Tire pressure -- who to believe?

The sidewall of my Subaru’s right front tire says “Inflate to 45 psi max”. The tire shop mechanic says I should go by the Subaru label on the door pillar, which says tires should be inflated to 34 psi. I think he’s full of it – the tire manufacture knows better than the car manufacturer what pressure a given tire wants. Am I right?

Sorry! You’re wrong.

The PSI you see on the sidewall of the tire is the max pressure the tire manufacturer recommends for that tire. The PSI you see from the auto manufacturer is the recommended pressure for the best handling, ride, life, and safety from the tires.

The tire manufacturer makes that tire for more than just your vehicle. So each vehicle has a different tire pressure spec. So don’t follow what’s on the sidewall of the tire, follow what the placard says on the door post.

Tester

That says it all - in case banjoist wants a second opinion.

Mechanic is right, go by placard on door.

That same size tire could be used on any number of brands and models of vehicles, each of which has its own unique handling characteristics and its own differing inflation guidelines. And, as was said, that information on the sidewall is there primarily as a warning against overinflation of the tire, and could not possibly reflect the ideal inflation pressure for every model of car that uses that size tire.

If you want your vehicle to handle properly, to ride like a car rather than a truck, and to give you the maximum tread life, you will follow the manufacturer’s guidelines–with the possible addition of a couple of psi over the manufacturer’s recommendations. If you insist on inflating those tires to 45 psi, besides the problems that I have already mentioned, you will wind up with excessive wear on the ball joints, due to the reduced cushioning effect of the tires when they are that hard.

Also–I am curious about another point. If you don’t believe the manufacturer’s recommendations regarding tire pressures, do you follow the manufacturer’s recommendations regarding maintenance intervals? In this type of situation, a “non-believer” like you could wind up with greatly increased repair costs over the life of the vehicle, and, in fact, you could wind up reducing the life of the vehicle.

Those who don’t follow the manufacturer’s recommendations on maintenance and on tire inflation invariably live to regret their mistakes.

"Inflate to 45 psi max ".

That word MAX says it all. It is not a recommendation, it’s a warring.

Yep, the tire has the maximum and the car’s manufacturer knows the best recommended tire pressure for the vehicle.

As a bit of an analogy, think of it this way–If your doctor told you that your cholesterol count should not be over 200, that does not mean that you should aim for a cholesterol count of 200. Like with the maximum inflation pressure figures listed on the tire sidewall, this number is the maximum that is considered to be safe, and a lower number is desirable.

As Mr. Meehan said, consider that number on the sidewall to be tantamount to a warning, not a recommendation for every-day driving.

I, too, had wondered about this myself and had done some asking around and got the same answer that these folks are giving you. Makes sense now, not that it did then. Go by the car manufacturers recommendation.

The tire pressure is also normally specified in the OWNERS MANUAL, the most unread best seller in the world. The door post or glove box location makes it easy for reference. Years ago, before fuel effiency standards, tires ran 24 psi to get that water bed ride. This resulted in premature wear, and less than optimal fuel economy, and mediocre handling. Most auto experts at that time recommnded and extra 4psi for handling, safety and better wear. The Lincolns running in the Mexican Road race had 35 psi in their tires, whereas the factory pressure was 24-26 lbs.

If you have a MINI SPARE, it will need much more air; mine says “Inflate to 60psi” right on the tire. You have to do this to get the load bearing ability out of this very small tire. This will also be in the owner’s manual somewhere. That is also the reason you can’t drive it more than 60 miles and no faster than 60 mph on a mini spare.

Subaru label is the best compromise between fuel economy (higher the inflation, less rolling resistance- better gas mileage); ride comfort (higher tire pressure = bumpier ride); and tire wear (greater tire pressure = premature wear on center of tread; too low pressure means premature wear on outer edges of tread); and probably other smaller factors as well. If your Subaru was built in the

last couple of years, go w/door pillar specs. If it’s much older, you could maybe add 3 psi MAX to compensate for the jump in gas prices in the last couple years.

Banjoist- unrelated comment- by your user name it seems that you play bluegrass. I saw Earl Scruggs a few years back and was floored by how well an 80 year old man could play. Oh well, just small talk. Good luck with your Subaru.

Ok, so my “low tire pressure” light came on in our '07 Jeep. (Had to read the “best seller” to figure out what the light was.) Light didn’t go off until all tires were over 40psi (tires show 44psi max., door says 35psi). So at what pressure does the light come on? (Don’t make me get the book out again.)… Gotta go let some air outta my tires now…

Tire pressure gauge accurate? I buy 2 different brand pencil pauges at a time- about $7 ea. If they both read withih a psi of each other I use them. If problem persists and your Jeep’s still in warranty ('07, must be); raise hell w/dealer. Well, not Literally.

I can’t speak for your Jeep. On my Sienna, that light indicates a difference between tires, not the pressure as such. Also, when changing anything, I have to go through the reset sequence which involves a reset button to the left of the steering wheel on the dash.

THe reason you have speed and distance limits on a ‘mini’ spare is that the difference in tire diameter from the wheel on the other side of the car will cause the differential to ‘differential’ (the wheels are then constantly turning at a different speed). This generates a fair amount of heat in the differential, and doing it continuously on a ‘mini’ spare, will eventually cause catastrophic failure of the differential. The limit that it can tolerate is (about) 50 miles at 50 mph. (you could allow it to cool down for at least 12 hours, and then do another 50 at 50… etc.

And go by the car’s placard, not the info on the tire, as everyone has rightly said.

The label on the door is what you should follow. 45 PSI is the MAXIMUM the tire can safely tolerate, but is NOT intended to be the every day inflation pressure.

Wussup, banjoist?

Thanks for the additional explanation. Yes, it will be hard on the differential as well. I stop at the closest shop and have the tire fixed, so I can take the mini spare off.

FWIW, load range C tires (4ply rated) have had the 45psi max rating for a long as I have paid attention to tires. The rims and tires are built to that standard and exceeding that pressure has caused beads to fail in the tires and rims to fail also. I am aware that manufacturers overinflate severely overinflate tires for shipment to the dealer with no apparent damage but in my little corner of the world I have seen several catastrophic tire failures and found other tires on the vehicle overinflated. Just installing load range D tires and operating at 45psi has caused many truck rims to split at the bead.