Tire pressure for up-sized wheels

[Sleipner]

Hi,

Have now had my new Project 6gr Sevens 20x11 + 20x10 lying around for more than a month and the new set of Pirelli Zeros too. Have been waiting for a set of TPMS that never showed up, and will finally get a set so I can mount the wheels so the next question is: Which tire pressure should I have?

New tires are 275/35R20 (102Y) front and 305/35R20 (104Y) rear.

Stock tires have a load index of 96Y up front and 101Y on the rear.

According to a standard table, using 32 PSI on both rear and front on the stock tires, it should translate to 26 PSI on the new front tires (1415 lbs) and 28-29 PSI (1647 lbs) on the rear tires.

Am I correct?

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[Strokerswild]

Interested in responses on this one....

I'm running stock pressures in 275/305-19s and wonder if it's proper. I don't see any odd wear, but....

 

[Sleipner]

I used this table but maybe people have more knowledge and have more useful info!

I don't think 1 or 2 PSI extra should give an extreme amount of extra wear, but my experience is that the ride will definitely be more bumpy and the grip suffers quite quickly from over-inflation.

 

[hlfbkd420]

How long did it take for your rims to show up?

 

[Sleipner]

How long did it take for your rims to show up?

More or less one week with delivery to Sweden.
Great service from Need4speed motorsports even though they forgot my lug nuts which cost me 70 bucks extra...

 

[GTthree50]

Hi,

Have now had my new Project 6gr Sevens 20x11 + 20x10 lying around for more than a month and the new set of Pirelli Zeros too. Have been waiting for a set of TPMS that never showed up, and will finally get a set so I can mount the wheels so the next question is: Which tire pressure should I have?

New tires are 275/35R20 (102Y) front and 305/35R20 (104Y) rear.

Stock tires have a load index of 96Y up front and 101Y on the rear.

According to a standard table, using 32 PSI on both rear and front on the stock tires, it should translate to 26 PSI on the new front tires (1415 lbs) and 28-29 PSI (1647 lbs) on the rear tires.

Am I correct?

I won’t say correct or not but the pressures you quote are fairly low. AFAIK the tire itself not the wheel size will be the determining factor for pressure, maximum pressure and load are listed on the tire sidewall. I have 20x11 F/R with 305-30/20 and run them about 10 pounds higher.

 

[Sleipner]

I won’t say correct or not but the pressures you quote are fairly low. AFAIK the tire itself not the wheel size will be the determining factor for pressure, maximum pressure and load are listed on the tire sidewall. I have 20x11 F/R with 305-30/20 and run them about 10 pounds higher.

I'm no tire expert but what you said is exactly what I've done so far

That is why I referred to the load index on the stock and the new tires when picking the pressure from the table that is based on exactly that. Load and load index. I based the load on what the corresponding pressure for the stock tires load index shows. It would help if you knew the correct load on each wheel as I don't have the car or manual available atm.

That is: The pressures indicated by the drivers door is 32 PSI all around (2.3 bar). That pressure, together with the load index of the stock tires indicate a load (xxx lbs) on the wheel.
This load is then found (or as close as possible) in the column for the new tires load index and from there I read the new (lower) pressures at the top of the diagram.

The size and width of the tire should matter IMO as a wider tire will become over or underinflated easier as the distance from the center to the sidewall is longer. At least that's what I assume, so I'm keen to get the correct pressure to save some rubber.

 

[VinnAY]

Youre making this horridly complicated...fill to 32psi and look at the tread wear across the tread, that will tell you where you're at. This trying to be scientific about load ratings and just nonsense.

 

[Sleipner]

Youre making this horridly complicated...fill to 32psi and look at the tread wear across the tread, that will tell you where you're at. This trying to be scientific about load ratings and just nonsense.

Yeah that's a great way of ruining a perfect set of tires... Thanks for the tip - not.
Do you fill all your tires on all your vehicles with 32 psi? Why not fill them to 40 PSI and check the thread wear?

It's not like parts of the tire is completely untouched after the first few yards... One way is to check the temperature over the surface to see if it's even. Or maybe, just maybe, fill them with the correct pressure from the beginning.... It's not magic or rocket science, it's just common sense.

The scientific nonsense with load rating is exactly what load rating is all about. Correct pressure and load capacity for a specific load.

 

[NightmareMoon]

Wider tires should probably run slightly lower pressures, but I don’t think you can trust that table. 26psi sounds very low to me. 30 psi maybe, but 26? I’ve never seen a mustang on wide tires run that low outside a drag racing scenario.

If you want to test pressures, look for sidewall wear after mild cornering (signs pressure is too low) and/or go get a laser thermometer and check temperatures across the tread immediately after driving ( before temps have a chance to even out). The center of the tread should be the same temps as the inside and outside tread regions if pressure is right.

 

[Sleipner]

Wider tires should probably run slightly lower pressures, but I don’t think you can trust that table. 26psi sounds very low to me. 30 psi maybe, but 26? I’ve never seen a mustang on wide tires run that low outside a drag racing scenario.

If you want to test pressures, look for sidewall wear after mild cornering (signs pressure is too low) and/or go get a laser thermometer and check temperatures across the tread immediately after driving ( before temps have a chance to even out). The center of the tread should be the same temps as the inside and outside tread regions if pressure is right.

Thanks for your input.
Yeah for me it seems logical that the pressure should be lower as the surface is bigger, but as you say, 26 psi is quite low.
Thermometer is an old classic and might just be the way to go...
Was looking for some more knowledge as I assume a standard table is just as good as it gets but 30 might be a good starting point.

Anyone else who drives the same setup with some input?

 

[GTthree50]

Here is another way to think about it. For a plus size tire, to maintain rolling diameter you generally go wider. this has the effect of having nearly the same volume as the factory size internally so pressure will remain constant.

 

[Sleipner]

Here is another way to think about it. For a plus size tire, to maintain rolling diameter you generally go wider. this has the effect of having nearly the same volume as the factory size internally so pressure will remain constant.

If you keep one value constant (diameter) and increase another (width) the volume will increase. That’s basic geometry. Also, volume is not connected to pressure unless you decrease the volume surrounding a fixed mass of air. This is where the load index comes in play.

 

[GTthree50]

If you keep one value constant (diameter) and increase another (width) the volume will increase. That’s basic geometry. Also, volume is not connected to pressure unless you decrease the volume surrounding a fixed mass of air. This is where the load index comes in play.

This thread is all in reference to plus sizing so in this case there wouldn't be an appreciable difference in volume.

 

[Sleipner]

This thread is all in reference to plus sizing so in this case there wouldn't be an appreciable difference in volume.

Since I started the thread, I know what it is about, thank you.

You claimed that the internal volume of a tire doesn't change as long as you keep the circumference of the tire the same. That is not true as the volume will differ between a 275-285-295-305 wide tire with the same circumference - again, that is basic geometry.

My question in the start was if the standard chart I posted is correct (the pressure should atleast be lower than stock, both according to the chart and according to common sence) but since the tires load index is a factor, and therefore also the internal construction of the tire might differ, and I'm not an engineer at Pirelli, I wan't insight from people who either run the same setup without uneven wear or other issues, or from people who actually know what they are talking about.
For some mysterious reason, my setup might even need HIGHER pressure than stock. I doubt it, but that's why I ask.

The reason our cars has 32 PSI both at the front and the rear is that Ford have picked tires with one certain size and load index, paired with a certain pressure which corresponds to the actual load from the weight of the car. I have driven many cars that are specified with different pressure on the front vs the rear too, so to "just fill 32 psi and be happy" is not a satisfying answer in my book.

"Learning by burning" is not the way I do things, neither at work as an engineer, nor with my personal belongings even though other people seem to enjoy destroying various things instead of reading the manual first...

I'm not trying to be rude, I just want proper answers to my initial question.

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