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Old 02-24-2011, 10:52 AM   #1
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Larger tires effects on the computer?

Besides the speedometer/odometer being off and the gearing difference does the check engine light or any other computer issue arise with a larger or smaller diameter wheel/tire combo? I know it can be reprogrammed at the dealer of with a hand held device. I was just wondering if it will set off the jeeps computer. I know I had issues with diesel trucks actually putting it into limp mode with a lower profile tire. I figure someone would know on the board from experience. Mine is a 2010 with an auto. Thanks in advance.

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Old 02-24-2011, 11:00 AM   #2
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As long as they are all the same size you will be fine.

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Old 02-24-2011, 11:10 AM   #3
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As long as they are all the same size you will be fine.

I'm going from a 32.06 inch diameter to a 32.99 inch diameter. About a 3% speedo error. Anybody do this without recalibrating?
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Old 02-24-2011, 11:11 AM   #4
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I drove on 37" tires without reprogramming the computer for a month, and never once had a check engine light or ESP light come on. The Jeep was seriously sluggish, and sometimes the engine would almost "diesel" for several seconds because the transmission didn't seem to know it should downshift. Fuel mileage was very poor, at 10 mpg. I reprogrammed the computer to 37" tires and the Jeep seemed to have more get up and go, and the transmission shifted normally again. Fuel mileage improved to 13 mpg. A few months later I changed to 5.38 gears in the axles, and programmed that into the computer. Performance improved dramatically (from the gear change I suspect), transmission shifting remained normal, and fuel mileage improved to 15 mpg.
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Old 02-24-2011, 11:13 AM   #5
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If you get bigger tires and have an Auto, watch your tach on the highway. Very easy to redline.
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Old 02-24-2011, 02:03 PM   #6
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Originally Posted by Grog
If you get bigger tires and have an Auto, watch your tach on the highway. Very easy to redline.
How? You have a rev limiter?
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Old 02-24-2011, 02:26 PM   #7
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No, the shift points have do not align with larger tires, Unless you update the shift points (procal, superchips) when you are at highway speeds, tranny takes a while to shift easy to redline at 70 (speed limit here)
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Old 02-24-2011, 03:12 PM   #8
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Originally Posted by Grog
No, the shift points have do not align with larger tires, Unless you update the shift points (procal, superchips) when you are at highway speeds, tranny takes a while to shift easy to redline at 70 (speed limit here)
Ive ran 35s on stock settings and I commute 40 miles a day on the interstate and Ive never redlined.
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Old 02-24-2011, 03:18 PM   #9
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I drove on 37" tires without reprogramming the computer for a month, and never once had a check engine light or ESP light come on. The Jeep was seriously sluggish, and sometimes the engine would almost "diesel" for several seconds because the transmission didn't seem to know it should downshift. Fuel mileage was very poor, at 10 mpg. I reprogrammed the computer to 37" tires and the Jeep seemed to have more get up and go, and the transmission shifted normally again. Fuel mileage improved to 13 mpg. A few months later I changed to 5.38 gears in the axles, and programmed that into the computer. Performance improved dramatically (from the gear change I suspect), transmission shifting remained normal, and fuel mileage improved to 15 mpg.
You were at an effective rear ratio of 3.55 with 37's and stock 4.10's and probably running a wider tire with more aggressive tread. I could see that becoming a dog. I'm trying an A/T tire getting away from the M/T.My final adjusted ratio will be a 3.98. I'm hoping with less rolling resistance even with 275's I will be ok. I might want to reprogram the shift points though and tweak the speedo for accuracy. Thanks for the help.
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Old 02-24-2011, 03:43 PM   #10
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Ive ran 35s on stock settings and I commute 40 miles a day on the interstate and Ive never redlined.
I was on the highway yesterday with 33s and came darn close yesterday. Maybe its the hills here in the Ozarks? I was told this in my club and I have seen it myself.
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Old 02-24-2011, 04:05 PM   #11
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As long as they are all the same size you will be fine.

Sorry Tom i just re-read. I'm interpeting that you mean all four of the new tires are the same size not the exact equivalent size of the old tires correct? I'm hoping to use a tire that is 3% larger in diameter.
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Old 02-24-2011, 04:26 PM   #12
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This is all very interesting, but


Quote:
Originally Posted by deepbluerubi View Post
I'm going from a 32.06 inch diameter to a 32.99 inch diameter. About a 3% speedo error. Anybody do this without recalibrating?
My main interest is-how do you comeup with 32.06" diameter and 32.99" ???

You must have a BIG SET of digital calipers and are measuring the tires-INSTALLED !!

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Old 02-24-2011, 05:02 PM   #13
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Quote:
Originally Posted by Grog

I was on the highway yesterday with 33s and came darn close yesterday. Maybe its the hills here in the Ozarks? I was told this in my club and I have seen it myself.
I live in the Ozarks too, im in arkansas lol, and I drive down I40 daily. Ive never had an issue of even coming close to redlining and Im running 35x12.50x17s.
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Old 02-24-2011, 05:15 PM   #14
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tire OD?

Quote:
Originally Posted by JIMBOX View Post
This is all very interesting, but




My main interest is-how do you comeup with 32.06" diameter and 32.99" ???

You must have a BIG SET of digital calipers and are measuring the tires-INSTALLED !!

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lol...
Measure with mics, mark with crayon, cut with a chainsaw....just sayin...
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Old 02-24-2011, 05:23 PM   #15
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Cutit off twice and it was still too short !!


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Originally Posted by Ratchet View Post
lol...
Measure with mics, mark with crayon, cut with a chainsaw....just sayin...
I can digit !

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Old 02-24-2011, 06:12 PM   #16
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Quote:
Originally Posted by JIMBOX View Post
This is all very interesting, but




My main interest is-how do you comeup with 32.06" diameter and 32.99" ???

You must have a BIG SET of digital calipers and are measuring the tires-INSTALLED !!

JIMBO

I'm glad I rounded it only to the 100th of an inch...........................
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Old 02-24-2011, 06:19 PM   #17
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Whew !


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Originally Posted by deepbluerubi View Post
I'm glad I rounded it only to the 100th of an inch...........................
I'm glad my Hankooks are onle 33.225", I'm getting tired of calibrating my speedo !!

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Old 02-24-2011, 06:41 PM   #18
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Wrap a string around the tire. Measure string. Divide by Pi. Viola!
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Old 02-24-2011, 06:43 PM   #19
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You could just lay a ruler on top of the tire and measure down. Thats the most accurate way to get a measurement for calibration.
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Old 02-24-2011, 07:00 PM   #20
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Quote:
Originally Posted by deepbluerubi View Post
Sorry Tom i just re-read. I'm interpeting that you mean all four of the new tires are the same size not the exact equivalent size of the old tires correct? I'm hoping to use a tire that is 3% larger in diameter.
You're original question stated "beside the speedometer". My answer addressed that. Other than the speedometer, which will in fact be off unless recalibrated, there will be no other computer related issues as long as all four tires on the ground are the same size.
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Old 02-24-2011, 08:38 PM   #21
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Originally Posted by MOPWR2U View Post
I drove on 37" tires without reprogramming the computer for a month, and never once had a check engine light or ESP light come on. The Jeep was seriously sluggish, and sometimes the engine would almost "diesel" for several seconds because the transmission didn't seem to know it should downshift. Fuel mileage was very poor, at 10 mpg. I reprogrammed the computer to 37" tires and the Jeep seemed to have more get up and go, and the transmission shifted normally again. Fuel mileage improved to 13 mpg. A few months later I changed to 5.38 gears in the axles, and programmed that into the computer. Performance improved dramatically (from the gear change I suspect), transmission shifting remained normal, and fuel mileage improved to 15 mpg.

What kind/size wheels are you riding on? Those 37" tires weigh 87 lbs! The stock rubi wheels and tires are only about 72 lbs combined.
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Old 02-24-2011, 10:00 PM   #22
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What kind/size wheels are you riding on? Those 37" tires weigh 87 lbs! The stock rubi wheels and tires are only about 72 lbs combined.
The wheels are aluminum allow 17x8, roughly the same weight as the OEM aluminum alloy 17x 7.5.

The biggest reason for being a dog on the highway is gearing though. Chrysler gears these things for max economy on flat pavement at sea level. Performance gearing would have to be far far lower than stock. The reason I say that is because of the horsepower curve these engine produce. Hypertech has a graph on their web page that shows how many HP you get at a certain RPM. At 2000 RPM our engines only produce 64 HP. With my Rubicon and its stock 4.10 gears turning 37" tires, my engine was only turning 1900 RPM at 75 mph. That means my Jeep was making a whopping 60 HP while trying to drive down the highway at 75 mph with 37" tires on it. No wonder it was a dog, and wouldn't stay in high gear unless I was going down hill or had a strong tail wind on level ground. By changing gearing to 5.38 it increases cruise RPM to 2600, which increases HP by 60 to 70 (going from memory here). In other words, it doubles your HP. That is why people are saying to regear your Jeep if you change to bigger tires.
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Old 02-25-2011, 08:52 AM   #23
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The wheels are aluminum allow 17x8, roughly the same weight as the OEM aluminum alloy 17x 7.5.

The biggest reason for being a dog on the highway is gearing though. Chrysler gears these things for max economy on flat pavement at sea level. Performance gearing would have to be far far lower than stock. The reason I say that is because of the horsepower curve these engine produce. Hypertech has a graph on their web page that shows how many HP you get at a certain RPM. At 2000 RPM our engines only produce 64 HP. With my Rubicon and its stock 4.10 gears turning 37" tires, my engine was only turning 1900 RPM at 75 mph. That means my Jeep was making a whopping 60 HP while trying to drive down the highway at 75 mph with 37" tires on it. No wonder it was a dog, and wouldn't stay in high gear unless I was going down hill or had a strong tail wind on level ground. By changing gearing to 5.38 it increases cruise RPM to 2600, which increases HP by 60 to 70 (going from memory here). In other words, it doubles your HP. That is why people are saying to regear your Jeep if you change to bigger tires.

Excellent info. Thanks I'm going to document some base readings today speed/rpms with my stock setup then see what i get with the new tires/wheels. I'll use my gps as the speedometer and the same stretch of road/conditions to keep it accurate. It's interesting stuff when you put in all the variables, unsprung weight, static weight, rolling resistance, tire diameter, rpm/hp/torque etc etc........... All good input thanks everyone.
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Old 02-25-2011, 07:28 PM   #24
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Excellent info. Thanks I'm going to document some base readings today speed/rpms with my stock setup then see what i get with the new tires/wheels. I'll use my gps as the speedometer and the same stretch of road/conditions to keep it accurate. It's interesting stuff when you put in all the variables, unsprung weight, static weight, rolling resistance, tire diameter, rpm/hp/torque etc etc........... All good input thanks everyone.

Ok, I just installed the 33" 275 michelin LTX AT/2 on my auto 2dr 10 Rubi. I set them at 37 psi. Looks like I'm at 60 mph at about 1900 RPM. This rig flys now. Smooth and quiet. I knew in the 30 feet I backed out of my garage these tires were for me. The wheel and tire combo came in at 7.5 lbs more than the stockers per corner. Had them in rain and then light snow already. Much more confident on pavement. I have a feeling that the gas mileage will be heading north also. Tires fit recessed under the top of the fenders and protrude aprox. about .75 inch front and back. (need to get a shorter straight edge and check again) Plenty of clearance on the inside with the 34.75 mm offset. Closest I'm near is in the rear with the upper arm being about .75 inch away from the side wall and it was at 1.25 inch with the old setup. New TPMs seem OK and no other issues/idiot lights. Opinions/reviews said they wear very well. Will see.

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