Tire change - calibration

wingnutdbq · **Posted:** Tue May 27, 2008 1:18 pm

I was just reading another post regarding cruise problems with larger tires. Someone said that the computer could be adjusted for revolutions per mile, modifying shift points, etc. Can the speedometer also be calibrated this way, without regearing?

DryBones · **Posted:** Tue May 27, 2008 1:57 pm

wingnutdbq wrote:

I was just reading another post regarding cruise problems with larger tires. Someone said that the computer could be adjusted for revolutions per mile, modifying shift points, etc. Can the speedometer also be calibrated this way, without regearing?

yep,
Just take it to a dealer with your new tire specs and they should be able to progam it for you.

kj924 · **Joined:** Wed Jan 16, 2008 11:01 pm **Posts:** 103

Yeah they re-cal the computer with a new pinion angle.

tan's2002kjlimited · **Posted:** Thu May 29, 2008 8:45 pm

Any idea what they charge for that? And can only the dealer do it?

modular · **Posted:** Thu May 29, 2008 9:05 pm

The proceedure is called a "pinion re-set". You need to find out what the revs per mile are for the size of the new tires on your KJ. When mine was done the tech said the pinion was set at around 750 revs per mile. I had it re-set to 682 if I recall correctly. I did the roll-out method to find the actual size. You can also use a tire calculator on the web to help with the calculations. Here is a couple popular ones...

http://www.miata.net/garage/tirecalc.html

http://www.discounttire.com/dtcs/infoTireMath.dos

The dealer should only charge you an 1/2 labor to do it. I was charged $42.

KY Liberty · **Joined:** Sat Jul 09, 2005 1:46 pm **Posts:** 1195

It's easy to get it very close if you have a gps. Have them read the revs per mile setting, multiply that by the mph the speedometer says you are going, then divide that by the speed the gps says your going. Have them program that as the revs per mile. The manufacturer's recommended revs per mile was about 5 mph off on mine. It's well inside of 0.5 mph now.

Cobalt-60 · **Posted:** Sat May 31, 2008 10:37 am

I finally went this morning to go get my pinion index reset done. The first dealer (Premier Chrysler/Jeep) I went to said it would take 2 hours to get to me and they would charge me $100. I turned around and walked out the door. :evil:

The second dealer (Stone Mountain Chrysler/Jeep) I went to I walked over to the service tech and said, "I need a pinion index reset." to which he replied, "Changed your tire size?" I knew I was in the right place. He disappeared for a few minutes and returned with a DRBIII Scan Tool and plugged it in saying, "I can't see the point of making you wait 2 hours for such an easy job." It took about 15 minutes (following the instructions found in another post found on this site that I had taken with me), and when I asked him what I owed, he said, "Nothing, you just know where to come for service."

I picked up his card and it said, "shop steward" on it.

Now, THAT is how dealerships should treat their customers!!! :lol:

Fulltimer · **Posted:** Sat May 31, 2008 10:58 am

Hmmmmm, I'll have to check and see how far it is from Saint Augustine, FL to Clarkston, GA. :lol:

My dealer said I would have to come in and the service tech would have to hook up the computer just to tell me IF they could do it or not! So, if they say "no can't do it" they are going to hand me a bill for checking. :evil:

Terry

Cobalt-60 · **Posted:** Sat May 31, 2008 11:10 am

Fulltimer wrote:

Hmmmmm, I'll have to check and see how far it is from Saint Augustine, FL to Clarkston, GA. :lol:

My dealer said I would have to come in and the service tech would have to hook up the computer just to tell me IF they could do it or not! So, if they say "no can't do it" they are going to hand me a bill for checking. :evil:

Terry

Well, if you were going to be able to make it up in August to wheel with us at the LOST ECS (and we are all very sorry that you are not :cry:

), I would say swing by on your way up and we will go see them. :wink:

I'd say, go find another dealer...

jason thompson · **Posted:** Sat May 31, 2008 3:19 pm

Why bother?
figure out how far off it is and then just drive slower and change your oil sooner
some quick math will give you a % of how far off you are and just use that

This is the kind of stuff that the guys on other boards laugh at
and if you ask me also the reason why the KJ gets so little respect from the rest of the Jeep world
This is not an issue that I have ever seen on other boards
also the MPG and the "my KJ mad a noise" posts are kind lame

Now if it were posts about "I bent this or broke that out on the trail" I could see the value
but worrying about the speed O being off?

modular · **Posted:** Sat May 31, 2008 3:29 pm

Cobolt-60, way to go! Sounds like a great dealer. I would definitely patronize that dealer when/if the time arises.

...and if the speedo is off enough (reading fast) you rack up more miles on the odo than you actually have traveled...especially if you do a lot of highway travel. I am very particular about my vehicle as others are and want things to be right which is why I had it done myself. I am really not too concerned with what people on "other" forums think...it is a personal preference.

jason thompson · **Posted:** Sat May 31, 2008 4:15 pm

modular wrote:

Cobolt-60, way to go! Sounds like a great dealer. I would definitely patronize that dealer when/if the time arises.

...and if the speedo is off enough (reading fast) you rack up more miles on the odo than you actually have traveled...especially if you do a lot of highway travel. I am very particular about my vehicle as others are and want things to be right which is why I had it done myself. I am really not too concerned with what people on "other" forums think...it is a personal preference.

Dont worry about you speedo reading fast
for it to do that you would have to put tires on that were smaller than stock
but I bet you knew that

tommudd · **Posted:** Sat May 31, 2008 4:29 pm

Have been running larger size tires for over 75000 miles and don't really think about it at all, not worth the time and the hassle of going to a dealer ( since mine has never been inside a dealers building yet! ) anyways.

kj924 · **Joined:** Wed Jan 16, 2008 11:01 pm **Posts:** 103

Here you go, this is to save face

I bent this getting out to the camp last night. Now I'm gonna have to upgrade the rear window suspension....maybe electric servos for window lift.

Cobalt-60 · **Posted:** Sat May 31, 2008 6:25 pm

Okay, think about it this way --

My 2004 KJ came from the factory with the god-awful 215/75R16 STs. I replaced them last year with 245/70R16 and did not get the pinion index reset.

Since then I have added hundreds of more pounds in weight to the vehicle (full skids, rock rails, front & rear tow hitches, etc.)

Now when I went and had the tech change the rev per mile to 706 (per Tirerack for the Destination ATs in the new size), I was looking over the techs shoulder. The rev per mile for the STs in the old size should have been 728 (again per Tirerack), but the setting that he change it from was something like 748!!! :shock:

Yes, this is (as expected) about a 6% change (748 to 706). What I did notice was that the minute I drove off the lot my shift points dropped by about 500 RPMs.

Since I am getting about 18 MPG on the highway and gas is about $4/gal (I spent $150 in gas last weekend going up to TN and back and only drove about 600 miles total!!!), I figure every little bit helps. :wink:

I am just trying to find the best balance between off-road functionality and practical daily use. :lol:

KY Liberty · **Joined:** Sat Jul 09, 2005 1:46 pm **Posts:** 1195

Mine came with 235/70R16 SRA's and was set to 766. According to the manufacturer it should have been 721. That's why I recommend the GPS and math method I listed above.

Inc · **Posted:** Sat May 31, 2008 8:30 pm

So I am late to this part. What do I need to take to me dealer to let them know how to do this?

snowsport · **Joined:** Tue Oct 18, 2005 11:52 pm **Posts:** 1211

kj924 wrote:

Here you go, this is to save face

I bent this getting out to the camp last night. Now I'm gonna have to upgrade the rear window suspension....maybe electric servos for window lift.

One of mine did that when a friend pushed down on the corner of the rear glass. I just bent it back and it's been working fine for over 3 years.

kj924 · **Joined:** Wed Jan 16, 2008 11:01 pm **Posts:** 103

Yeah I didn't do it, I was just informed that "something" happened to the window. I'm gonna try to bend it back for now and see what happens. I figure I can't hurt it ant worse than it is. :lol:

Tire change - calibration

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