Tire Pressure Inquiry

[WildHorse]

When going from stock 235/50R18's to 255/40R19 fronts 325/30R19 rear, should the tire pressure be the same? Or more? If more, how much? Thanks.

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[Stuntman]

Wider tires will have the same size contact patch as a narrower tire (if the tire pressure & weight of the vehicle remain constant). The shape will just be shorter and wider with a wider tire.

Wider tires allow you to run lower pressures on track since the load is spread over a larger area and thus, the lower pressures now make a larger contact patch.

For the street, you can leave them the same, in the ballpark of 35-40psi for both front and rear.

 

[WildHorse]

Ahhh thanks. I got em set at 33 psi which IIRC is 1 psi over stock.

 

[Stuntman]

Ahhh thanks. I got em set at 33 psi which IIRC is 1 psi over stock.

That's fine too. Nothing wrong with stock or 33psi. You might get a little better gas mileage at 35-40psi on the street, but the car will handle better if you started at 32-33psi. On track, you don't want them to get above 38-40psi.

 

[Assh]

Purchase a tread depth gauge it will tell you what air pressure you need run in your tires.

 

[WildHorse]

Purchase a tread depth gauge it will tell you what air pressure you need run in your tires.

How the heck is that gonna tell you what pressure to run?

 

[NightmareMoon]

32 front, but you could drop the rears 1-2 psi with the larger tire.

Only run 40 if you want to wear the middle of the tire early and have less grip.

 

[Assh]

How the heck is that gonna tell you what pressure to run?

Here's how I do it. Start with the Mustang placard tire pressure on a new set of tires. Measure tread depth all along the width of the tire, write that down. After some mileage, remeasure tread depth and then compare to what you wrote down. If the middle tread depth wear is greater than the outside worn tread depth then decrease the cold air pressure 1 or 2psi till the wear is the same across the width of the tire. The opposite is true if the outsides are wearing more more than the middle.

The tread depth gauge will tell you what your tire is doing quicker than your eyeball. The gauge will tell you if you need to align the vehicle and also if the tires need balancing. It tells you the trend of the tire.

 

[FruityJudy]

Here's how I do it. Start with the Mustang placard tire pressure on a new set of tires. Measure tread depth all along the width of the tire, write that down. After some mileage, remeasure tread depth and then compare to what you wrote down. If the middle tread depth wear is greater than the outside worn tread depth then decrease the cold air pressure 1 or 2psi till the wear is the same across the width of the tire. The opposite is true if the outsides are wearing more more than the middle.

The tread depth gauge will tell you what your tire is doing quicker than your eyeball. The gauge will tell you if you need to align the vehicle and also if the tires need balancing. It tells you the trend of the tire.

You could just draw a couple of chalk lines across the tire and roll the car enough to make a couple of rounds and see how the chalk line disappears

 

[WildHorse]

Here's how I do it. Start with the Mustang placard tire pressure on a new set of tires. Measure tread depth all along the width of the tire, write that down. After some mileage, remeasure tread depth and then compare to what you wrote down. If the middle tread depth wear is greater than the outside worn tread depth then decrease the cold air pressure 1 or 2psi till the wear is the same across the width of the tire. The opposite is true if the outsides are wearing more more than the middle.

The tread depth gauge will tell you what your tire is doing quicker than your eyeball. The gauge will tell you if you need to align the vehicle and also if the tires need balancing. It tells you the trend of the tire.

Wow. Thanks for that!

Seems no matter what the tire size is, the psi is all the same. For example:

2017 Ford Mustang Shelby GT350R Tire Pressure
Recommended tire pressure for 2017 Ford Mustang Shelby GT350R tires (1 original equipment tire size set).
Rear Tire Size 315/30R19
Recommended Rear Tire Pressure 32 psi (2.2 bar or 221 kPa)
Tire Size Type Metric
Tire Load Index 100
Tire Load Capacity 1764 lbs (800 kg)
Tire Speed Rating Y
Maximum Tire Speed 186 mph (300 km/h)
Tire Load Range SL (Standard Load)
Maximum Load Tire Pressure 36 psi @ 1764 lbs

So I guess I'll lower my psi from 33 to 32 cold.

 

[NightmareMoon]

Yup Ford does recommend the same pressure for pretty much every Factory tire you could possibly find on a Mustang.

Pressures range a lot while you’re driving, every time the weather changes etc, so don’t get too concerned about always being at a specific number. When racing I’ll definitely fine tune pressures and may run the rear tires at slightly different pressures than the front, but whatever.

New tires don’t all have exactly even tread on the inside versus the edges, so not sure how you could use ‘even tread’ to judge pressures on tires like that, so YMMV

 

[WildHorse]

Didn't Ford provide some psi recommendations when tracking the Mustang?

 

[Assh]

Yup Ford does recommend the same pressure for pretty much every Factory tire you could possibly find on a Mustang.


New tires don’t all have exactly even tread on the inside versus the edges, so not sure how you could use ‘even tread’ to judge pressures on tires like that, so YMMV

Yes, this is true, and by writing the depth down at the start you can keep that relationship as the tire wears, if you like.

I'm talking best/longest wear, not so much performance.

The placards and tables are all generic, not specific. Just like each tire is different, each tire manufacturer is different, and each Mustang is different. For me the tread depth gauge tells me what is trending on my specific tires, my specific Mustang and I adjust the pressure accordingly to get the best wear.

To much pressure and the tire wears in the middle and to little pressure, the tire wears on the outsides. The tread depth gauge tells me which direction to go with the tire pressure.

 

[Assh]

I just had a thought for you track guys and I realize track/carving corners is way different than mostly city or highway driving.

What if you track guys measure your tread depth at your standard track pressure, do your track day and then remeasure. You could then tweak your pressure and pickup some time maybe. Might be worth a try.

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