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Unread 06-09-2012, 09:51 AM   #1
BioTex
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Reprogramming the computer for larger tires

A friend of mine swears that when you change tire size, you need to go to the dealership to have them reprogram the computer to re-educate it for optimal mileage. I'm calling BS, but....

It kind of makes sense, but then again if the speedo is calibrated, isn't that enough? What's the true scoop?

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Unread 06-09-2012, 09:55 AM   #2
Ironhead
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looks like you have a rubi, so its an electronic signal.
many aftermarket options,

speedo healer
yellow box
truspeed

and many more.

or you could go to a dealer, i've heard the snap on scanner works as well

your speed is used by the ecu to make calculations, so it is a good idea to do it
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Unread 06-09-2012, 10:40 AM   #3
BioTex
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Quote:
Originally Posted by Ironhead_Jed View Post
looks like you have a rubi, so its an electronic signal.
many aftermarket options,

speedo healer
yellow box
truspeed

and many more.

or you could go to a dealer, i've heard the snap on scanner works as well

your speed is used by the ecu to make calculations, so it is a good idea to do it
We're not talking just reprogramming the speedo here. He swears some of the other sensors are involved. Don't know which ones, but he's saying there is more to reprogram than just the speedo. I haven't done my speedo yet because I'm about to change tire size.
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'85 CJ7, BDS 4" lift, 1" Body lift, 33x12.5, Shrockworks Sliders, 304 V8 with RV cam., T-176, D300, Dana 30, AMC 20.
1986 CJ10-A SD-33 Diesel/727/np208
1971 800B with 345/T-19
06' TJ Rubicon, 4" R.C. springs, BFG/AT 35s M.C. 6" fenders, rockers and surrounds, Currie front & rear adj. tracbars, tattons DC rear shaft, adj. upper - lower CA's. Bilstein 5100's
YJ Buggy Build Current project. Stroker/FI ?
1990 MJ Comanche 4.0L AX15/np231
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Unread 06-09-2012, 10:43 AM   #4
Ironhead
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i'm pretty sure your friend is wrong. what other sensors would control speed other than your speed sending unit? the ones i listed plug in in-line and change the output signal, telling your ecu the correct speed
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Unread 06-09-2012, 10:53 AM   #5
BioTex
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Quote:
Originally Posted by Ironhead_Jed View Post
i'm pretty sure your friend is wrong. what other sensors would control speed other than your speed sending unit? the ones i listed plug in in-line and change the output signal, telling your ecu the correct speed
I'm with you on this line of thinking, but since I come from a non computer era, I just don't yet have the experience to know for sure.

It seems to me that the computer only needs to know the correct speed, and rpm to couple with the MAP, O2, TPS, CPS inputs, and the air/fuel will be optimized. I'm sure there are others, but these are the ones I'm familliar with so far.

I'm thinking some dealership gave him a line of crap just to overcharge him for recalibrating his speedo.

Now another question. If the dealer reprograms it, it is not the same as aftermarket options is it? They provide a hardware solution that stays on the Jeep correct. It is not reprogramming so to speak, but rather correcting the signal right?
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1986 CJ10-A SD-33 Diesel/727/np208
1971 800B with 345/T-19
06' TJ Rubicon, 4" R.C. springs, BFG/AT 35s M.C. 6" fenders, rockers and surrounds, Currie front & rear adj. tracbars, tattons DC rear shaft, adj. upper - lower CA's. Bilstein 5100's
YJ Buggy Build Current project. Stroker/FI ?
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Unread 06-09-2012, 10:56 AM   #6
Ironhead
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no, they re-program the ecu, and there are lots of stories about the re-programming not sticking and having to go back multiple times.

the hardware option allows for easy re-calibration if you change tire size or re-gear
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Unread 06-09-2012, 11:13 AM   #7
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When I had access to a Jeep programmer (DRB3 I believe) yes you can go into the PCM and change the tire size from 245/75R16 to 33x12.5R15, and you can also change the gear ratio. This is only on a Rubicon, but 03-06 Rubicon's can be programmed at the dealer for larger tires/gears.
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Unread 06-09-2012, 01:15 PM   #8
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AEV procal will do everything you need, its only 129 bucks and its plug and play after a beep from the horn n and you can reuse it after switching tire size or gears
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Unread 06-09-2012, 01:45 PM   #9
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I called my local dealer about this and they flat out refused, saying they would only re-program it to another factory tire size
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Unread 06-09-2012, 01:51 PM   #10
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Your friend is wrong on everything but it needing to be reprogrammed to recalibrate the speedometer. The dealer can only reprogram your Rubicon for larger tires, nothing else. None of the other sensors need anything done or reprogrammed, the computer has an adaptive learning ability so it relearns everything it needs to on its own. But better than paying the dealer to reprogram the computer to recalibrate the speedometer, just install an electronic speedometer calibrator. That way you can recalibrate it yourself as you change tire sizes. Like this one... http://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=126/category_id=287/home_id=59/mode=prod/prd126.htm
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Unread 06-09-2012, 02:06 PM   #11
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Quote:
Originally Posted by Best4x4 View Post
When I had access to a Jeep programmer (DRB3 I believe) yes you can go into the PCM and change the tire size from 245/75R16 to 33x12.5R15, and you can also change the gear ratio. This is only on a Rubicon, but 03-06 Rubicon's can be programmed at the dealer for larger tires/gears.
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Unread 06-09-2012, 02:15 PM   #12
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Quote:
Originally Posted by Y1ann188 View Post
AEV procal will do everything you need, its only 129 bucks and its plug and play after a beep from the horn n and you can reuse it after switching tire size or gears
Thanks for the tip, I'll check into it. I will end up with a mounted set of 33's, and a set of 35's, so nice to be able to recalibrate on the fly.

Quote:
Originally Posted by Jerry Bransford View Post
Your friend is wrong on everything but it needing to be reprogrammed to recalibrate the speedometer. The dealer can only reprogram your Rubicon for larger tires, nothing else. None of the other sensors need anything done or reprogrammed, the computer has an adaptive learning ability so it relearns everything it needs to on its own. But better than paying the dealer to reprogram the computer to recalibrate the speedometer, just install an electronic speedometer calibrator. That way you can recalibrate it yourself as you change tire sizes. Like this one... http://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=126/category_id=287/home_id=59/mode=prod/prd126.htm
I kind of felt like he was wrong, but thought I better check on it anyways. So much new stuff to learn, but it is all good.
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1986 CJ10-A SD-33 Diesel/727/np208
1971 800B with 345/T-19
06' TJ Rubicon, 4" R.C. springs, BFG/AT 35s M.C. 6" fenders, rockers and surrounds, Currie front & rear adj. tracbars, tattons DC rear shaft, adj. upper - lower CA's. Bilstein 5100's
YJ Buggy Build Current project. Stroker/FI ?
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Unread 06-09-2012, 03:31 PM   #13
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TheBoogieman when I had the DRB3 in my hands and I changed the tire size on my Rubicon from 245/75R16 to 33x12.50R15. One of the tabs on the list was gear ratio. It of coarse had mine set at 4.10, but it had the options for 3.73 and 4.56. I was the Co Organizer of HJP and we had a DRB3 in our club. Not sure where that scantool is now, but I had it for over 3 weeks and I went through all the options. Most were blocked out due to it not being connected to the main PC, but Tire Size/Gear Ratio were two of the options I remember seeing. I was also able to see the boo boo the local Jeep dealership did when they replaced my PCM after getting countless P0700 faults. The idiots never put the correct mileage on the new PCM. While I had the DRB3 I corrected that so that both the gauge cluster/PCM had the same exact mileage readings.

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Unread 06-09-2012, 05:40 PM   #14
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I didn;t mention to you, that SpeedoHealer has two setting so you can press one button and switch between tire sizes. Convenient for your application.

http://www.healtech-electronics.com/
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Unread 06-09-2012, 06:28 PM   #15
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Hey, I'm only running 31s. Are any of these solutions worth any amount of benefit that I might see?

Thanks in advance.

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