size upgrade

Hi Guys,

Long time reader and first time poster here.

My wife and i bought a 1998 Jeep Wrangler Sahara (6cyl 4.0 Manual) last summer which is all stock (Tires, no lift, etc). The tires are worn out and it's time for us to get new tires this spring as we prepare for the summer and the fun rides that are ahead. The current tires look like the original stock/manufacturer size (not sure what they are exactly -215?). We don't like the look of the tires as they look very small on the jeep and are very narrow.

We would like to buy new tires and get ones as big as possible for this stock jeep without doing any type of lift or anything like that (Unless we are able to install a small lift ourselves - not sure how easy this is for a DIYer). I've read a few posts mentioning that 31x10.5R15 tires for this car, as well as adding a couple of washers underneath the steering stops.

My questions is - do you guys think this is the best option? If we go with this tire size what brand should we go with, and will we experience any performance issues? Should we possibly consider a lift? We're not sure about a lift because we figure that the new tires alone are gonna cost at least $800+ - not sure how much a small lift would even cost or if its possible to do it DIY?

Thanks

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post #2 of 44 Old 03-17-2020, 03:42 PM
2jhanna
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As you have read here, the largest size on a stock TJ is 31x10.5x15. A factory option was 30x9.5x15. So either of those would work.

If you have 3.73 gears, you won't suffer much performance issue, if you have 3.07 gears, you will feel it.

You will have to change the transfer case speedo gear to correct speedometer and odometer.

Unless you plan mud in your future, I'd stay away from MT tread, it won't be as smooth of a ride and will be noisier. Stick with a street tread or a AT tread.

Here is my stock '02 on 31 ATs. No lift.
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post #3 of 44 Old 03-17-2020, 05:17 PM Thread Starter
RedStar1991
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Quote:
Originally Posted by 2jhanna View Post
As you have read here, the largest size on a stock TJ is 31x10.5x15. A factory option was 30x9.5x15. So either of those would work.

If you have 3.73 gears, you won't suffer much performance issue, if you have 3.07 gears, you will feel it.

You will have to change the transfer case speedo gear to correct speedometer and odometer.

Unless you plan mud in your future, I'd stay away from MT tread, it won't be as smooth of a ride and will be noisier. Stick with a street tread or a AT tread.

Here is my stock '02 on 31 ATs. No lift.
Thank you for the response - it's very helpful. Love the look on your Jeep. What exactly do you mean by 3.73 vs 3.07 gears? Sorry if this is a dumb question - as we're still fairly new Jeep owners
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post #4 of 44 Old 03-17-2020, 09:18 PM
2jhanna
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Originally Posted by RedStar1991 View Post
Thank you for the response - it's very helpful. Love the look on your Jeep. What exactly do you mean by 3.73 vs 3.07 gears? Sorry if this is a dumb question - as we're still fairly new Jeep owners
Not a dumb question, and don't take this the wrong way, but you got a lot to learn.

I give this a try, others can correct/add as needed.

Gears, gearing, axle ratio, "3.07, 3.73" are just 2 common examples of axle gear ratios.

Your front and rear axles each contain a set of gears, pinion gear, ring gear, spider gears. The number of teeth on the ring gear, along with the number of teeth on the pinion gear, establish a ratio.

Think about any mechanical knowledge you may have. If 2 gears, each with the same number of teeth, are messed together so that when 1 turns, the other turns, this is called a 1:1 ratio.

If 1 gear has twice the amount of teeth as the other, this is called a 1:2 or 2:1 ratio, depending on which gear is used to turn the other gear.

In an axle with 3.07:1 gearing, it takes a little over three (3.07) full rotations of the driveshaft to rotate the tire 1 full rotation.

When you change tire size to a larger size, you are, in effect, changing this dynamic of power between the engine and tires. (Wow, did I say that right). Therefore changing vehicles performance .

Another way to realize this, your jeep has a 5 speed manual trans. This means you have a set of 5 forward gears, each with a different number of teeth. You can see/feel the difference if your in 1st vs 5th gear. When you shift gears from 1, 2, 3 and so on, your choosing a different physical gear with a different number of teeth, and you can feel the difference in power, so to speak.

I am multi-talented, I can talk and piss you off at the same time.

jhanna
denton county, tx
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post #5 of 44 Old 03-17-2020, 10:28 PM
TheBoogieman
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No lift and 1.25" ($100) body lift.
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Quote:
Originally Posted by jeepinmichguy_ View Post
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post #6 of 44 Old 03-18-2020, 10:43 AM Thread Starter
RedStar1991
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Quote:
Originally Posted by 2jhanna View Post
Not a dumb question, and don't take this the wrong way, but you got a lot to learn.

I give this a try, others can correct/add as needed.

Gears, gearing, axle ratio, "3.07, 3.73" are just 2 common examples of axle gear ratios.

Your front and rear axles each contain a set of gears, pinion gear, ring gear, spider gears. The number of teeth on the ring gear, along with the number of teeth on the pinion gear, establish a ratio.

Think about any mechanical knowledge you may have. If 2 gears, each with the same number of teeth, are messed together so that when 1 turns, the other turns, this is called a 1:1 ratio.

If 1 gear has twice the amount of teeth as the other, this is called a 1:2 or 2:1 ratio, depending on which gear is used to turn the other gear.

In an axle with 3.07:1 gearing, it takes a little over three (3.07) full rotations of the driveshaft to rotate the tire 1 full rotation.

When you change tire size to a larger size, you are, in effect, changing this dynamic of power between the engine and tires. (Wow, did I say that right). Therefore changing vehicles performance .

Another way to realize this, your jeep has a 5 speed manual trans. This means you have a set of 5 forward gears, each with a different number of teeth. You can see/feel the difference if your in 1st vs 5th gear. When you shift gears from 1, 2, 3 and so on, your choosing a different physical gear with a different number of teeth, and you can feel the difference in power, so to speak.
That makes perfect sense. I assume this is difficult to tell beforehand if there would be performance issues? The Jeep runs great and is in great shape, so I'm assuming I won't know if there will be any performance changes until it's installed.

And when you said 'You will have to change the transfer case speedo gear to correct speedometer and odometer.', does this need to be done only if there are performance changes or it should be done regardless? I'm not sure how to do this, or if I should take it to a mechanic/try to do it myself?

Thanks again in advance, your answers have been very helpful.
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post #7 of 44 Old 03-18-2020, 10:44 AM Thread Starter
RedStar1991
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Quote:
Originally Posted by TheBoogieman View Post
No lift and 1.25" ($100) body lift.
Looks awesome and very clean! I may decide to go that route or no body lift at all. Which tires do you have on there?
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post #8 of 44 Old 03-18-2020, 12:35 PM
BM3
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Quote:
Originally Posted by RedStar1991 View Post
And when you said 'You will have to change the transfer case speedo gear to correct speedometer and odometer.', does this need to be done only if there are performance changes or it should be done regardless? I'm not sure how to do this, or if I should take it to a mechanic/try to do it myself?
.
The speedo gear is based off of tire size, and very easy to change yourself.
Wrong chart was uploaded, my bad sorry
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post #9 of 44 Old 03-18-2020, 05:07 PM
2jhanna
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Quote:
Originally Posted by RedStar1991 View Post

And when you said 'You will have to change the transfer case speedo gear to correct speedometer and odometer.', does this need to be done only if there are performance changes or it should be done regardless? I'm not sure how to do this, or if I should take it to a mechanic/try to do it myself?

Thanks again in advance, your answers have been very helpful.
The transfer case speedo has no real effect on performance, only perceived. If incorrect, it will make you think you're going faster or slower than you actually are. Same with the odometer.

Think about it, if you put a larger tire on your jeep, that larger tire has a larger circumference. One revolution of this larger tire will move you future down the road than one revolution of your original smaller tire. The t-case speedo gear is what "calibrates" your speed and odometer, whenever a tire size and/or axle gearing is changed. As an example, without the correct speedo gear, your speedometer could read 60mph, but your actual speed could be 55mph or 65mph, depending on tire size and gearing.

As far as your current axle gearing, most 6cyl TJ came from the factory with either 3.07 or 3.73 gears. There are some exceptions. If you crawl under the rear of your TJ with a flashlight, look at the rear axle cover. Around the circumference of the cover are bolts, at the 9 o'clock position, there should be a metal tag, held in place by 2 bolts. This metal tag will have different sets of numbers/codes. Look for a number seguence of 3.07 or 3.73. That's the gear ratio of your front and rear axles.

As I stated earlier, if you having 3.73 gearing, you won't notice much performance difference with 31 tires. If you have 3,07 gears, you probably will notice a difference.

I am multi-talented, I can talk and piss you off at the same time.

jhanna
denton county, tx
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post #10 of 44 Old 03-19-2020, 08:45 AM
TheBoogieman
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Quote:
Originally Posted by RedStar1991 View Post
Looks awesome and very clean! I may decide to go that route or no body lift at all. Which tires do you have on there?
Stock Goodyear MTR/K tires equivalent to 31". Most 31" tires are closer to 30-30-5".

p.s. It doesn't matter if you have 3.07 or 3.73 gears. Either is ok with 31s. I bought a 98' Sahara with 33s and NO LIFT. I was shocked that it only rubbed when turning on the street. My 2000' Sahara had 33s and a 3" lift with heavy bumpers and a winch.
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Quote:
Originally Posted by jeepinmichguy_ View Post
TheBoogieman is a jerk.
03' Rubicon/Zone 4" coils with 1" spacers up front/Rancho shocks/5.13 gears-35" MTZs/Black Magic brakes. ***SOLD*** for Covid-19 pandemic rent money. :(
99' XJ /4.0L-AW4/ 5.5" lift / Rancho RS5000X shocks/33" DC Extreme country MTs. Jeep #18
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post #11 of 44 Old 03-19-2020, 08:53 AM
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Quote:
Originally Posted by TheBoogieman View Post
Stock Goodyear MTR/K tires equivalent to 31". Most 31" tires are closer to 30-30-5".

p.s. It doesn't matter if you have 3.07 or 3.73 gears. Either is ok with 31s. I bought a 98' Sahara with 33s and NO LIFT. I was shocked that it only rubbed when turning on the street.
Wow those look stuffed in there. Surprised it didn't rub if you hit a pebble in the street.

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post #12 of 44 Old 03-19-2020, 09:00 AM
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Wow those look stuffed in there. Surprised it didn't rub if you hit a pebble in the street.
So was I. Especially with stock wheels with their backspacing. The tires didn't even stick out. I bought it in Washington D.C. and had a 2 hour drive home at 75 mph. I was happy when I made it home safe and didn't have any sharp turns. I thought the rear tires would scrape the spring perches bad. They never did. My driveway was nearly vertical and it still didn't rub. My GF drove it for a couple months before I sold it.
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Quote:
Originally Posted by jeepinmichguy_ View Post
TheBoogieman is a jerk.
03' Rubicon/Zone 4" coils with 1" spacers up front/Rancho shocks/5.13 gears-35" MTZs/Black Magic brakes. ***SOLD*** for Covid-19 pandemic rent money. :(
99' XJ /4.0L-AW4/ 5.5" lift / Rancho RS5000X shocks/33" DC Extreme country MTs. Jeep #18
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post #13 of 44 Old 03-20-2020, 09:05 AM Thread Starter
RedStar1991
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Originally Posted by 2jhanna View Post
The transfer case speedo has no real effect on performance, only perceived. If incorrect, it will make you think you're going faster or slower than you actually are. Same with the odometer.

Think about it, if you put a larger tire on your jeep, that larger tire has a larger circumference. One revolution of this larger tire will move you future down the road than one revolution of your original smaller tire. The t-case speedo gear is what "calibrates" your speed and odometer, whenever a tire size and/or axle gearing is changed. As an example, without the correct speedo gear, your speedometer could read 60mph, but your actual speed could be 55mph or 65mph, depending on tire size and gearing.

As far as your current axle gearing, most 6cyl TJ came from the factory with either 3.07 or 3.73 gears. There are some exceptions. If you crawl under the rear of your TJ with a flashlight, look at the rear axle cover. Around the circumference of the cover are bolts, at the 9 o'clock position, there should be a metal tag, held in place by 2 bolts. This metal tag will have different sets of numbers/codes. Look for a number seguence of 3.07 or 3.73. That's the gear ratio of your front and rear axles.

As I stated earlier, if you having 3.73 gearing, you won't notice much performance difference with 31 tires. If you have 3,07 gears, you probably will notice a difference.
I went under the Jeep and I don't think I was able to find the gearing tag this time. I did find two other 'tags' which looked different from the one that you described. I'm not sure if I was looking in the wrong place or not. The one mentions a ratio of '272' - not sure what that means.
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post #14 of 44 Old 03-20-2020, 09:15 AM Thread Starter
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Also - these were the tires that I was gonna buy. Let me know if you guys think they are good. We are mostly going to be driving on the road and will probably not be going off road anytime soon.
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post #15 of 44 Old 03-20-2020, 09:18 AM
2jhanna
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The top picture is of the axle cover. Is that the front or rear axle ? It looks like the front axle.

Did you read my post, rear axle cover, tag at 9 o'clock position.

Your lower picture is of the transfer case.

Neither of these pics reference your gear ratio.

I am multi-talented, I can talk and piss you off at the same time.

jhanna
denton county, tx
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