Tire PSI question

[Lazerred98]

so, on my 98 base model, the original factory size tires is 205-65-15 with a SL load rating. I have changed to 16 inch wheels, and now have 205-55-16 tires with XL load rating. My question is, do I still use the factory PSI of 35, or do I need more pressure with these new tires??? The car is a weekend driver, and does NOT get raced or anything. I just want good tir life, and also as soft of a ride as possible.

 

[white95]

You should run the same air pressure as before.

 

[Lazerred98]

That is what I thought, but I asked on another forum, and many people said that since the new tires are XL and the original tires are SL, that I should have about 5 more PSI. Trying to get some opinions from people who have the same cars. Think I will just try the stock PSI, and play by ear.

 

[white95]

That is what I thought, but I asked on another forum, and many people said that since the new tires are XL and the original tires are SL, that I should have about 5 more PSI. Trying to get some opinions from people who have the same cars. Think I will just try the stock PSI, and play by ear.

Essentially the inner structure of the tire is reinforced with the XL vs the SL. You can run more maximum pressure in the XL rated tires but it’s a Mustang. You aren’t going to exceed the load rating of your tires.

 

[Lazerred98]

That was my thinking about the differences, that you CAN run more pressure, but not required, especially since the weight of the car loaded is not near the max load for the tire.

 

[b1pig]

The specs that come from the manufacturer are specific to the brand, model and size of tire your car CAME WITH off the assembly line.

Once you change to another brand and or size of tire, that label can be tossed in the trash can.
I won't complicate my answer to your question with specifics, but you need to look at the TIRE. Every tire comes with a "Max Load: XXXXlbs @ XXpsi Cold". That means each tire is rated to a certain max weight at a certain pressure. Always base your rule on what is on the tire sidewall. Due to modern tire constructions, the specs of a new tire are very unlikely to match what is on the label Ford put on your car.
Once you change WHEEL size, its almost a guarantee that the load/psi rating of the tires you get will not be the same as OEM.






Copied image from somewhere on the internet... used for explanation...


Just to use the pictured tire for understanding.
So, in theory if you have 4 of the same tires, the 4 tires are rated to work on a car that weighs 5,908 pounds. (1477x4) Methmatitians please refrain from references of front/rear balance, etc.
If you car weighed 5,600 pounds full of fluids, passengers and baggage then you really should be running the 50psi this tire is rated for. Running low pressure and high loads, this will cause excessive heat, excessive wear and really bad handling. Anyone remember the Firestone/Explorer (Exploder) stuff from years back?

The truth is that MOST cars rarely ever come close to hitting that maximum load rating on a tire. Regardless. You NEVER want to run more than the maximum recommender pressure. When you drive, the tires heat up. When tires heat up, air expands. Air expansion increases the pressure inside the tire. This affects the wear and handling of the tire and can lead to premature tire failure in severe cases.

If your car is really light compared to the tire's weight rating, running the max pressure CAN result in abnormal wear, which will be evident by the center of the tread being worn out with lots of tread left on the outer edge. High pressure makes the tire harder, and therefore a lower rolling resistance which can improve fuel economy slightly.


There is no PERFECT answer without going down a rabbit hole.... One that gets deeper when you start looking at light truck and offroad tires. The BEST rule is to refer to what the tire's spec is. When tire technology advances and you make mods, the label in your door jam doesn't get updated.


For reference...

Label in my Jeep says "255-75-17, 40psi"
Tires are 35-12.50-17, 80 psi. I have kept these tires around 40 psi since they were installed in Feb2018. Perfect treadwear and perfect handling (for a lifted 4wd @ 5100 pounds). I went down the rabbit hole.
I don't run them at 40 because of the label in the Jeep. Its because the original tires had almost half of the load rating that the current tires have. Half the load rating... half the pressure... it works IN THIS CASE.
So, just to show there is NO PERFECT answer. Just a good grasp of the details and a little research on what you're running.

 

[Lazerred98]

The specs that come from the manufacturer are specific to the brand, model and size of tire your car CAME WITH off the assembly line.

Once you change to another brand and or size of tire, that label can be tossed in the trash can.
I won't complicate my answer to your question with specifics, but you need to look at the TIRE. Every tire comes with a "Max Load: XXXXlbs @ XXpsi Cold". That means each tire is rated to a certain max weight at a certain pressure. Always base your rule on what is on the tire sidewall. Due to modern tire constructions, the specs of a new tire are very unlikely to match what is on the label Ford put on your car.
Once you change WHEEL size, its almost a guarantee that the load/psi rating of the tires you get will not be the same as OEM.






Copied image from somewhere on the internet... used for explanation...
View attachment 20137

Just to use the pictured tire for understanding.
So, in theory if you have 4 of the same tires, the 4 tires are rated to work on a car that weighs 5,908 pounds. (1477x4) Methmatitians please refrain from references of front/rear balance, etc.
If you car weighed 5,600 pounds full of fluids, passengers and baggage then you really should be running the 50psi this tire is rated for. Running low pressure and high loads, this will cause excessive heat, excessive wear and really bad handling. Anyone remember the Firestone/Explorer (Exploder) stuff from years back?

The truth is that MOST cars rarely ever come close to hitting that maximum load rating on a tire. Regardless. You NEVER want to run more than the maximum recommender pressure. When you drive, the tires heat up. When tires heat up, air expands. Air expansion increases the pressure inside the tire. This affects the wear and handling of the tire and can lead to premature tire failure in severe cases.

If your car is really light compared to the tire's weight rating, running the max pressure CAN result in abnormal wear, which will be evident by the center of the tread being worn out with lots of tread left on the outer edge. High pressure makes the tire harder, and therefore a lower rolling resistance which can improve fuel economy slightly.


There is no PERFECT answer without going down a rabbit hole.... One that gets deeper when you start looking at light truck and offroad tires. The BEST rule is to refer to what the tire's spec is. When tire technology advances and you make mods, the label in your door jam doesn't get updated.


For reference...
View attachment 20138
Label in my Jeep says "255-75-17, 40psi"
Tires are 35-12.50-17, 80 psi. I have kept these tires around 40 psi since they were installed in Feb2018. Perfect treadwear and perfect handling (for a lifted 4wd @ 5100 pounds). I went down the rabbit hole.
I don't run them at 40 because of the label in the Jeep. Its because the original tires had almost half of the load rating that the current tires have. Half the load rating... half the pressure... it works IN THIS CASE.
So, just to show there is NO PERFECT answer. Just a good grasp of the details and a little research on what you're running.

But what are you running on your 94 GT?

 

[b1pig]

40 in the front. 35 in the rear. My car came with two different brands of tires on the front and back. They are the proper size, but they have different specs. I believe the front tires max load/pressure rating is 40 and has a lower weight capacity than the rear. The rears have a higher load capacity at 40, but its lighter in the rear anyway. Both brands are off-brand tires... I can't even begin to tell you what they are without looking.

 

[RAU03MACH]

you could over inflate but then you would have center of the tire wearing faster
under inflation the outer edges would wear
just better to leave it at called for specs

 

[TrickVert]

My question is why anyone would sell the OP XL-rated tires for a Mustang. That's an "extra load" rating for light trucks.

And yes, changing tire ratings and/or sizes will most often change the recommended or optimal tire pressures.

 

[TrickVert]

you could over inflate but then you would have center of the tire wearing faster
under inflation the outer edges would wear
just better to leave it at called for specs

... and the car could feel like you're driving on a slick surface when you're not. Dropping my tires from 35 to the factory pressure of 32 changed (improved) the handling of my '95 dramatically.

 

[badass98svt]

... and the car could feel like you're driving on a slick surface when you're not. Dropping my tires from 35 to the factory pressure of 32 changed (improved) the handling of my '95 dramatically.

Yuppers.

Also, 32 would offer a smoother ride as well.
I was actually going to suggest 32.

32-35 PSI is the sweet spot

 

[ttocs]

tire wear will tell you if it is on straight and aired right. Unfortunately that will not tell you unless something is wrong but start by paying attention to if is leaving rubber on the driveway/garage as slow speeds and then look at how the tire is wearing once it is being used that it is even.