jkozlow3

Ok, I'm switching from the stock 215/50/17 MXM4's to the new Michelin Pilot Sport A/S in size 225/50/17.

The stock tires have a load index of 93 XL and the new tires have an index of 94 SL.

I need to know what pressure to run the new tires at, and I'm receiving conflicting advice from everyone. I called several tire manufacturers/retailers trying to get some good advice and they're all coming up short.

For example, Michelin said I should decrease pressure to 27-28 PSI, TireRack wondered why I'd change the pressure at all, Yokohoma told me to decrease pressure to 31 PSI and Goodyear didn't even seem to know what the hell I was talking about.

Please help!!

daverman

Why would you change the tire pressure at all? At first thought, I'd retain the stock tire pressure.

jkozlow3

I was just under the impression that the recommended tire pressure was only applicable for the stock size and that it needed to be altered when changing sizes. Am I wrong?

EricL

Start with the stock pressures, then you could always monitor the condition of the tread (with time).

Some tires (like mine -- Toyo T1S 235/45-17) want more pressure than stock. I run 40 front / 39 rear -- the tire dealer set the tires up with 40/40 and I talked to a few Toyo engineers to confirm this. (These tires have a 50psi max pressure rating and a load rating of 97W)

You will get a million and one replies on this issue -- sorry.


BTW -- The new Michelin A/S you're running are 94 SL AND the 94 load rating is at 51 PSI !!!! The stock tires supported the 93-load rating at 35 PSI, with a max pressure around 40 PSI. Something you might want to consider...

jkozlow3

The max pressure on the stock tires is 41 vs. 51 on the Pilot Sport A/S. I was under the impression that the load rating was always calculated at the max pressure. If not, how do you know that the load rating for the MXM4's was calculated at 35?

That would mean I should inflate the tires higher than 32 PSI in order to maintain a similar load rating, right? 39-40 should do it, wouldn't you think?

Also, what are the advantages of running your tires at 1 lb. less in the rear? Thanks.

chrism7855

so is it bad to run at 40psi in the front? cause i have the stock tires. and well. if not at 40psi.. i was running at like 35. they just look flat to me. so i pushed it up to 40psi. any drawback in that?

Edward'TLS

Your ride will be bumpy like hell, especially on the sport-tuned TL-S.

jkozlow3

I just read this on the Tirerack:

"Note: 35 psi is the assigned "maximum load" pressure for standard load tires and 41 psi is the assigned "maximum load" pressure for extra load tires."

The stock tires are extra load (XL) tires, so the max load of 1433 was determined at 41 PSI. My new tires are standard load (SL) tires, so the maximum load of 1477 was determined at 35 PSI.

Which means that my new tires can handle more load than the stock tires at a lower inflation pressure. So it looks like the load is one thing I won't have to worry about. I'm still not sure what the optimal pressure is gonna be for ideal ride/handling/tread wear, but at least I won't have to worry about the load!!

Any more thoughts on optimal pressure guys?

EricL

The load rating is calculated at max pressure for "most" tires (SL rated tires). However a "P-Metric" XL rated tire has its load rating (93 in the case of the 215/50-17 MXM4) calculated at 35 PSI (not at its max pressure of 41 PSI)!

So, I didn't mean that this was the only issue or parameter to use in determining what pressure you should use.

I said to start with the "stock" 32 PSI (make sure it is dead cold -- you would be surprised how much the pressure will go up after a few miles of driving).

Bump up the pressure as needed (or to suite your ride). You may also discover more information from Michelin and/or other engineers "in the know" about your tires.

I wanted to make sure you understood that the 93 XL rating of the stock tire is obtained at 35 PSI (I'm going to include a link to a .pdf file -- ok?)

In the case of my Toyos (XL rated, not P-metrics, with 50 PSI max pressure), I use 40 front and 39 back. You should understand that I have experimented with the pressures, called the Toyo engineers, and have seen other Toyo T1S Proxies (on the CLS in the same 235/45-17) that were driven with 32-35 PSI (and suffered from signs of under-inflation). I can tell you what to do with your stock tires and Toyos – but, you will probably have to “mess around” with your Pilot A/Vs to get the best results…


LINK to Michelin MXM4 spec page: http://tires.michelin-us.com/assets/..._pilotmxm4.pdf

Check out the row with: P215/50R17 XL 93V 1433@35PSI


The following link (look down the page) explains how the P-Metric XL vs. SL ratings work. If you look carefully, you will see that P-Metric tires support their rated load at only 35 PSI, but have a max pressure that is 41 PSI (and support even more load at that higher pressure).

http://www.tirerack.com/tires/tirete...especskey.html


Here is the link to the Pilot A/V – you can see that it is load rated at 51 PSI (unlike the stock tire):

http://tires.michelin-us.com/assets/...lotsportas.pdf

EricL

You need to read what I posted...

The XL (stock tires) support the full load at 35PSI NOT 41 PSI.

YOUR TIRES NEED TO HAVE 51 PSI IN THEM TO SUPPORT THAT 94 LOAD RATING!!!!!!

So, your tires WILL NOT SUPPORT the same weight at 32 PSI as the stock tires.

Please read the post I placed for you above...

EricL

Yea, it's called lack of headroom!

The tires support a max pressure of 41 PSI (the stock MXM4s). IMO -- running the tires at the absolute max cold inflation pressure is not the best idea. Try dropping them down a bit. The car is very heavy in front and even my 97 XL rated tires look a bit round in the front (and they are made for 50 max PSI and have a reinforced casing)...

jkozlow3

WOW!! Thanks a lot EricL.

I didn't realize that Michelin used such different pressures to calculate the max load. It doesn't seem right that they can do that.

Now I'm worried that my new tires won't be able to support the load unless I pump them up to 40+ PSI. I don't really want to do that however, because the ride is a little stiff for my taste at 40 PSI.

I wonder how necessary the high load rating is on the stock tires. I mean, does our car need a tire with that high of a load rating, or is that just the load rating that the tire happens to have? I'd really like to run these tires around 32-35 PSI if possible.

Also, Michelin just returned an e-mail I sent them 2 days ago asking them what the new inflation pressure should be with the Pilot Sport A/S and they told me 30 PSI front & rear. That's funny, cause the guy I talked to on the phone at Michelin earlier today told me 27-28 PSI. Both numbers seem too low now that I know about the "true" difference in load rating.

What a headache!!

EricL

Try the 32-35PSI to start with and see how it goes…

Sometimes if you "suspect" that the data your getting is "funny" (I'm not saying it is), you might want to try to talk to an engineer and explain how the car weighs over 3500 lbs, with more than 60% of the weight on the front, and mention the 93 XL rated tires the factory put on (the MXM4s).

Hmmm... Perhaps you could make 5 calls to them and use a 3 out of 5 voting system (just kidding)

bhall01

One thing I think you should consider is what Acura wants as well as what the tire manufacturer says is correct. God forbid you crash and the tire is at fault you will have to prove that you know the correct tire pressure per the manufacturer. Besides knowing what is correct start at stock use your butt and see how it feels. Most folks will increase tire pressure slightly on the track and will reduce pressure slightly when desiring comfort or traction on unstable surfaces, I.E. sand or something like that.

jkozlow3

Ok, well I FINALLY got this whole mess straightened out with Michelin.

It turns out that the load rating for the Pilot Sport A/S was NOT determined at 51 PSI as their website states. The load rating was in fact determined at 35 PSI which is the same pressure that the MXM4's load rating was determined at.

Since the Pilot Sport A/S has a load rating of 94 and the MXM4's have a load rating of 93, I could potentially run the Sports at a lower air pressure. 29-30 is the minimum air pressure Michelin recommended and suggested starting at 30 and going up from there until I found the ideal combination of comfort/handling/wear.