Taller rear tire -- tune required?

[mikeyjobu]

I've been thinking of putting a taller tire in back to help me achieve a one-shift to 60 proposition. Looking at wheel packages on CJ's site, it says a tune adjustment is required to the speedometer? Is that true? If so, why? I thought speed was determined by the front wheel -- not the rear? I'd also read that Whipple was the only company that was currently able to tune for the rear-end and probably tire diameter with the S550 currently? If it's a foregone conclusion that I need to tune the car anyway -- voiding my warranty -- I'd like to know so I can consider tuning first versus a new tire-wheel setup.

From CJ Pony Parts:

Please Note: We have found that 275/35-20 tires tend to be on the slightly skinny side ONLY on the 20x10.5" Velgen rear wheels. Even though that tire size is perfectly fine, the 275/35-20 Nitto INVO tire will help cover and protect the wheel lip far better than the other 275/35-20 tire options. For a much more aggressive fit, we now offer the 295/35-20 Nitto INVO and Nitto 555 G2 tires which offer a great fit on any 10.5" wheel but are slightly taller than your original sizes. This will require proper adjustment do your speedometer which can be done with many of the handheld tuners on the market today. This may also cause interference issues on cars that are extremely lowered. Correct alignment and proper adjustment to rear suspension geometry is a must if you choose to have either of the available 295/35-20 Rear Tires installed.

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[CJ Pony Parts]

I've been thinking of putting a taller tire in back to help me achieve a one-shift to 60 proposition. Looking at wheel packages on CJ's site, it says a tune adjustment is required to the speedometer? Is that true? If so, why? I thought speed was determined by the front wheel -- not the rear? I'd also read that Whipple was the only company that was currently able to tune for the rear-end and probably tire diameter with the S550 currently? If it's a foregone conclusion that I need to tune the car anyway -- voiding my warranty -- I'd like to know so I can consider tuning first versus a new tire-wheel setup.

From CJ Pony Parts:

Please Note: We have found that 275/35-20 tires tend to be on the slightly skinny side ONLY on the 20x10.5" Velgen rear wheels. Even though that tire size is perfectly fine, the 275/35-20 Nitto INVO tire will help cover and protect the wheel lip far better than the other 275/35-20 tire options. For a much more aggressive fit, we now offer the 295/35-20 Nitto INVO and Nitto 555 G2 tires which offer a great fit on any 10.5" wheel but are slightly taller than your original sizes. This will require proper adjustment do your speedometer which can be done with many of the handheld tuners on the market today. This may also cause interference issues on cars that are extremely lowered. Correct alignment and proper adjustment to rear suspension geometry is a must if you choose to have either of the available 295/35-20 Rear Tires installed.

It is more a recommendation then it is a requirement. For example, if you went from a 275/35-19 rear tire to a 295/35-20 rear tire, your speedometer would be 3.79mph off. It would read 65mph when you were actually doing 68.79mph.

 

[GoBlues38]

Speed has always been configured from final rpms out of the transition to the rear wheels. IN old fox body days, it was a little plastic gear. Now a days, it is done in software.

To many variables if you pull from a particular wheel. inside wheel goes slower then outside wheel.

 

[mikeyjobu]

Thanks for the clarification. Forgive me - new to tuning with a tuner, can you select the diameter without flashing the rest of the parameters too? Will making this one change tip off a dealer being asked to do warranty service - as in will that one change increment the number of times the car has been flashed?

 

[mikeyjobu]

FYI, last I checked against a speedometer app on my phone, I hit fuel cutoff in second at 59.8 mph.

 

[mikeyjobu]

[MENTION=20475]Dave@CJPP[/MENTION] -- thanks for that quick response! What tuner works best for adjusting the tire diameter? I'd been considering going to a Lund tune -- BAMA is the one I'd first really heard of years ago. Some of my buddies tell me to just go get a dyno tune.

 

[BC]

....What tuner works best for adjusting the tire diameter?...........

My speedo is off by about 2% and I was under the impression tuners could adjust due to advertised claims. I bought an SCT and tune from Steeda but they did not want to use the tuner to change wheel diameter setting, saying results undesirable and it was an SCT issue.

I emailed SCT and they told me the proper setting was in the ABS system and not the ECM. They said they were trying to figure out how to do it. I haven't emailed them back in awhile...might do so.

It's no big deal but does bug me a little knowing the speedo is off.
If SCT can't adjust maybe a dealer could change it but that would probably be a minimum $150 service charge...not worth it.



EDIT: This is what I got from SCT.


From: Franklin Allen [mailto:[email protected]]
Sent: Monday, January 11, 2016 9:50 AM

We are sorry to inform you that the tire size and axle ratio is currently not available for your vehicle. This is why. This is no longer controlled by the PCM this is controlled by the ABS system. We are currently seeing if we can correct this problem.

Thank you,
Franklin Allen
Sales Development Associate
407-774-2447 EXT. 2157
[email protected]
***** To Reduce Reply time, please reply with history to all emails. *****
Does your device serial number begin with XP? click here for update instructions: http://www.sctflash.com/PDF/AutoUpdate.pdf
All other devices, click here for updating instructions: http://www.sctflash.com/PDF/UpdatingFW_TR.pdf
How to Load Custom Tunes on all Devices, click here: http://www.sctflash.com/PDF/LoadingCustomTunes.pdf

 

[mikeyjobu]

[MENTION=12757]BC[/MENTION] - Thanks a million - great info!

 

[evo8904]

You will either have to go to a Ford dealership to have them recalibrate the speedo or get a whipple flare device. I use a whipple flare device to recalibrate whenever I change my tires.

 

[mikeyjobu]

You will either have to go to a Ford dealership to have them recalibrate the speedo or get a whipple flare device. I use a whipple flare device to recalibrate whenever I change my tires.

How much is a Whipple flare device? And does doing that void your engine warranty?

 

[mikeyjobu]

So this is it:
https://whipplesuperchargers.com/index.php?dispatch=products.view&product_id=354
Still unanswered: will this void the powertrain warranty?

 

[Need4SpeedMotors]

I have members running fat tires in the rear, the difference is very minor. No need for a tune unless you want to do that.

Also any local dealership can adjust for you for the cost of 1 hr of labor

 

[mikeyjobu]

You will either have to go to a Ford dealership to have them recalibrate the speedo or get a whipple flare device. I use a whipple flare device to recalibrate whenever I change my tires.

So as I understand the instructions on Whipple's page, if you aren't reprogramming the PCM, you'd install the software, and skip right to step 25? I'm assuming you don't have to flash the whole thing to get at the axle ratio/tire size part?

 

[BC]

I emailed SCT again and they got back to me saying you can now correct the rear wheel size for speedo. It requires latest SCT x4 update.

IMPORTANT:
Before you update your SCT X4 device return your vehicle to stock.
See below:


From: Franklin Allen [mailto:[email protected]]
Sent: Thursday, May 26, 2016 12:50 PM
Yes please return the vehicle back to stock before updating.
Sent: Thursday, May 26, 2016 3:48 PM
To: Franklin Allen <[email protected]>
One more question. Should I return to stock tune first, then update the device, then reload my custom tune? Or can I just update the device and good to go with changing wheel diameter setting?
From: Franklin Allen [mailto:[email protected]]
Sent: Thursday, May 26, 2016 12:22 PM
Hello Bob,
Yes this can be corrected through our tool. Have you updated the device?
Sent: Tuesday, May 24, 2016 6:13 PM
To: Franklin Allen <[email protected]>
Hello Franklin,
I am just checking back to see if there has been an update since last January for the SCT X4, to adjust wheel size diameter to correct speedo. 2015 Mustang GT.

 

[mikeyjobu]

I emailed SCT again and they got back to me saying you can now correct the rear wheel size for speedo. It requires latest SCT x4 update.

IMPORTANT:
Before you update your SCT X4 device return your vehicle to stock.
See below:


From: Franklin Allen [mailto:[email protected]]
Sent: Thursday, May 26, 2016 12:50 PM
Yes please return the vehicle back to stock before updating.
Sent: Thursday, May 26, 2016 3:48 PM
To: Franklin Allen <[email protected]>
One more question. Should I return to stock tune first, then update the device, then reload my custom tune? Or can I just update the device and good to go with changing wheel diameter setting?
From: Franklin Allen [mailto:[email protected]]
Sent: Thursday, May 26, 2016 12:22 PM
Hello Bob,
Yes this can be corrected through our tool. Have you updated the device?
Sent: Tuesday, May 24, 2016 6:13 PM
To: Franklin Allen <[email protected]>
Hello Franklin,
I am just checking back to see if there has been an update since last January for the SCT X4, to adjust wheel size diameter to correct speedo. 2015 Mustang GT.

Great news! Have you already otherwise tuned your car? Or in order to make this one parameter change, are you essentially performing a tune? It seems like it should be something more akin to reading/clearing codes -- you're updating a setting to reflect changes, not hunting for more performance messing with timing, RPM limits, air-to-fuel, etc.

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