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Plus Sizing Perils: TPMS Adds More Complexity to Aftermarket Tire/Wheel Installation

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Over the next couple of months, I’ll be tackling the issue of how tire pressure monitoring systems (TPMS) affect the aftermarket and discussing strategies that can help tire dealers protect themselves and their customers.

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First off, what does NHTSA say about TPMS and aftermarket tire and wheel installation? Here is the excerpt from the latest ruling:

Under 49 U.S.C. 30122(b), “A manufacturer, distributor, dealer or motor vehicle repair business may not knowingly make inoperable any part of a device or element of design installed on or in a motor vehicle or motor vehicle equipment in compliance with the applicable motor vehicle safety standard prescribed under this chapter, unless the manufacturer, distributor, dealer or repair business reasonably believes the vehicle or equipment will not be used (except for testing or a similar purpose during maintenance or repair) when the device or element is inoperative.”

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However, when aftermarket tire and wheel assemblies are installed, and the MIL (malfunction indicator lamp) is illuminated as a result of the difference in replacement size or construction, “such equipment arguably has not damaged the TPMS itself, but, instead, has hindered its low-pressure detection capability…Once the TPMS MIL illuminates, the consumer would be warned that the equipment (tire/wheel) has caused a TPMS malfunction, and the consumer could substitute other equipment that would permit the TPMS to resume normal functioning.”

So, we can surmise that aftermarket wheels are exempt from TPMS; still, it is recommended that you indicate to customers that their systems may be inoperable and that neither you nor the customer can disable the MIL on the dashboard.

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I have heard stories about some dealers requesting that customers sign forms stating that the dealer has returned the OE tires/wheels to the customer and that the customer can install them at any time. The forms also express that it is the customer’s choice to install an aftermarket wheel and/or tire and that the dealer has attempted to use the original TPMS on the aftermarket wheels, but the sensors either will not fit into the specific wheel or cannot be reset to the higher air pressure required to maintain load capacity. In some states, inspections are necessary, and the system may have to operate properly before it will pass muster.

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Here’s the reality in my world. Take the 2005-present Mustang. This car is an aftermarket installer’s dream. Have you noticed that the OE tire is a P235/55R17 98W as opposed to the P245/45R17 95V from the last decade? Ford figured out that many of its earlier models were being fitted with 20s, even though the tires were much taller than the OE overall diameter. The new tire size has an OD of 27 inches, which just happens to be the exact OD of several 20-inch tire options. The recommended inflation pressure is 32 psi front/rear, which equates to 1,587 pounds of load capacity.

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What does all of this have to do with TPMS, you ask? Hang on – this is where things get dicey. The two most common and popular tire and wheel sizes in 20 inches for cars are 255/35R20 and 20×8.5 inches with a 5×4.5 bolt pattern and a high offset – a perfect fit for the Mustang.

But wait a second. Take a look at the load index on that 255/35R20 tire. A standard load tire with a load index of 93 will only carry 1,433 pounds at 35 and 36 psi for the P-metric and Euro-metric sizes, respectfully. The new Mustang is about 500 to 600 pounds heavier than the previous generation, so weight is now a critical issue. The only choice in this tire size is the Euro-metric extra load tire with a 97 load index. However, if we inflate to 42 psi, we can attain slightly more load-carrying capacity – 1,609 pounds – than the OE tire.

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Now, how do we tell the TPMS that the new psi is 42 and not the OE 32? NHTSA has mandated a 25% loss of pressure before triggering a warning, which we all know is ridiculously low. Now, we’ve installed a tire that needs 42 psi, and the system doesn’t know it. This means there would be no warning to alert the driver to a low-pressure situation until the tire drops below 24 psi, 25% of the original 32 psi!

I contacted two of the largest suppliers of TPMS to the OE market, and they told me that they build the units to manufacturer specs and don’t have any control over how the computer is programmed or which parameters – such as psi – are used.

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John Maxgay, lead engineer for chassis electronics with General Motors, shares some insight into how vehicle manufacturers are addressing this issue. “Many older systems that were installed early in the rise of TPMS didn’t have the ability to be programmed for anything other than stock OE settings,” he says. “Over the last 24 months, GM has launched its own accessory line, including plus-sized tires and wheels. We have also introduced high-performance options on certain vehicles, such as the Cadillac CTS-V, which inherently has larger wheels and low-profile tires.”

Vehicles with plus-sized tires and wheels require a change in psi ratings, which means that their TPMSs have to adapt. And, new systems coming online have the ability to be reprogrammed for higher pressure if needed, Maxgay says.

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This leads to another question: How do we reprogram the systems? At present, OE dealers have the tools to perform the procedure. But good luck finding one that is willing to perform the service or is even aware that TPMSs can be reprogrammed.

I randomly called the service manager at several OE dealers and asked the question: Can you reprogram the TPMS for higher pressure? I didn’t find anyone who either knew that they could or would be willing to find out. I did have one tell me that I could call the company’s customer-service line and ask for help. Yeah, right.

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Taking it a step further, I contact Alldata and Mitchell1. Currently, they have the same information that is supplied to OE dealers, which, at present, is the standard re-learn procedures needed for repair, replacement and rotation, not changing inflation parameters. Rich Diegle, communications manager and technical editor at Alldata, did tell me that as the OE manufacturers release new information about reprogramming the newer systems Alldata will incorporate the procedures into future updates. He explained that their data is about six to 12 months behind new vehicle releases. That’s likely the situation at Mitchell1, as well.

All of this means that tire dealers selling aftermarket upgrades on new vehicles will probably have to refer customers to the car dealer to get the system reprogrammed. How does that make you feel?

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The whole process hinges on the tire dealer and its staff being able to install the correct tire and wheel properly. Even more important is knowing and setting inflation pressure to the psi necessary to meet OE load-carrying capacity.

We covered this extensively in the March issue of Tire Review, so if you missed it, check it out and learn how this is done.

Don’t miss the second installment this article, coming in October. We will showcase some unique products and applications for aftermarket wheels and TPMSs that are designed to be adaptive.

For those of you clever enough to see beyond the horizon, we’ll show you how to turn the tables and actually profit from TPMS.

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