I would think that a tire with a Max PSI of 44 would have a different “factory recommended” pressure than a tire with a Max PSI of 51. If a tire has a 20% higher max PSI, I’d think that it’s recommended PSI would be 20% higher.
But I recently had this conversation with a tire shop.
“What’s the best pressure to keep these tires at?”
“Well, your door says 35, so we’re adjusting your tires to 35”
“But these tires didn’t come with this car. This car came from the factory with 16” normal tires. These are 17" low profile performance tires. Shouldn’t that make a difference? These tires did not come stock on this model"
“Yeah, but the do come stock on the BOSS and some GTs”
“Well wouldn’t those cars say something different on the door?”
“We always go by what the door says”
Are they right? I realize they’re the experts here. But I no longer believe any claim a person makes unless there’s a consensus to the truth on the SDMB.
No, they are most assuredly NOT right. The shorter the sidewall, the more pressure you need in order to effectivley keep the tire flat on the road and keep the sidewall from flexing. So if you switched from 16" tires (say 215/70R16) with a high sidewall to tread ratio, to low-profile 17" tires (say 225/50R17) you would definitely need to increase the tire pressure. By how much, I can’t say for sure. But if you don’t you’re going to wear the outsides of those tires like there’s no tomorrow.
FWIW, I have 17" low profile tires (Stock) on my PT Cruiser GT. Recommended tire pressure is 38, and I always put in 40, and I STILL wear the damn outside edges faster than the middle.
Right now I have 275 40 R17s with a max PSI of 51. I switched from 235 55 R16 with a max PSI of 44. I kept the old tires at 35. Up until yesterday, I was running the new tires at 45.
The door is right if the tires have the same weight rating as the specced tires.
If the tires DON’T have the same weight rating, you’re operating outside of your automaker’s design spec, and thus the door placard is invalid.
I saw a formula one time for calculating your inflation pressure based on axle weight and tire rating. I think it gives “lowest safe pressure”.
Want me to dig that up?
Maximum pressures indicated on the tire are a safety limit determined by the tire manufacturer. They are not meant to be, and are not, in any way a guide to what pressure the tire should be inflated to - other than very simply what it says, a maximum not to be exceeded.
Correct tire pressures for a given tire on a given car are determined by the car manufacturer. In a case like this, where you’re using a tire that is not listed on the car’s inflation decal, you’re in a position of not knowing the recommended pressure. You’re probably right in suspecting that it would be higher with this type of tire, and in suspecting that a similar model that lists this type of tire would have an applicable recommended pressure on its decal. But you’re not right in assuming that it’s simply proportionate to the maximum indicated on the tire, and the tire shop is not right in assuming that an inflation decal for different types of tires should be followed.
Call a dealer and find out the right specs for your vehicle/tire combination.
PS- I was reading an article provided to a web site by Bridgestone. Bridgestone advised you to call their 800# for tire pressure recommendations for aftermarket-sized tires they’d manufactured, and to call any other tiremaker in a similar situation.