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Old 09-18-2011, 08:08 PM   #1
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Larger tires- Do you NEED to re-callibrate?

I currently have the stock 16" steel wheels on my sport. I will soon be putting on Rubi takeoffs. I know that this will affect my speedometer. Can anyone tell me how much and the calculation I need to do to figure my true speed?

Also If I don't go to the dealer to re-calibrate the speedometer to the new tire and wheel size, will that negatively affect the jeep performance or the mpg?

Thanks

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Old 09-18-2011, 09:16 PM   #2
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Yes you need to make changes to correct for proper speedo changes. It is all done in the computer unlike earlier models were you had to change gears. As long as you don't go bigger then the largest stock tire size, then the dealer should be able to correct that for you. You can also do it by getting a Pro-Cal tuner or a Superchips Flashpaq. . If you do it yourself the size you actually program should be about 1" smaller then actual tire size. This is to compensate for compression of the tire due to the weight of the Jeep. 1" is just a good starting point. It may need to be .25" either side of that depending on the PSI you set the tire too. Use a gps if you have one to check the speed.

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Old 09-18-2011, 09:18 PM   #3
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Great idea with the gps. If I don't get it calibrated is it okay or will mpg and performance suffer.
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Old 09-18-2011, 09:22 PM   #4
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Well, you need to know the tire diameter's, so as a mental excercise go to tirerack.com and find the outside diameter of your existing tire. Assume 30", now move to a 32" tire. Thats 32/30 or abbout a 7%. So if the speedo says 70, you're going almost 75. If you tire is 31, it's only about 2 mph off at 70.

You can/should verify this with a GPS.

Depending on gearing, you might lug the engine going to a larger tire, thi will result in sligtly lower mpg. Compared to an all season or AT tire, the MT tire will negatively affect your mileage. My diesel Ram runs an LTX A/S highway tire and averages 22mpg freeway, loses about 1.5 with an MT in the same size. The highway tire has lower rolling resistance (especially at 65-70 psi), running and MT at 45-50 psi grabs better but is tougher on the wallet. These are hand calc figures with 148,000 on the odometer. I have a Rubicon that has run bot the stock MT and sahare AT's and am sure the same applies, though I haven't put the miles on to quantify.
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Old 09-18-2011, 09:23 PM   #5
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This chart will show how far off you will be: Car / SUV / Van / Light Truck Tire Calculations
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Old 09-18-2011, 09:27 PM   #6
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Quote:
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Great idea with the gps. If I don't get it calibrated is it okay or will mpg and performance suffer.
Shift points are affected if you have an auto
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Old 09-18-2011, 09:30 PM   #7
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Quote:
Originally Posted by CG3

Shift points are affected if you have an auto
I do have auto. So what does that mean. Bad thing?
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Old 09-18-2011, 09:39 PM   #8
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It means it won't shift when it's supposed to. Is it bad? It is my understanding that over time it's not good for the transmission. And in the short term, it's not as efficient... power/gas mileage/etc. Someone please correct me if I'm wrong.
You can EASILY remedy this. Buy a AEV Procal Module. Program it per directions, plug it in, and voila'.... speedo calculated properly, miles driven showing on odometer are correct, shift points accurate... and it's so simple to do. Plus the module does other cool stuff too....
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Old 09-19-2011, 07:58 AM   #9
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The AEVPM is the best way($149) bucks, to correct your speedos--GPS will tell you actual miles, but

Doesn't do anything for the vehicle and if you're gonna correct the speedo,using the AEVPM--don't go by charts--THEY'RE ALL WRONG, in respect to actual tire installation--

Put your USUAL air pressure in the tires and MEASURE THE ACTUAL HEIGHT of the rear tires--write it down and the AEVPM instructions will inform you how to set the DP switchs for the AEVPM computer use-any other way is a GUESS-

It's also against the law to sell your jeep, if you haven't corrected the speedo and DON'T INFORM THE BUYER--you've alter'd the speedo and thats a FED offense-

Heh Heh, I doubt if anybody has ever been charged withit though-

Good morning

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Old 09-19-2011, 08:07 AM   #10
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Before I put the new rims and 33's on my 2007 Wrangler the speedo was off by about 5mph - If I was going 50 my true speed was 55. That never changed when I put the bigger tires on. Still 5mph faster than the speedometer says. I shecked this by using the "speed limit is xx your speed is xx" things that the local PD put out.
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Old 11-09-2013, 08:12 AM   #11
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AEV Procal Module

Do they have a module for the 2004 Jeep
with auto trans.
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Old 11-09-2013, 08:29 AM   #12
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I didn't recalibrate... my speedo is about 3.5 mph off...so what...
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Old 11-09-2013, 09:04 AM   #13
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Mine is about 5 or 6 off at 60MPH, I haven't recal'ed yet.
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Old 11-09-2013, 09:19 AM   #14
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This is an old thread, but they SHOULD be recal'ed.

It corrects the:
Speedometer
Odometer
Shift points (auto tranny)
Computer adjustments (items that automatically turn on/off at certain speeds like ediscos, lockers, hill descent, ect)

If you sell the vehicle within 10 years, TIMA (Truth In Mileage Act) makes it illegal to not disclose the actual mileage.

Why spend the money on wheels and tires (and sometimes a lift....which could run thousands of dollars in total), but skimp on the recalc?
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Old 11-09-2013, 09:36 AM   #15
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It is all done in the computer unlike earlier models were you had to change gears.
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Old 11-09-2013, 10:35 AM   #16
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Quote:
Originally Posted by JIMBOX View Post
The AEVPM is the best way($149) bucks, to correct your speedos--GPS will tell you actual miles, but

Doesn't do anything for the vehicle and if you're gonna correct the speedo,using the AEVPM--don't go by charts--THEY'RE ALL WRONG, in respect to actual tire installation--

Put your USUAL air pressure in the tires and MEASURE THE ACTUAL HEIGHT of the rear tires--write it down and the AEVPM instructions will inform you how to set the DP switchs for the AEVPM computer use-any other way is a GUESS-

It's also against the law to sell your jeep, if you haven't corrected the speedo and DON'T INFORM THE BUYER--you've alter'd the speedo and thats a FED offense-

Heh Heh, I doubt if anybody has ever been charged withit though-

Good morning

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* Okay, Ive worked all night and probably not thinking clearly.......so if I buy this I'm only correcting the speedo? Does it or does it not fix shift points? Will my mpg change? Will it show better or worse? I have 35's on at the moment.....Auto tranny too. Thanks..
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Old 11-09-2013, 05:20 PM   #17
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I had the 16" steel wheels with normal aspect ratio tires (what, 26 inches? 2 inches?). I put 33s on my jeep. The jeep drove fine (albeit now lacking low end torque due to improper gearing). The speedometer was 7 MPH off according to GPS. It read 7 mph low. The result? Excessive speeding.

Do you like getting tickets? No? Then get it calibrated.

my jeep drives exactly the same after recalibrating it. Did nothing to shift points whatsoever, still shifts like crap.

By the way, my dealer refused to do the job.

Your mpg will change because without recalibrating it you have no way to accurately detect miles. Your mpg will go up because the miles will go up and gallons will stay the same.

Superchips is *WAY* better than AEV, just because it has a screen. It cost less too, for me anyway.
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Old 11-09-2013, 05:23 PM   #18
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Quote:
Originally Posted by JIMBOX View Post
The AEVPM is the best way($149) bucks, to correct your speedos--GPS will tell you actual miles, but

Doesn't do anything for the vehicle and if you're gonna correct the speedo,using the AEVPM--don't go by charts--THEY'RE ALL WRONG, in respect to actual tire installation--

Put your USUAL air pressure in the tires and MEASURE THE ACTUAL HEIGHT of the rear tires--write it down and the AEVPM instructions will inform you how to set the DP switchs for the AEVPM computer use-any other way is a GUESS-

It's also against the law to sell your jeep, if you haven't corrected the speedo and DON'T INFORM THE BUYER--you've alter'd the speedo and thats a FED offense-

Heh Heh, I doubt if anybody has ever been charged withit though-

Good morning

JIMBO
Lots of people have been charged with tampering with odometers. But they intentionally rolled them back, its not like they put bigger tires on their jeeps and didnt think about it.

I had a car with 230,000 miles on it once and replaced the speedometer just because it had an 85 mph speedo and i found one with a 120 mph speedo..that had 150,000 miles on it. I suppose i could have gotten in trouble over that, if i hadn't informed the buyer (which i did).....
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Old 11-11-2013, 02:40 AM   #19
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* Okay, Ive worked all night and probably not thinking clearly.......so if I buy this I'm only correcting the speedo? Does it or does it not fix shift points? Will my mpg change? Will it show better or worse? I have 35's on at the moment.....Auto tranny too. Thanks..
If you call AEV and talk with the ProCal tech he will all but walk you through what you need to do with the ProCal. When I talked to him last week he told me if I was planning to regear DO NOT use the ProCal on the transmission until after you re-gear, just add/factor in the larger tire size in the ProCal.

Just a suggestion give AEV a call they are great to talk to and will answer all your questions. There customer service in my experience.
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Old 11-11-2013, 07:20 AM   #20
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slightly larger tires

Just put P245/75R16 tires on my new 2013 unlimited sport. The stock tires were P225/75R16. Since I didn't change rim size and diameter of tire only increased by 1.2 inches to 30.5 should I worry about shift points or anything else detrimental to the Jeep? I realize speedo is off just a bit. It's an auto tran.
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Old 11-11-2013, 07:24 AM   #21
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* Okay, Ive worked all night and probably not thinking clearly.......so if I buy this I'm only correcting the speedo? Does it or does it not fix shift points? Will my mpg change? Will it show better or worse? I have 35's on at the moment.....Auto tranny too. Thanks..
Yes, you can adjust shift points. MPG will change because of the tire size. Any calculations that you do with an incorrect speedometer/odometer will not be accurate for your MPGs.

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