lex430

this may sound like a stupid question.., but as the subject suggests?

I recently got some 18's and I have couple of questions:

1 the shop told me I need to keep 36 psi in all the tires. I learned a while back to measure your tires a little warm (less than 2 miles of driving) to obtain the reading. Is that correct?

2 I have one of the pressure readers that pops out at one end, how do you take readings with it?? do you read the pressure while it stays in contact with the tire stem (is that what its call)? Or the reading that you get when you pull the pressure reading stick away from the stem after you take the measurement?

3 I tried both ways (measure cold/warm) above, when my tires are cold, and I keep the measure on the tire, it reads 40-42, and if I pull the measure off the stem, then the reading is at ~35, so now I am not sure if I need to let air out of the tire, or put more in! help please!

bitkahuna

iTrader: (20)

I measure when cold. It's OIL you're supposed to measure when the car's warmed up a bit.

As far as your pressure guage, you're supposed to be able to take it off the stem and the reading should be right. I know the kind you mean, they're pretty cheesy. Sounds like your one is defective.

damnboy

I always measure my tires when they're cold for consistency and I keep their pressure to 36 psi.
Try get a digital pressure gauge for easy reading.

lex430

you're kidding right, bit?? I thought oil you HAVE to measure when cold!

back to the dealer

RON430

Well this is interesting. Normally, tire manufacturers give pressure readings for warm tires. From cold, a tire will pick up anything up to a couple of pounds pressure depending on a lot of things. I don't think there is too much problem measuring cold as most street tires are being set more for consistency than anything. If you autocross you are used to chasing tire pressures. Long times between runs let them cool down and during the run you heat up quickly, easier to gauge a full race car. Might want to check cold and warm just to see the impact of the temperature and how you like the ride. I don't think I have seen engine oil level change much with temperature. Now automatic transmission has to be checked hot and a few other levels, like coolant and power steering, are usually marked for both cold and hot ranges but I am not sure engine oil levels change all that much. Problem measuring engine oil hot is that you have to wait for the oil to drain back to the pan to get an accurate reading. If you don't wait for it to drain back, you think that when the engine gets hot the oil level drops, it doesn't, it is just that some of the oil is in the engine and hasn't drained back to the sump. Cold level, or several minutes after a slight warmpu, on engine oil is probably the best time to check.

itsallgood

Just kick it with your big toe, if your toe bounces off then its good to go. I learned this from watching HUDINI check his tires

kevs

Hey, I've got a GS4 with 235/35/18 and have no idea what the ideal tire pressure should be. Does anyone have a similar setup or know a site with a tire pressure calculator? Thanks!

bitkahuna

iTrader: (20)

That's a small diameter tire!!!! Must look very small in the wheel well. Probably need 40lbs!

garnet92

Tire pressure must be checked when tire is cold (not driven, even a mile).

From the Toyo tire website:
“Tires must be checked when they are cold (before they have run a mile). If you must drive over a mile for air, measure and record the underinflation amount of each tire. Upon arriving at the service station, measure each tire's inflation again and if the pressure has increased, adjust the amount of additional air pressure needed. For example, if cold pressure should be 35PSI, cold pressure was 28PSI, and current pressure is 33PSI, you should inflate the warm tires to 40PSI and recheck them again when cold.”

From Bridgestone/Firestone:
“For accuracy, check your air pressure with a tire gauge when tires are cold. Driving heats up tires and makes the reading incorrect.”

From Rubber Manufacturers Assn. (linked from Yokohama website):
“When you check the air pressure, make sure the tires are cool - meaning they are not hot from driving even a mile. (NOTE: If you have to drive a distance to get air, check and record the tire pressure first and add the appropriate air pressure when you get to the pump. It is normal for tires to heat up and the air pressure inside to go up as you drive. Never "bleed" or reduce air pressure when tires are hot.)”

RON430

OK, so what they are saying is that all of the factory recommended pressures are cold.

miko400

I was also told to always check when the tires are cold. You will not get an accurate reading after running the car.. SO ONLY WHEN COLD...

lex430

thanks for all the replies, now question is this, I went out and got a digital tire pressure reader (pep boys, ~$10), and when I measured my tires cold (overnight), all of tires are at 42lbs.

Should I let air out since the shop told me to keep the pressure at 36lbs, or is that the weight of the car. I tried the reader on two other older cars, and it seems to be giving the right reading. So, I am just confused bout the 245/40, 275/35 18' tires that I have.

RON430

No, if you want 36 lbs, measure cold and let air out to hit the pressure. As long as you don't exceed the tire load rating, very difficult to do on a passenger car, the factory spec is with the tires mounted and the vehicle on the ground, and cold. You might notice a bit less handling precision but the ride will probably improve. If you have a tire tread depth gauge, or depth reading caliper, you can watch tread wear. Higher wear in the center means overinflated, higher wear on the outer edges, underinflated. Higher wear on inner edge (or outer edge) usually means alignment problem. The reason for using a gauge is because if you wait till you can see it with the naked eye, the tire has usually had it.

lex430

ron,
Thanks for the great info,

my confusion is, when I had the new 18' tires mounted, the shop had it at 36ps, I have never heard of tires getting inflated by themselves (at least not without heat). I am at 42ps now when cold. I am a bit hesitant to let air out cause I hear that tires wear out more when underinflate, especially with low profile tires. But
36 - 42 to me is quite a gap in difference. So, leave it alone? let air out?

mooretorque

AH, GRASSHOPPER WHO ONCE WAS lex430: YOU HAVE COME FOR INSTRUCTION. YOU WANT THE ULTIMATE TRUTH. "PLEASE LET THE SCALES FALL FROM MY EYES THAT I MIGHT SEE!!!!!"

and so it will be. Underinflate from factory recs, floopy tire, short life. Comfy, sure. Happp, NO!!!!
Inflate to fact specs, obtain balance handling, comfort and tire life. Sort of. Happpp, NO!!!!!!!!
Inflate to fact specs for 100+ mph sustained, much better balance, much better handling, probably ok comfort if your not cutting diamonds while driving around. Tire life, probabaly short, worn in center, great, when inside wears down enough I'll start on the inner and outer rims (Hurrah!!!!!)

In short, different inflations change the handling/ride/comfort characteristics remarkably, and may or may not make a large difference in tire life except at extremes of pressure (either way). So why don't yoiu set up to drive, say150 of your typical route at a given pressure. Note down your impressions, the steering, the ride, the initial turn in, stopping, the noise. Now, next day, do it again, 4# firmer. And again, till you reach the max pressuse that is recommended. (This is called primary research and you are obtaining a first approximation.)

Now, proceed to your nearest purveyor of canned, carbonated alcoholic beverages, purchase some and begin the cogitation. Much canned lubrication may be required. But you should be able to at least narrow things down. You are then ready for th process of 2nd approximation, which is much like the first but in my experience requires more lubrication. Considering the amount of lubricant I've consumed this evening, this first approximation wiill be a good one if the post is even legible, much less cogent.