Upgrade Tires to 245x75r17 - Need to flash ECU? "Relearn transmission?" - Jeep Commander Forums: Jeep Commander Forum
Wheels / Tires This section contains discussion about wheels and tires for your Jeep Commander.

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post #1 of 12 (permalink) Old 04-09-2018, 05:58 PM Thread Starter
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Upgrade Tires to 245x75r17 - Need to flash ECU? "Relearn transmission?"

As the title says, I went to buy new tires tonight. Specifically, the BFG KO2's in 245x75r17, which according to the sticky just barely fit with minimal rubbing.

Anyways, as I was about to check out the manager came out and asked me where I was getting my "transmission re-learning" and I had no idea what he meant. According to him, by going from 245x65r17's to 245x75r17's I have to reprogram the ECU and I have to get a "Trans re-learning procedure" or my vehicle won't function anymore. I feel like he was being a bit dramatic, but there might be some truth to it. So I walked away for now, and came here to verify what I need to do.

So, has anybody (I'm guessing on this forum there will be) changed from the stock tire to 245x75r17's ? Did you need to reprogram anything? Speedo? Transmission? ECU? PCM????

All help greatly appreciated!

Last edited by valicor; 04-09-2018 at 06:28 PM.
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post #2 of 12 (permalink) Old 04-09-2018, 06:51 PM
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Going to a different tire size without reprogramming will affect the way your vehicle drives, but it won't break anything. A bigger tire does fewer revolutions per mile, so your speedometer will read low by a small amount, around 7% with the size you're looking to move to. This means when your speedometer reads 60 mph, you'll be going closer to 64 mph, and the faster you go, the bigger the gap will be.

The transmission also tends to shift based off speedometer readings, and the shift points will feel different with a bigger tire. I've gone to the extreme end of this, running 33s on a stock tune, and the shift points always seem a bit high or low, but nonetheless it's still driveable. A tuner, like the Superchips Flashpaq, can adjust tire size in the computer and get everything back to normal, but it's not necessary.

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post #3 of 12 (permalink) Old 04-09-2018, 08:07 PM Thread Starter
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Quote:
Originally Posted by consumedbywater View Post
Going to a different tire size without reprogramming will affect the way your vehicle drives, but it won't break anything. A bigger tire does fewer revolutions per mile, so your speedometer will read low by a small amount, around 7% with the size you're looking to move to. This means when your speedometer reads 60 mph, you'll be going closer to 64 mph, and the faster you go, the bigger the gap will be.

The transmission also tends to shift based off speedometer readings, and the shift points will feel different with a bigger tire. I've gone to the extreme end of this, running 33s on a stock tune, and the shift points always seem a bit high or low, but nonetheless it's still driveable. A tuner, like the Superchips Flashpaq, can adjust tire size in the computer and get everything back to normal, but it's not necessary.
Thanks for the info. I really care about hurting the vehicle first (as he implied). I have a QT1 so I'm not looking for a ton of performance, but I'd like to be able to handle moderate trails with some decent tires. Anyways, thanks again for the info. I think I'll do it, but if the cost to reprogram is cheap I'll do it.
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post #4 of 12 (permalink) Old 04-09-2018, 08:58 PM
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Quote:
Originally Posted by valicor View Post
As the title says, I went to buy new tires tonight. Specifically, the BFG KO2's in 245x75r17, which according to the sticky just barely fit with minimal rubbing.

Anyways, as I was about to check out the manager came out and asked me where I was getting my "transmission re-learning" and I had no idea what he meant. According to him, by going from 245x65r17's to 245x75r17's I have to reprogram the ECU and I have to get a "Trans re-learning procedure" or my vehicle won't function anymore. I feel like he was being a bit dramatic, but there might be some truth to it. So I walked away for now, and came here to verify what I need to do.

So, has anybody (I'm guessing on this forum there will be) changed from the stock tire to 245x75r17's ? Did you need to reprogram anything? Speedo? Transmission? ECU? PCM????

All help greatly appreciated!
Quote:
Originally Posted by consumedbywater
The transmission also tends to shift based off speedometer readings, and the shift points will feel different with a bigger tire. I've gone to the extreme end of this, running 33s on a stock tune, and the shift points always seem a bit high or low, but nonetheless it's still driveable. A tuner, like the Superchips Flashpaq, can adjust tire size in the computer and get everything back to normal, but it's not necessary.
As cbw said, the only thing really noticeable is that your speedo will be off a little bit - but you won't be doing any harm to your XK with the different tire size.

You can buy a tuner - I have the superchips F5 3874 Flashpaq which adjusts the speedo for the larger tire size - it also gives you a separate menu to manually adjust the transmission shift points if you feel the need to;

I never messed around with the transmission shift points, I just adjusted the tire size from the stock 29 inch tire diameter to the 31.5 inch tire diameter of the 265/70R17's I put on, so my speedo is accurate.

And YES....that manager was being extremely dramatic IMHO.


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Last edited by Big Blue; 04-09-2018 at 09:36 PM.
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post #5 of 12 (permalink) Old 04-09-2018, 09:30 PM
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You might also get the occasional traction control activation due to larger tires when turning hard as the outside wheel has to travel a longer distance than the inner wheel and the computer may misconstrue this as tire slip. I had a tuner when I changed my wheels and tires, but a lot of people do not.
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post #6 of 12 (permalink) Old 04-10-2018, 12:45 AM
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Quote:
Originally Posted by Big Blue View Post
You can buy a tuner - I have the superchips F5 3874 Flashpaq which adjusts the speedo for the larger tire size - it also gives you a separate menu to manually adjust the transmission shift points if you feel the need to;

I never messed around with the transmission shift points, I just adjusted the tire size from the stock 29 inch tire diameter to the 31.5 inch tire diameter of the 265/70R17's I put on, so my speedo is accurate.
The stock shift points will work fine with a larger tire. What I was referring to when talking about the transmission was the fact that the transmission tends to shift based on speedometer readings, so for instance with a 33 you won't get into fifth gear until you're going around 70 mph. I'm not sure exactly how the computer decides when to shift, but I've noticed that when I switch between tires the truck seems to lug down a lot more often before downshifting, and other times will downshift at unexpected times when the computer may feel throttle application isn't properly affecting speed. Just adjusting the computer and speedometer for a larger tire size irons out most of these kinks.
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post #7 of 12 (permalink) Old 04-10-2018, 05:49 AM
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Quote:
Originally Posted by consumedbywater View Post
The stock shift points will work fine with a larger tire. What I was referring to when talking about the transmission was the fact that the transmission tends to shift based on speedometer readings, so for instance with a 33 you won't get into fifth gear until you're going around 70 mph. I'm not sure exactly how the computer decides when to shift, but I've noticed that when I switch between tires the truck seems to lug down a lot more often before downshifting, and other times will downshift at unexpected times when the computer may feel throttle application isn't properly affecting speed. Just adjusting the computer and speedometer for a larger tire size irons out most of these kinks.
I hear you.....that makes sense.


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post #8 of 12 (permalink) Old 04-10-2018, 09:08 AM Thread Starter
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Thanks for all of the input. I figured it was fine, but that manager was POSITIVE I'd be damaging something. Almost to the point where he refused me as a customer.

With everything I read, I think I'm going to get the 245x75r17 tomorrow. I'll probably try to find somebody that can program the speedo at least, just so I have that accurate.

If I find somebody with one of those programmers/tuners do they just change a setting and that's all? Or do I need to physically change out an ECU or something? Assuming of course I want to do it "properly" and have all speedo and shift points accurate.
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post #9 of 12 (permalink) Old 04-10-2018, 04:54 PM
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Quote:
Originally Posted by valicor View Post
Thanks for all of the input. I figured it was fine, but that manager was POSITIVE I'd be damaging something. Almost to the point where he refused me as a customer.

With everything I read, I think I'm going to get the 245x75r17 tomorrow. I'll probably try to find somebody that can program the speedo at least, just so I have that accurate.

If I find somebody with one of those programmers/tuners do they just change a setting and that's all? Or do I need to physically change out an ECU or something? Assuming of course I want to do it "properly" and have all speedo and shift points accurate.
I can tell you that the superchips flashpaqs are VIN specific - meaning that once they read & store an engine file - they can only be used on that specific vehicle unless you purchase an unlock code from the manufacturer.

I don't know about the other types of scammers on the market.


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post #10 of 12 (permalink) Old 04-10-2018, 06:00 PM
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Find someone to do it or spend $500 for a tuner to change your speedo by a few mph.With your tire change you're going to have close to no change in shifting.

I've had more than a few vehicles with larger tires than stock and never reprogrammed anything.

Here's a size reference for your tire change. Not really worth messing with a programmer IMHO.

https://www.tacomaworld.com/tirecalc...5r17-245-70r17

At 55mph you'll actually be going 57mph. That guy was really full of it and I'd suggest buying your tires elsewhere.

Last edited by Legofan; 04-10-2018 at 06:02 PM.
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