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Tire Pressure Recommendation Needed for New Wheel and Tire Set Up

Mr2raw

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Hey guys,

So I have a 2017 Gt California Special; on the door jam, it says the tire pressure for the stock tires 255/40/19 should be 32 PSI cold all around.

My New tires set up will be Michellin PS4S 285/35/19 front and 295/35/19 rear.
Should I follow the recommendation on the door jam and keep the new tires at 32 PSI cold? Or do I go with something different?

Appreciate the help
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MRGTX

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Maybe some folks will disagree but I think the door jamb numbers are still applicable regardless of tire size.

In any case, you might want to go up a couple PSI from the recommended numbers for better responsiveness (at the cost of slightly harsher ride) and adjust as necessary. If you find the front end to push in the turns, drop a couple PSI at the front. If the tail wants to come around too easily, drop a couple in the rear.

Either result is going to be hard to induce safely on the street...so maybe test this out in a big open parking lot or something... :)

After a few thousand miles, take a good look at your tire wear. Excessive wear in the middle usually indicates too much air pressure. Excessive wear on the outside edges/shoulders usually indicates insufficient pressure. Excessive wear on the rear tires and flecks of rubber on the bodywork aft of the rear tires indicates that you are a hooligan.
 

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anything between 28-34 will be fine. Start with say 30 and go drive normally - eg. your favorite canyon road. What does the TPMS say after 20-30 minutes of earnest driving? If it's over 35 maybe start with 32 next time and re-run experiment.
 
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Mr2raw

Mr2raw

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anything between 28-34 will be fine. Start with say 30 and go drive normally - eg. your favorite canyon road. What does the TPMS say after 20-30 minutes of earnest driving? If it's over 35 maybe start with 32 next time and re-run experiment.
Any specific reason as to why there is a limit on 35 PSI?
 
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Mr2raw

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Maybe some folks will disagree but I think the door jamb numbers are still applicable regardless of tire size.

In any case, you might want to go up a couple PSI from the recommended numbers for better responsiveness (at the cost of slightly harsher ride) and adjust as necessary. If you find the front end to push in the turns, drop a couple PSI at the front. If the tail wants to come around too easily, drop a couple in the rear.

Either result is going to be hard to induce safely on the street...so maybe test this out in a big open parking lot or something... :)

After a few thousand miles, take a good look at your tire wear. Excessive wear in the middle usually indicates too much air pressure. Excessive wear on the outside edges/shoulders usually indicates insufficient pressure. Excessive wear on the rear tires and flecks of rubber on the bodywork aft of the rear tires indicates that you are a hooligan.
So more PSI = harsher ride but more responsive in what way?
So less PSI = better cornering but is there a drawback?
Lastly, which will cost easier reduction in treadwear? over or under recommended PSI?
 

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shogun32

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Any specific reason as to why there is a limit on 35 PSI?
35/30=1.16, 16% change. Unless you're flogging the car at a track, 10% delta is the general rule of thumb for street work.
 

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So more PSI = harsher ride but more responsive in what way?
So less PSI = better cornering but is there a drawback?
air supports the sidewall. more air = less rolling resistance due to smaller contact patch, stiffer side-wall = faster response to turn - tire doesn't roll over as much. But less outright traction possible because of smaller contact patch.

less air is opposite.

That said, tire construction varies wildly between brands and models - eg. Bridgestone tend to have very firm side-walls and Continentals are very soft.
Lastly, which will cost easier reduction in treadwear? over or under recommended PSI?
Under-inflated is as a rule is considerably worse than over-inflated but there are limits to everything. At STP if your cold pressure is 32 +/- 3 you're "just fine".

A particular Pirelli PZero might feel "best" at 30psi but the Conti Extreme SPort might work/feel better at 33. Experiment. Or just set it to 32 and 'forgedda-bout it'.
 
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air supports the sidewall. more air = less rolling resistance due to smaller contact path, stiffer side-wall = faster response to turn - tire doesn't roll over as much. But less outright traction possible because of smaller contact patch.

less air is opposite.

That said, tire construction varies wildly between brands and models - eg. Bridgestone tend to have very firm side-walls and Continentals are very soft.
Under-inflated is as a rule is considerably worse than over-inflated but there are limits to everything. At STP if your cold pressure is 32 +/- 3 you're "just fine".

A particular Pirelli PZero might feel "best" at 30psi but the Conti Extreme SPort might work/feel better at 33. Experiment.
thanks for the insight!
 

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air supports the sidewall. more air = less rolling resistance due to smaller contact patch, stiffer side-wall = faster response to turn - tire doesn't roll over as much. But less outright traction possible because of smaller contact patch.

less air is opposite.

That said, tire construction varies wildly between brands and models - eg. Bridgestone tend to have very firm side-walls and Continentals are very soft.
Under-inflated is as a rule is considerably worse than over-inflated but there are limits to everything. At STP if your cold pressure is 32 +/- 3 you're "just fine".

A particular Pirelli PZero might feel "best" at 30psi but the Conti Extreme SPort might work/feel better at 33. Experiment. Or just set it to 32 and 'forgedda-bout it'.
Thanks. You beat me to it. :)

I’m not a tire engineer so I don’t know everything but all of what you said matches my understanding and experience.
 

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Hey guys,

So I have a 2017 Gt California Special; on the door jam, it says the tire pressure for the stock tires 255/40/19 should be 32 PSI cold all around.

My New tires set up will be Michellin PS4S 285/35/19 front and 295/35/19 rear.
Should I follow the recommendation on the door jam and keep the new tires at 32 PSI cold? Or do I go with something different?

Appreciate the help
Ignoring the speed rating letters, your OE tires are load index 96 in SL (standard load table applies). That's good for 1411 lbs @ 32 psi. You shouldn't run at a lower load rating than 1411 lbs without evaluating your actual vehicle GVW as driven. Note that 96 in SL compares to 100W in XL (most people aren't aware of the roughly 4 load index offset between the SL and XL ratings, let alone that XL is the higher number at the same load and inflation pressure).

285/35-19 in PS4S is load index 103 in XL
295/35-19 in PS4S is load index 104 in XL

From a load capacity point of view (that 1411 lbs thing), you could run the PS4S tires as low as 29 psi up front and 28 psi out back. So you need to be running at least that much. That said, turn-in response and steering precision may feel "softer" or less precise even though ultimate cornering grip might go up a little.

Are you going to be putting the PS4S tires on wheels the same widths as your OE wheels, or are you also going to be going with wider wheels? If wider, I'm curious as to how wide.

FWIW, I tend to run rear tires at 3 or 4 psi less than the front tires for handling reasons, and the fronts at slightly higher than the pressure needed for load rating reasons. Wheel width matters aside, I'd probably run your tire choices at something like 32 or 33 up front and 29-ish out back.


Norm
 
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Mr2raw

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Ignoring the speed rating letters, your OE tires are load index 96 in SL (standard load table applies). That's good for 1411 lbs @ 32 psi. You shouldn't run at a lower load rating than 1411 lbs without evaluating your actual vehicle GVW as driven. Note that 96 in SL compares to 100W in XL (most people aren't aware of the roughly 4 load index offset between the SL and XL ratings, let alone that XL is the higher number at the same load and inflation pressure).

285/35-19 in PS4S is load index 103 in XL
295/35-19 in PS4S is load index 104 in XL

From a load capacity point of view (that 1411 lbs thing), you could run the PS4S tires as low as 29 psi up front and 28 psi out back. So you need to be running at least that much. That said, turn-in response and steering precision may feel "softer" or less precise even though ultimate cornering grip might go up a little.

Are you going to be putting the PS4S tires on wheels the same widths as your OE wheels, or are you also going to be going with wider wheels? If wider, I'm curious as to how wide.

FWIW, I tend to run rear tires at 3 or 4 psi less than the front tires for handling reasons, and the fronts at slightly higher than the pressure needed for load rating reasons. Wheel width matters aside, I'd probably run your tire choices at something like 32 or 33 up front and 29-ish out back.


Norm
They will go on 19x10 front. 19x11 rear. Would have gotten 305 but ps4s not avaliable in 305/35/19
 

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They will go on 19x10 front. 19x11 rear. Would have gotten 305 but ps4s not avaliable in 305/35/19
Good. "Measuring width" up front and half an inch wider then measuring width out back is a decent place to be.


Norm
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