Okay, I have a question about tire pressure. I run 245/40/18 and 275/40/18's. The side wall says max pressure is 51 lbs.

What pressure should I run? One tire shop told me 45 lbs, one told me 40 lbs, and one said 35 lbs.

Thanks in advance for the help.

 

Max pressure is for maximum load.

"Door" pressure is generally for "comfort and ride" (read: way too soft).

The best pressure to use is the one that wears the tires evenly across the tread. A pretty good place to start would be 35-40, and check the wear regularly to adjust the tire pressures to get even wear.

 

Take a look at this to figure it out...

The load index refers to the load-carrying capacity of a tire, or how much weight a tire can support. For example, if a tire has a load index of 89, it can support 1,279 pounds at maximum air pressure. Multiply that by four (4 x 1,279 = 5,116 pounds) to get your maximum load-carrying capacity.

Find the load index of your O.E. tires on the door placard and the recommended air pressure and go across the chart to determine what it will hold. Then look at the load index of your new tire and find the same weight on the chart and go with the air pressure that the chart recommends.

Or, you can just PM me the 2 load indexes, and approx. how much your car weighs, and I'll figure it out for you.



Chris
www.discounttiredirect.com

 

There is nothing soft about the Z's ride.

Please don't just start picking a pressure because you like the way it rides or gives an even wear, use what the manufacturer recommends. It's based on the design of the vehicle and the capabilities of the tires.

Didn't the Ford Explorer exploding tire problem provide enough of an example of what happens when you don't follow recommended inflation values?

 

Whoah, slow down the rant a bit. I appreciate your concern for others safety but it's lost a bit in your response.

Tire pressures can make more of a difference in the way your car handles than any other component. By adjusting the pressures and experimenting one can find the ideal compromise for their specific driving style. I agree tire pressures should not be picked out of thin air, so when testing out new pressures only go up or down by one or two psi at a time and drive long enough to determine if the ride got better worse or no change. For autocross competitions it is common for skilled competitors to raise their tire pressures by 5 - 10 psi to reduce the amount the sidewall rolls over, thereby increasing steering response. This would not necessarily be a good idea for street driving because the higher your tire pressure, the harsher the ride is especially potholes. It is much like an overinflated basketball versus and under or properly inflated one; it will bounce much higher.


Don't be misleading. The Ford Explorer/Firestone tire probelm occurred because the tires were both incorrectly manufactured (had defects) as well as run well under the reccommended inflation level. Most of those tires that exploded were run 8-10psi lower than the recommended level. On the street, having underinflated tires will also cause dangerous handling on wet roads because of an inability to evacuate water.

Thanks for the concern, it is appreciated on this forum.

 

Thanks for all the responses. However, some of them raise more questions.

Several people say to follow the manufacturer's recommendation. I assumed the information provided with the car was applicable to OME tires. Is that not correct?

By giving my tire size in the originail post, I tried to indicate that I have aftermarket tires and wheels.

Chris@DTD. Thanks for the chart. I will give that a try and see what I come up with.

If anyone else cares to comment, I would appreciate the input.

 

OEM recommendations on tire pressure are definitely for stock equipment and take a bunch of items into consideration. Like others mentioned they are looking at ride quality, gas mileage, tire life, handling, etc.

I would stay away from the 45 psi, just because it can get awfully close to the 51 psi max pressure on a hot day and driving time. I suspect you are going to want to run somewhere between 35 to a little over 40 psi depending on how you are driving (driving speed, auto cross, daily driving, etc.).

You might want to pick the brains over at Tire Rack or shoot an email to the tire manufacturer/distributor for their thoughts.

 

Give you an example. When at the track I set my Michelin Pilot Sport Cups to 26psi front and 27psi rear. This is the cold temp psi. at the end of the session I check the pressire and they are almost always at 35psi front and 34psi rear. These settings come from Michelin's racing division Tech white paper, "the Care and Feeding of your Michilen Pilot Sport Cups"

For the ride home I inflate them to about 35-37 psi and my OEM every day tires are set at 35psi. I have never had an issue with the S0-3s set at 35psi.

 

35 -40 is good. Try not to go over 40psi, you will wear out the center of the tire too fast. You did not buy the car to save gas. So I would run 34-36

 

I'm having a similar problem just got some Toyo 235/50/17 on the back and when I drive highway speed the car is all over the place. Any suggestions?

 

35PSI is it, do not go into the 40s, it will have premature tire wear in the center. I just had my rear 275/40/18 eaten out in the center because of over inflations. My tires were 320 a piece, not a good thing to change all the times

 

Chris from Discount Tire is correct! I looked them up for you on a more detailed chart than he posted above.
Here is what I found out:
33psi front and 32psi rear will give you the correct load carrying capabilities for the tires you are running. The "best" inflation may vary from there due to the rim size you have the tires on. If you are running the O.E. wheels with these tires they will probably wear prematurely in the center, due to the sidewalls being suck in. This will crown the tire at correct inflation for the application and will also make the car sway around on the tires.

 

Discount tire is the best!
I like them a lot. I run 275/40/18 with 18X9.5. It is the perfect set up, I just need to be at 33-34 psi, I had them at 40 one time, that is when it screw me. I will bet.

 

I guess I'm missing something but I don't see any air pressures listed on that chart? Chris, can you clarify?

 

Thank you for everybody's help. I set the tires down to around 35-34 psi seems to be riding better. I tried taking it on the hgwy, but couldn't get a good run. To be continued.....