Detroit Bureau Chief Todd Lassa recently asked me to cover a “#TireWise’ Twitter chat organized by the National Highway Traffic Safety Administration, which was gathering “subject matter experts” to answer tire-related questions. Most people would rather watch paint dry, but I’m a tire geek and the opportunity sparked my interest. I won’t bore you with all the details because it turned out I should have gone to my local hardware store, purchased a can of paint, applied it to a wall in my house, and then sat and observed until all was safe to the touch, but the Twitter chat did inspire me to do research the subject further and it’s clear there is far more to tire pressure than you may think.
Every car comes with an NHTSA-mandated tire pressure label, usually found inside the driver’s door, that lists a set of tire pressures as well as the maximum combined load for passengers and cargo. NHTSA will simply tell you to run those max-load cold tire pressures, but there’s much more to inflation specs. Some car companies list multiple recommended inflation pressure combinations based upon the load the vehicle is carrying, speed, tire size, and what type of tire is fitted—winter, summer, etc.
Take the 2014 Mercedes-Benz SLS AMG Black Series. If you were to adhere to the NHTSA placard inside the driver’s door, you’d set the tire pressures to 35 psi front and 41 psi rear. But Mercedes recommends 32/32 for driving up to 155 mph, 35/38 for over 155 mph, and 33/41 psi if you’re crazy enough to cover the gullwing Mercedes with salt and run it on winter rubber. This information is found on a far more detailed Mercedes placard that lives inside the fuel filler door (the 2017 Mercedes-AMG GT S has a similar label). When I asked the NHTSA via Twitter about these tire pressures provided by the manufacturer, they simply recommended running the (high) pressures listed on their mandated decal.
The Europeans seem far more trusting of their drivers, even drivers of more pedestrian automobiles. My mother-in-law’s 2016 BMW 330d wagon in England carries a label listing 8 sets of tire pressures—both part-load and full-load tire pressures for four different tire combinations. This would be handy information for Americans to have affixed to their vehicle—smart people who live in cold climates run winter tires; with many cars, you can save money and obtain superior winter performance by using smaller wheels and narrower/taller winter tires. Some owners also change their wheel and tire size from stock.
Although the NHTSA’s mandated label does note to “see owner’s manual for additional information” and some companies in the USA list more tire pressure data in the manual, it’s both inconsistent and not nearly as handy as having the information easily accessible via a label affixed to the vehicle.
Adding to this rather confusing setup is that some car manufacturers don’t allow U.S. owners to reset their tire pressure monitoring system (TPMS). NHTSA requires that vehicles carry a TPMS that alerts the driver to a pressure loss of 25 percent or more. Ford, for example, runs a fixed TPMS threshold on their vehicles. The door jam decal on a Ford Focus RS in the U.S. recommends 46 psi front and rear. Those numbers are for max load; the part-load pressures supplied to European buyers are 41/38.
Unfortunately, Focus RS owners in the States can’t set the part-load pressures and then reset the system because it’s fixed to illuminate the TPMS light at 25% below 46 psi. This means owners who run the part-load tire pressures may find their TPMS warning burning their retinas after a minimal pressure loss—say on an unseasonably cold morning. Clearly, the TPMS setup is why Ford only supplies one set of tire pressures to U.S. buyers. Seeing that the Focus RS rides like a brick, why should Americans be kept in the dark, forced to run the higher, full-load tire pressures even when carrying a light load? Most Focus RS owners surely run a limited number of people and amount of cargo, fitting into the part-load parameters the majority of the time. Ford also fails to supply information on the recommended tire pressures for the factory 18-inch winter wheel and tire package.
Here’s the solution that I propose:
First, an important point is that there aren’t any safety issues if you follow the NHTSA’s recommendation and run the max-load tire pressures, but you may experience a harsher ride and tire wear may be affected. It’s also possible to damage a tire more easily when running higher tire pressures. I understand that many American buyers aren’t particularly tire savvy and I appreciate NHTSA’s desire to have one set of max-load pressures clearly listed in the door jam that are safe for all conditions, but a second decal on the car should list both the part and full-load pressures for all original-equipment tire sizes as well as the tire pressures for the factory-recommended winter wheel and tire package(s). Some may think the owner’s manual is the proper place for this additional information, but they can get misplaced or lost and do you really want a tire shop employee digging through your car to find the books?
An additional change I propose is for all vehicles to carry a user-resettable TPMS. Owners can then properly set tire pressures based upon wheel and tire size as well as load and not worry about TPMS woes. Then we need to make sure dealerships and tire shops are properly educated to both adhere to this proposed system and educate owners.
Some companies come close to utilizing my dream system. Mercedes-AMG is the rock star, with its system ticking all the boxes. Porsche comes close and provides tons of information, but it chooses to put that data in the owner’s manual. At least the info is there. Audi has a similar setup to Porsche. BMW falls a tick behind due to a rather strange “over and under 100 mph” full-load pressure list (no part-load pressures in the USA), but it still gives owners far more helpful information in the owner’s manual than most car companies. Strangely, non-AMG Mercedes don’t offer enough information, especially the SUVs. Let’s hope both consumers and the car companies get smarter about tire pressures and the NHTSA begins understanding the need to provide owners with more information and they realize the importance of a resettable TPMS system.