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Tire pressure for up-sized wheels

robwlf

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im still having good reaction and grip running 32 psi cold.. they heat up to about 34 or 35 on a 90 degree day .. highway they stay pretty much 34 ..so ill stick with that
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Sound like you're at where my calculations say we should be, more or less.
Atm I run 32 cold and as the next person say they heat up to 33-34 on a warm day. Might lower the pressure some but it feels good at present pressure.

Depth gauge might be good to buy just to ensure everything is good. I have a little too much negative camber on rear right (-2.0) but other than that the alignment is spot on.

Thanks for the feedback!

Buy a tread depth gauge, check your tire tread depth regularly across the width and adjust pressure accordingly.

275/35/20 F=30 psi, 305/35/20 R=27psi seem to be the normal setting for these tires on my Mustang GT. G-Force comp-2 A/S BF Goodrich.

I will adjust the psi, up or down, depending on tread depth wear across the width of the tire. I have gone as low as 29psi on the fronts and 26psi on the rears to get the tread depth to equalize across the width.

These tires have 10k miles on them and the tread depth across the width of the tire are within .5mm of each other.

Also by doing this you can tell quickly if your alignment is correct/incorrect.

This seems to work for me, I'm no expert.
 

Rash

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Seems to me I remember reading on this board somewhere that the factory recommended tire pressure for the 20 inch option was 35.
 
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What I've heard they recommend 32 on all versions but I don't know. What I do know however, is that manufacturers doesn't recommend pressure for traction / performance but rather comfort / fuel economy and emissions...

My calculations is based of the stock 32 psi recommendation, not for the actual weight of the car that I don't have any figures for at the moment. That gives that for a given weight (real or not real) the pressure shall be lower with a tyre with higher load index as for my previous examples regardless the size. 35 psi is therefore high any way you look at it.

Seems to me I remember reading on this board somewhere that the factory recommended tire pressure for the 20 inch option was 35.
 

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Here ya go: https://www.mustang6g.com/forums/threads/tire-pressure-for-conti-dws-06-20.98239/#post-2151239

If factory recommends it, I doubt it's too high.

I run my Cont DW06 275/35/20 at 35 cold. I can tell when it drops below 33 because the car feels squirrelly. Best I can tell tread wear is fine.

Maybe the calculator assumes equal aspect ratio. If aspect ratio is smaller, more PSI would be needed to provide equal load bearing.
I don't say its too high for everything but as i wrote earlier, Ford's only worry is to provide a car that ticks the boxes for gas mileage/exhaust limits and will of course have to compensate for any non-standard spec car by inflating the tires more. No manufacturer gives tire specs for best grip / least wear. Today, emissions are everything and running on stock spec will many times give premature tire wear, but that's not the manufacturers problem, it's the customers when he realizes that the good gas mileage comes at a cost.
It's a known fact.

Also, 20" doesn't say much unless you get the full specs on the tire size. I've got 305/35 in the rear and 275/35 up front with load index 102 XL front and 104 in the back, what are the 20" edition specs?
On top of that, I run Pirelli Zero's (EU) and not Continental...

As Assh said, he runs even lower PSI than I do with even wear. I'm not saying the car won't run fine with 35 psi, I reach 34 warm and it feels good, however at 31-32 it sticks to the ground like glue without being soggy. I've got the lowering springs too and BMR kit in the rear so it's stable as h*ll. Just by the looks of it, the contact surface is right on point, but I will buy a gauge to double check.
 

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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?
According to tiresize.com it seems you have to compare oem tire size, load rating, and psi from on the door jam, with the new tire size and load rating. The tool on their site spits out the calculated psi you should be running to come as close as possible to supporting the vehicle's weight load. I punched in the oem numbers for the 19s @ 32psi:

OEM Size (255/40R19 XL) Load Capacity: 1418 lbs. @ 32 psi.

New Size (275/35R20 XL) Load Capacity: 1431 lbs. @ 30 psi.


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?
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?

Punched in on tiresize.com here are the scientifically correct psi's to run:

Front OEM Size (255/40R19 XL) Load Capacity: 1418 lbs. @ 32 psi. New Size (275/35R20 XL) Load Capacity: 1431 lbs. @ 30 psi.
Rear OEM Size (275/40R19 SL) Load Capacity: 1647 lbs. @ 32 psi. New Size (305/35R20 SL) Load Capacity: 1660 lbs. @ 29 psi.

My setup is Nitto 555 G2's sizes:
F 275/30/20
R 285/30/20

I bought the car this way and only recently dawned on me that these tires may take a different psi than the oem sizes. I punched my numbers in and got
F psi - 36
R psi - 41
Those numbers seemed too high to me so I filled F32/R34, looks good, feels good, only more miles will tell me the tread's wear pattern, and if this tiresize calculator is real-world-helpful.

Meantime hope this tiresize.com tool is helpful, cheers
 
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According to tiresize.com it seems you have to compare oem tire size, load rating, and psi from on the door jam, with the new tire size and load rating. The tool on their site spits out the calculated psi you should be running to come as close as possible to supporting the vehicle's weight load. I punched in the oem numbers for the 19s @ 32psi:

OEM Size (255/40R19 XL) Load Capacity: 1418 lbs. @ 32 psi.

New Size (275/35R20 XL) Load Capacity: 1431 lbs. @ 30 psi.






Punched in on tiresize.com here are the scientifically correct psi's to run:

Front OEM Size (255/40R19 XL) Load Capacity: 1418 lbs. @ 32 psi. New Size (275/35R20 XL) Load Capacity: 1431 lbs. @ 30 psi.
Rear OEM Size (275/40R19 SL) Load Capacity: 1647 lbs. @ 32 psi. New Size (305/35R20 SL) Load Capacity: 1660 lbs. @ 29 psi.

My setup is Nitto 555 G2's sizes:
F 275/30/20
R 285/30/20

I bought the car this way and only recently dawned on me that these tires may take a different psi than the oem sizes. I punched my numbers in and got
F psi - 36
R psi - 41
Those numbers seemed too high to me so I filled F32/R34, looks good, feels good, only more miles will tell me the tread's wear pattern, and if this tiresize calculator is real-world-helpful.

Meantime hope this tiresize.com tool is helpful, cheers
I'm still running 32 psi cold and got great grip and handling as well as completely even wear so seems to be the sweet spot.
 

mustanghammer

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Interesting thread and frankly I have never paid much attention to load ratings on tires. In my experience, tires will tell you what they want with respect to tire pressure from a ride and handling perspective. Also that is not possible to ruin a street tire in a few hundred or even a thousand miles while trying to figure this out.

The "correct" tire pressure is determined by the construction of the tire and the car it is being installed on. Load specs while valuable are essentially created in a vacuum because the tire manufacturer doesn't know what car the tires will be mounted on.

Changes to the car can also dictate tire pressure. Fill the trunk for a vacation and it becomes necessary to add air to the rear tires. Change springs, sway bars, etc and tire pressure requirements change because the car is working the tires differently. It is possible that the optimum pressure is different front to back. All of my race cars have had different front and rear pressures even though the tires were the same front to back. This is true even on my RX7 and it has a perfect 50/50 weight balance.

How the car is being used will determine tire pressure settings. For example, running a car in an autox on street tires will typically require cold pressures to go up to keep the tire sidewalls from rolling over excessively. A spirited drive in the country can do the same thing because the car is being worked harder. In a competition environment, air pressure changes can be used to "fix" handling issues too.

The good news is that that the tire pressure monitors in a S550 is pretty freaking accurate. So it is possible to watch pressures go up and down as you drive the car. Combine this with a tire pyrometer (I like the prob style) and this sort of thing can be determined pretty fast.
 

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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?
Holy haha. Fill your tires to 32 PSI, or whatever euro specs call for and call it a day. As another poster stated, this ain't rocket science. But if you wanna turn it into rocket science continue on. I run 32 psi on my fronts (245/45/R19) and 32 psi on my rears (325/30/R19) have have noticed ZERO unusual wear.
 

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Holy haha. Fill your tires to 32 PSI, or whatever euro specs call for and call it a day. As another poster stated, this ain't rocket science. But if you wanna turn it into rocket science continue on. I run 32 psi on my fronts (245/45/R19) and 32 psi on my rears (325/30/R19) have have noticed ZERO unusual wear.
Good for you - no-one asked.
 

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I saw the forum and became curious as well. I started looking around and found this tire pressure calculator online. The idea is the same but the results make more sense than the chart in the first page. My stock setup was 235/50R18 (97w). Now I’m running 275/35/20(102w) squared as you can see the recommended air pressure is 31 psi. As it states in the calculator you’d need to accurately pick the specs of your tires.
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