Tire Pressure vs Tire Size

[WildHorse]

Can't find a definitive answer. Base GT, stock tire pressures for my 235/50ZR18 square was 32 PSI cold. Since upgrading to 19 x 8.5 front / 19 x 10 rear wheels with 255/40/ZR19's & 305/35/R19's respectively, do those tire pressures still apply for street application? TIA!

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[69mach1-395]

I like to run the rears a couple of psi less than the fronts for 2 reasons...more weight up front and don't want the wider rears to wear more in the center, but that's just me. 33 front, 31 rear

 

[Norm Peterson]

Can't find a definitive answer. Base GT, stock tire pressures for my 235/50ZR18 square was 32 PSI cold. Since upgrading to 19 x 8.5 front / 19 x 10 rear wheels with 255/40/ZR19's & 305/35/R19's respectively, do those tire pressures still apply for street application? TIA!

Not necessarily.

Your OE tires are Load Index 97 SL, and are rated for 1455 lbs @ 32 psi.
To match that with the 255/40-19's, which are typically 100 XL or possibly 96 SL, you'd need to run 33 psi.
To match that with the 305/35-19's, which are 102 SL, 32 psi is too high by several psi, probably by enough to have uneven tread wear issues (centers wearing faster than the shoulders). I'm uneasy about suggesting anything lower than 30 or possibly 29, though even those are higher than necessary for matching the rated load of the OE tires at OE placard pressure.

You may want or need to experiment a little from those pressures, same as you might from 32/32 if you were keeping with 97SL tires.

There are tables for the above situation where the new tires do not match the old tires in terms of Load Index, and they can be a bit confusing. A 97 SL is not the same as a 97 XL, and the 97 SL is actually the larger tire size.

You'll want to drive cautiously enough while you're learning any differences in handling and possibly adjusting pressures away from 33/30 or 33/29.


Norm

 

[Norm Peterson]

I like to run the rears a couple of psi less than the fronts for 2 reasons...more weight up front and don't want the wider rears to wear more in the center, but that's just me. 33 front, 31 rear

This works very well as long as the front and rear tires are the same size. Actually, I tend to run a slightly greater front vs rear pressure difference, more like 35/31 or 35/32 with 101XL tires (same load capacity as 97 SL tires at those pressures).


Norm

 

[69mach1-395]

Looks like one would need to know a few more things before settling on a final answer, like actual weights, particular tire loading tables and tire pressures at running temps, front vs. rear in all cases.

 

[Norm Peterson]

Even that only covers you for the actual load (this would probably be gross vehicle weight and axle loads on the psi placard) vs rated load @ spec psi, with some sort of factor to account for load transfers due to acceleration/braking/cornering.

Don't worry about rated load @ hot psi. It's going to be higher than the rated load @ cold psi, and the psi rise from normal driving is considered when the cold psi on the placard is posted.

Here's what parts of the SL and XL tables look like. The red lines are showing how you'd adjust inflation pressure when changing to tires with different Load Indexes or from SL to XL.

There may be understeer/oversteer handling balance considerations for the OE tires and inflation pressure causing them to be inflated to a slightly different value from what a table lookup based solely on GVW, front and rear axle weights, and that unknown safety factor would give.


Norm

 

[WildHorse]

@Norm Peterson rears are P305/35R19 94 S. The door jam decal says GVWR 4720 lb - GAWR FR 2335 lb / RR 2440 lb - MAXIMUM LOAD OCCUPANTS + LUGGAGE = 668 lb.
So because of my ignorance, what would be a good starting pressure be? Thanks.

 

[Norm Peterson]

That sounds like a LL (light load) tire, possibly something like an NT05R drag radial. Unfortunately, there does not seem to be any specific load vs inflation table for LL tires that everybody adheres to (I've asked an ex-tire engineer, who would have known). Best bet is probably going to be to contact the mfr directly.


Norm

 

[WildHorse]

Yeah it is NT05R's. The most useless tire ever except in a straight line. Time for a real set of DD's I think.

 

[69mach1-395]

For tire pressure at temp I was more concerned with starting too low and then getting too much change in pressure from cold to hot...

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