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I have an 2005 STS4 1-SG with the 18" wheels and Michelin HX MXM4 W rated tires, 235/50R18 fronts and 255/45R18 rears. Factory recommended tire pressures are 30 psi all around. Due (I guess) to the very low profile, tire heat gain results in pretty quick pressure increases up into the 34-35 psi range after a bit of normal driving, according to the Tire Pressure Monitor.
After about 15,000 miles I started noticing edge wear on the fronts, not so much on the rears. I also noticed that the front end felt a little mushy. To counteract the edge wear (probably too late) I bumped the fronts to 32 psi and noticed a marked handling improvement. I have started checking air pressure more often using a digital gauge (to the 1/10th psi) and also noticed that the Tire Pressure Monitor doesn't always agree. (And, yes, I always checked first thing in the A.M. after a slow 3/4 mile drive to the station.)
I'm now at 18,000 miles and have eased the pressure up to 33 psi front and 31 psi rear and it makes all the difference in handling. It is much crisper at the higher pressures. I think it's too late for the OEM tires. I doubt I'll see 25k.
Has anyone else had a similar experience and reacted more quickly than I and saved the tires? Has anyone else experimented with tire pressures with any success?
In terms of replacements, after the Formula 1 fiasco at Indy, I'm ready to chuck the Michelins (on principle) and go with Bridgestones, who appears to be the only other manufacturer with the same size/rating tire. Has anyone else tried these tires or know of any others?
PS: I have no interest in 20's or 22's or whatever with some showboat tire. I have enough faith in Cadillac engineering that the car was delivered with the right wheel/tire combination. Bling ain't my thing.
Any thoughts?
 

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I have the 1-SG also. The pressure at delivery was 35 on all 4 which dealer recommended. Ride was too harsh (My last car was Lexus LS430 and I am used to soft ride) so I reduced to 30. I love the ride at this pressure. But I am going to try 33-31 like you to prolong tread wear. I have a 3 year lease with 36000 miles allowed and that is what I expect to accumulate. I anticipate buying one set of tires during the lease. So I really don't care if all I get is 20000 miles out of the first set. In fact, if I get 20000 I will be surprised. Never got more than 20000 miles on the Lexus Michelin tires.
 

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i also have 1SG .... i currently have pressures all at 28 ... looking for smoother ride .... wife currently having cervical/spinal troubles ....

Goodyear also makes these sizes ... and they are getting surprising review at TireRack ...!!!??

i notice about a 2 psi difference (lower) from TPMS and my tire gauge ...??

hopefully they'll were out soon , so i can try something else ...
 

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I have a 01 STS and installed new tires about 6 months ago. I went to Tire Warehouse and they had a booklet on using N2 to inflate your tires. They say all the race car drivers use N2 because it will not jump up 5Psi during heatup/cooldown. Also they say the tires wear is better and the ride will improve. I laughed but felt why not try it. 7.95 a tire for life time refill as long as i own it. Funny thing is I like it tire pressure goes up aboy 2psi but a noticeable ride difference. Just my 2 cents worth of info..

Ketcha L8A Mike
 

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The factory specs are there for reasons only engineers know, but I am sure if you had one in front of you he could point out things until he was blue in the face.
Just like any vehicle you can play with the pressure until you get the "road feel" and driving characteristics that are suited to you.

Front tire edge wear is normal due to the design of the radial tire. The tires roll back and forth as you turn which really causes this. Newer radial tires are not as suceptible to edge wear due to inflation pressures unless they are quite low. The real problem is that those tires can't be rotated. Tire rotation makes the wear more even since you take the fronts to the rear and vice versa, they get a chance to wear nice and square on the back.
Without the benefit of tire rotation the tires will wear where they sit.
Trying a different tire may bring you some satisfaction, but playing with the tire pressures, if you have the time and the inclination, should help.

BTW, my impression of the tire monitors is that they are to help you know if a tire is going low, not to give you a hyper-accurate measure of the tire pressure. And let's not forget that the gage you use to check the pressures has to be "in calibration" also. Most retail stuff is not hyper-precise.
You'd have to buy a specialty gage, like the guys from NASCAR have, in order to be really accurate.
 

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That would be Nitrogen. Compressed air will expand/retrack more with heat & cold. Sorry.. Don't hit me to hard :thepan: Ketcha L8A Mike


Soggy Dollars said:
Hey Sharc....
What is N2

Soggy
 

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I, too, noticed terminal understeer at the factory-recommended 30 psi front and rear (06 STS V8 1SG with PHP). I put the fronts up to 32 psi cold and left the rears at 30 psi cold, giving me crisper turn-in and more neutral handling at the ragged edge. I'll try 33 front and 31 rear.
 

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I have 30 PSI in my Vogue Tires and I noticed what's stressed on the profile is that max approved is 35 . As for ride I am quite happy with them both soft and comfortable
 

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Can anyone explain to me why there are two different sized tires on my STS. I also have problems with the front tires. What problems would I have if I replaced all four tires with the same size?
 

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Discussion Starter #11
30 psi ate my front tires for lunch. I replaced them with the matching tire, but the same size as the rears (Michelin HX MXM4 P255/45ZR18 99W $285 plus add-ons!), bumped the front to 40 psi cold and the fronts to 34 psi cold. Handling is much more precise, but a little more harsh. I'm monitoring for center wear in case I need to lower the front pressure a bit.

I have All Wheel Drive, but checked the rolling diameter of the original front against the rears and it is the same.

I haven't tried it in Performance mode yet, but will this weekend at the YearOne Experience at Road Atlanta.
 

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In my opinion, front outer edge tire wear is accelerated by aggressive cornering. I aired up my tires yesterday to 35PSI on my 2006 STS V-8 and the car tire monitors registered 32,32,32,31PSI.
 

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you'll find the fronts wear faster mainly because we can't rotate due to the staggered tire sizes. I will replace the fronts twice to every rear change. anyone do anything different? I keep pressure around 31-32...
 

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30psi would give you the best ride comfort but I set mine at 32-33psi because I like a more firm ride anyway and better handling. I also have the PCZ option at 255 tires front and rear. Also, it is perfectly normal for the tires to increase at least 5 psi after they warm up in the summer time.
 

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I run 35 psi in the fronts and 30 psi in the rear. (All 4 are Michelin HX MXM4 P235/50/17) Maybe a little harsh on the ride, however wear and handling is good.
 

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Lower tire pressures don't necessarily hurt performance. You've heard the old adage 'horses for courses'? Tire pressures are the quickest and least expensive adjustment you can make to differences in road surface and handling requirements. (Next would be the tires themselves.) Lower pressures might decrease 'road feel' and make it easier to 'roll the tire over' onto the edge or even the sidewall, but it can also keep more of the tire in contact with the pavement, depending upon the exact conditions.

After 51,239 miles on a set of Conti DWS 235/50R18 97W's, my 2006 AWD 1SG is now wearing DWS 255/45ZR18 103Y XL. I wasn't down to the wear-bars and could easily have gone another Summer because regular tire rotation usually extends the life of tires. I'm still experimenting with pressures for the new tires, there's a lot more rubber with the XL option (extra load) and TiresDirect.net beat TireRack shipping to me by half.

Anyway, the old Conti 235/50R18 set seemed best for day-to-day use at 30 psi front and 29 psi back (TPMS cold). I had to be vigilant, alert for temperature swings, because a drop of more than 20 degrees could put pressures lower than I'd like to ever see. For that reason, I'd often run 31/30 or even 32/31. But around SE Wisconsin, in the Winter, potholes start to dominate the roads and the AWD w' MRC (like almost all 1SG's) rides firm. I chose the Conti 235/50 to mitigate that. I rarely went as high as 35 psi; Interstate long-distance, maybe.

The new set, 255/45, must have enough more to it that they just eat up those imperfections in the roads. 'Course it could just be the new tire affect. They feel great, look great & I want to fly!
 
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