How to adjust tire pressure when plus-sizing?

rockthebeef · Joined: 01 Oct 2007 Posts: 371 Location: San Diego, CA

The stock 205/55/16 tires should be at 35/32 (front/rear) PSI.

Are there any general rules to getting the right pressure when going up a size? For example, 225/45/17, 225/40/18, etc.

Better yet, is there a formula I can apply to make sense of all this? Maybe I can figure something out but I just don' t know all the variables, so I'm stuck. Thanks.

IH8ONYU · Posted: 8/11/08 3:12PM Post subject:

yes there is a simple formula, read the recomended PSI on the new tire you want to use. They are always printed on the sidewall of the tire.

you can go to the tire manufacturer's web site to find "recomended" PSI for each tire, or ask your local tire shop, but with all the different rubber compounds out there and sidewall streangth, there is no standard calculation you can make.. Just dont fill to the "Max" pressure...

rockthebeef · Joined: 01 Oct 2007 Posts: 371 Location: San Diego, CA

Thanks. I understand what you're saying but those can only be guidelines, right? Don't we have to account for our cars' specific weight distribution? I guess I can just go proportionally from the stock pressure. Maybe to 38/35 front/rear on 17s.

Maybe the chalk test would work? You cover your tread with chalk and drive a tiny bit. If you've still got chalk on the outsides, then you're overinflated, etc. It would just take trial and error until you can get it even.

Maytag · Joined: 28 Jun 2007 Posts: 155 Location: Palmyra, PA

Guys, the indicated pressure on the tire sidewall represents that tires MAXIMUM recommended tire pressure, not what's ideal for your particular application. Most passenger car tires will be stamped with between 44 and 50 PSI on the sidewall.

IH8ONYU · Posted: 8/11/08 5:47PM Post subject:

i mainly adjust based on my comfort preference, never exceeding the max pressure of the tire. i generally like tire that doesn't feel like it drags, is excessively loud, or responds slugishly, i ususlly drive with only myself in the car, at times with 1 other passenger no heavy loads, etc.. i dont like a overinflated tire either, you can get a little better mpg (less contact patch = less friction?) but at the same time you have less contact patch = less traction for accelleration and breaking. i use the manufac suggested psi as a baseline and add / remove based on the way the tire responds.

rockthebeef - i hope i didn't come across as rude, i re-read my post and thought it could have been taken that way.
I do like your chalk test, though i have never heard of it before, it sounds like something i will be trying this weekend.
most manufacturers will list the recomended psi for their tires on their website usually in the "specs" section for each tire, but i dont think that recomended psi changes with tire size, stock 205/55/16s recomended 35/32 psi, my 225/35/18's call for the same recomended psi.....

rockthebeef · Joined: 01 Oct 2007 Posts: 371 Location: San Diego, CA

IH8ONYU wrote:

rockthebeef - i hope i didn't come across as rude, i re-read my post and thought it could have been taken that way.

Didn't take it that way at all. No worries.

Now for the tires, I guess it'll be a bit of trial and error before I get the pressures right.

Leo_EdgeRacing · Joined: 20 Nov 2007 Posts: 418 Location: Miami, FL

I have found that if you just set the PSI to what the sticker of your door sill says, 35 front/32 rear, you will be fine. I use to bump the pressure 2 psi for the fronts of my tC when I went up to 18's and 2 psi in the rear and I found out I had uneven tire wear. I then lowered it 1 psi in the front and rear and still got uneven wear. I then decided to stick to what Scion recommends for the 17's and it fixed the problem.

I say just start with what Scion recommends and then check your tires and go from there.

BBN-Boys · Joined: 04 Jul 2008 Posts: 262

Wow!If 1 psi can make that much difference :shock: ...... I should check my tires more often to see if the wear if even or not. I think I pump'em up to about 40 to 42 psi. I'm running 235/35-19 tires. Tires are rated at 50 psi max.

Leo_EdgeRacing · Joined: 20 Nov 2007 Posts: 418 Location: Miami, FL

BBN-Boys wrote: Wow!If 1 psi can make that much difference :shock: ...... I should check my tires more often to see if the wear if even or not. I think I pump'em up to about 40 to 42 psi. I'm running 235/35-19 tires. Tires are rated at 50 psi max.

That seems way too high. I would take one fron tire off and check the wear. You are probably wearing out the middle part of the tire faster than the edges.

roXor_boXor · Joined: 01 Dec 2004 Posts: 719 Location: Packerland

For the TRD 19's there is a new sticker that goes on the car in place of the stock one. As noted in this thread:
http://www.scionlife.com/forums/viewtopic.php?t=233114&highlight=psi
TRD Paul posted that it recommends
Front 39psi
Rear 37psi

I have never seen one of those stickers and have looked around the web for a pic of one without any luck. It would be nice if someone who has the TRD wheels could post a pic of the sticker on their car. If they were installed by the dealer correctly the sticker will list the 19's and the Scion recommended pressures for them.

FWIW I run the 39/35 psi in my 235/35/19's with no sign of abnormal tire wear.

BBN-Boys · Joined: 04 Jul 2008 Posts: 262

I noticed the Falken tire web site has recommended tire pressures for all their tires on specific vehicles. They even shows PSI when going up or down in wheel size. REALLY GREAT!!! Unfortunately, 2nd Gen XB is not listed.
Although, I think if you know of a vehicle that is very similar in design and weight as our XB, the recommended PSI would be close to what we should be running. I think this is better than guessing.....

Does anybody know of any vehicles that is very similar to our XB in weight, tire requirements, and size??

Here's the Falken Link:

http://ftcdev.innovapp.com/plusSize2/plusWizard2.cfm

:)

roXor_boXor · Joined: 01 Dec 2004 Posts: 719 Location: Packerland

This was on a Matrix.
Quote: Tire - Azenis ST115
Size - 235-35-19
Speed Rating -W
Load Index -
Front 91 @ 35 psi
Rear 91 @ 35 psi

Quote: Tire - Azenis ST115
Size - 225-35-19
Speed Rating -W
Load Index -
Front 88 @ 39 psi
Rear 88 @ 39 psi

Interesting that they recommend higher psi on the 225-35-19. :ponder:

BBN-Boys · Joined: 04 Jul 2008 Posts: 262

roXor_boXor wrote:
Interesting that they recommend higher psi on the 225-35-19. :ponder:

I think the higher PSI on the 225-35-19 is correct. Compared to the 235/35, it's smaller in diameter, section, and sidewall. Thus, it need more pressure to keep this tire safe. Take a look at your tiny spare tire. It's recommended PSI is probabaly very high also.

225/35-19
Tire diameter 641 mm (25.2'')
Section width 225 mm (8.9'')
Sidewall 79 mm (3.1'')

235/35-19
Tire diameter 647 mm (25.5'')
Section width 235 mm (9.3'')
Sidewall 82 mm (3.2'')

:)

roXor_boXor · Joined: 01 Dec 2004 Posts: 719 Location: Packerland

I guess I thinking more about how much higher. 4 psi difference is pretty significant for that little difference in tire size.

Regardless, I stick with the car manufacturer's recommendations myself. One reason I say that is that I cannot remember any front wheel drive Toyota product I have owned calling for the same psi front and rear. Always higher in the front due to the weight.