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TPMS Reset - no 20” option

Doanster1

Active Member
Feb 14, 2018
1,034
558
Oregon
Probably innocuous, but when resetting the TPMS for my new wheels, the car only asks whether I have 19” or 21” wheels. No option for any other size (20” in my case). I wonder if the trigger points for the warnings to pop up are hard-coded based on the factory 45psi/42psi for the 19s/21s respectively?
In other cars, when you perform a reset, you’re telling the car that your currently set pressures are now the “right” values so it shouldn’t warn you until they drop by 25% (or whatever) from those settings. That is, if I intentionally want to run much different pressures, I tell the car that by performing a reset.
After leaving the tire shop last night with my new setup, the car instantly recognized the new TPMS and wanted me to reset, which I did. The shop had set the pressures to 42 front and only 36 rear. Driving this morning, I got a warning about the rears being low, so I’m thinking my theory about hard-coded values is true. Otherwise why would they ask what size wheels are installed?
Unnecessarily inflexible IMO.

ps yes, I put more air in the rears to bring them up to 42psi...
 

mongo

Well-Known Member
May 3, 2017
13,310
40,730
Michigan
Probably innocuous, but when resetting the TPMS for my new wheels, the car only asks whether I have 19” or 21” wheels. No option for any other size (20” in my case). I wonder if the trigger points for the warnings to pop up are hard-coded based on the factory 45psi/42psi for the 19s/21s respectively?
In other cars, when you perform a reset, you’re telling the car that your currently set pressures are now the “right” values so it shouldn’t warn you until they drop by 25% (or whatever) from those settings. That is, if I intentionally want to run much different pressures, I tell the car that by performing a reset.
After leaving the tire shop last night with my new setup, the car instantly recognized the new TPMS and wanted me to reset, which I did. The shop had set the pressures to 42 front and only 36 rear. Driving this morning, I got a warning about the rears being low, so I’m thinking my theory about hard-coded values is true. Otherwise why would they ask what size wheels are installed?
Unnecessarily inflexible IMO.

ps yes, I put more air in the rears to bring them up to 42psi...

Interesting, I've never heard of a car that lets you adjust the tire pressure. Seems like it violates the door sill tire and pressure placard and FMVSS requirements:
Specifically, the TPMS must warn the driver when the pressure in one or more of the vehicle’s tires is 25 percent or more below the vehicle manufacturer’s recommended cold inflation pressure, or a minimum level of pressure specified in the standard, whichever pressure is higher.
See also:
49 CFR 571.138 - Standard No. 138; Tire pressure monitoring systems.
 

Doanster1

Active Member
Feb 14, 2018
1,034
558
Oregon
Tesla is the “only” car that has OTA updates so they have the ability to change/hard code values on the fly.
TPMS sensors are just reporters of data. It’s up to the car to decide when the pressures have dropped more than 25%. Other manufacturers cannot hard code values because warnings would pop up all over the place due to the wildly fluctuating pressures people safely run.
For my recently gone ‘09 C-class, it worked exactly as I described. Even for newer cars like our F15 X5, you drive around for a while while the sensors recalibrate to your current settings. iDrive actually shows you the progress of the reset procedure in percentage points. It doesn’t just ask for which size wheels you’re running. I run 20” daily and 18” winters so pressures vary quite a bit. Just think of the thousands of different wheel/tire setups the 3-series crowd runs.
Tesla obviously is not a mod or indie-repair friendly make right now, hence the hard-coding. Hopefully that changes over time. They should know that many don’t want to run 21” and like the compromise 20” gives.
 

mongo

Well-Known Member
May 3, 2017
13,310
40,730
Michigan
Tesla is the “only” car that has OTA updates so they have the ability to change/hard code values on the fly.
TPMS sensors are just reporters of data. It’s up to the car to decide when the pressures have dropped more than 25%. Other manufacturers cannot hard code values because warnings would pop up all over the place due to the wildly fluctuating pressures people safely run.
For my recently gone ‘09 C-class, it worked exactly as I described. Even for newer cars like our F15 X5, you drive around for a while while the sensors recalibrate to your current settings. iDrive actually shows you the progress of the reset procedure in percentage points. It doesn’t just ask for which size wheels you’re running. I run 20” daily and 18” winters so pressures vary quite a bit. Just think of the thousands of different wheel/tire setups the 3-series crowd runs.
Tesla obviously is not a mod or indie-repair friendly make right now, hence the hard-coding. Hopefully that changes over time. They should know that many don’t want to run 21” and like the compromise 20” gives.

TIL :)
FMVSS 138 final rule:
We have decided to deny SEMA’s request that we amend FMVSS No. 138 to require TPMS reprogrammability, because there is no evidence to demonstrate an actual problem in this area. We believe that vehicle manufacturers installing TPMSs that may require reprogramming in certain situations are well aware of this issue and will provide this feature, as necessary. Thus, in the final rule, we expressly stated that TPMSs are permitted to be reprogrammable. Once again, although we are uncertain as to the exact details of systemreprogrammability, we assume that it will be fairly easy for the service industry to reprogram TPMSs to accommodate different tires and rims.
 

Ande

Member
Jul 28, 2017
740
543
Norway
What is this 19/21" selection used for anyway ?
I use 22" with 21" setting, and speed is spot on Vs GPS speed.
Does tires for rim size of 20 or 22" wheels have the same circumference anyway ?
 

mongo

Well-Known Member
May 3, 2017
13,310
40,730
Michigan
What is this 19/21" selection used for anyway ?
I use 22" with 21" setting, and speed is spot on Vs GPS speed.
Does tires for rim size of 20 or 22" wheels have the same circumference anyway ?

The tire diameter/circumference is very close. The image of the car on your display should change based on the wheel installed. Also the pressure is different as noted by OP.
 

Doanster1

Active Member
Feb 14, 2018
1,034
558
Oregon
The tire diameter/circumference is very close. The image of the car on your display should change based on the wheel installed. Also the pressure is different as noted by OP.
Does the image of the wheel change if you select one size or the other via the TPMS reset screen? I thought the SC had to change the cfgs for it to show up on the app/MCU?
My original point is that even with OEM setups, the TPMS screen is lacking available options, such as 21” square vs staggered (unless, of course, the recommended pressures are the same for both?).
 

artsci

Sponsor
May 10, 2012
6,260
3,257
Timonium, Maryland
Probably innocuous, but when resetting the TPMS for my new wheels, the car only asks whether I have 19” or 21” wheels. No option for any other size (20” in my case). I wonder if the trigger points for the warnings to pop up are hard-coded based on the factory 45psi/42psi for the 19s/21s respectively?
In other cars, when you perform a reset, you’re telling the car that your currently set pressures are now the “right” values so it shouldn’t warn you until they drop by 25% (or whatever) from those settings. That is, if I intentionally want to run much different pressures, I tell the car that by performing a reset.
After leaving the tire shop last night with my new setup, the car instantly recognized the new TPMS and wanted me to reset, which I did. The shop had set the pressures to 42 front and only 36 rear. Driving this morning, I got a warning about the rears being low, so I’m thinking my theory about hard-coded values is true. Otherwise why would they ask what size wheels are installed?
Unnecessarily inflexible IMO.

ps yes, I put more air in the rears to bring them up to 42psi...

As TPMS measures pressure the wheel size hardly matters. Pressure is pressure. When I change out to the 20s in the winter I set them for 21" No problems of any kind.
 

Doanster1

Active Member
Feb 14, 2018
1,034
558
Oregon
Yep, was never concerned about the actual pressures. Was wondering whether the software set the thresholds based on how you answer, e.g. my hard-coded values comment. Why even ask the question when performing the reset? A case of over-thinking things IMO.
 
Last edited:

artsci

Sponsor
May 10, 2012
6,260
3,257
Timonium, Maryland
Yep, was never concerned about the actual pressures. Was wondering whether the software set the thresholds based on how you answer, e.g. my hard-coded values comment. Why even ask the question when performing the reset? A case by vet over-thinking things IMO.

I think Tesla offers those settings just because it looks good. In the end I don't think it makes any difference, at least on the basis of my experience.
 

Russell

Supporting Member
Supporting Member
Sep 25, 2012
918
703
Silicon Valley
I believe Roadside Assistance will lookup your car and use that number to confirm your wheel size.
I had a flat tire over 1.5yrs ago and he knew I had 19" even though from the factory my car came with 21".
 

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