2010 Suzuki Kizashi GTS: Tire Pressure Problem

adamb1: Aren't you supposed to set the pressure when the tire is cold. During cold months, I set the PSI in the morning to 29PSI @ 35F and in the afternoon it'll be 33PSI@50F.

I don't know the optimal temperature to set tire pressure. I always believe cold tire meant not hot from driving.

I adjust pressure for winter and summer. This week, it's 100F here during the day. Next month, temps will top out in the 80's.

I wonder if the almost-empty gas take reduced the weight of the car enough to also reduce the tire pressure enough to go from "on the border" of tripping the TPMS to actually tripping it. Sounds crazy but who knows.

Gas tank, that is. Where's that Edit button again?

Gasoline weighs about 6.1lb/gal and the tank weight difference is not going to increase the volume of the tire more than a small fraction of. that large a percentage change in pressure.

Checking pressure when cold means at the day's ambient temperature before driving. Pressure is directly proportional to absolute temperature (i. e., degrees above absolute zero) and will change when the temperature changes significantly--if my physics and arithmetic are correct, dropping from 72 deg F to 50 deg F, e. g., is approximately a 4% drop in pressure or 1-2 psi for typical car tires. I'm assuming sidewalls are stiff enough that volume change is negligible.

I vote for service guy's mistake. My cars tires are supposed to be 32 psi all around and I frequently find them at 28 psi after maintenance. Previous car was worse as the fronts and rears had different recommended pressure but service people set them all the same and usually still incorrect for both axles.

lots of current cars have readings for all four tires in the instrument panel so you dont have to measure each one.

Tires naturally seep a little air over time, but not all the way from 38 to 31 in the amount of time we've had this car. Methinks someone didn't agree with Suzuki's 38 psi recommendation and decided something lower sounded better. I've found that if you randomly ask someone what their tire pressure should be, those who don't look at you funny will answer 32 psi. It seems to be some sort of a weird default.

Yea I work in a tire shop and every car we set the presure at 32 and for trucks/suv's we set it at 35 unless its 3/4 ton or up then its 55 in the front and 80 on the rear

"Yea I work in a tire shop and every car we set the presure at 32 and for trucks/suv's we set it at 35 unless its 3/4 ton or up then its 55 in the front and 80 on the rear"

Why wouldn't you set it to the manufacturer's spec that's on the driver's door jamb? My car requires 38/41. 32 would be way too low.

I can tell you exactly what the recommended pressures are for all of my vehicles, even the recent ones I don't own anymore. Few things irritate me more than a shop REMOVING AIR FROM MY TIRES to set it at what they believe is the "right" pressure, without taking a few seconds to read the door jamb or look it up in their system. Makes me wonder if they make sure the oil weight is right. Yet another reason to DIY.

I've never seen a car call for a pressure above 35psi. That sounds really high. I was surprised that my 05 CTS-V wants 30 front and rear, but the manual mentions high speed and/or high load driving requires a little more pressure, with a rearward bias.

The heavier the vehicle, and the narrower the tires, and the flimsier the sidewalls, the higher the pressure that is required to maintain a proper contact patch.

I remember when some tire place dude put only 20 psi in my 80 psi E rated tires just because "they look massively over inflated."

i agree with the comments here about techs/mechanics who think they know "best" for your driving. Like what? I go into a shop and I'm a moron just because someone else is doing the work? I do like the nitrogen in the tires that my shop has put in. I used to always be constantly adjusting the pressures with regular air. I am not one to fall for the "marketing hype" til I tried it. 100% nitrogen and then measured the air over time. It hardly ever fluctuated as wildly as regular air. maybe it's the weather here in the upper midwest.

And the pressures going down evenly for the original post. Isnt this car on a roadtrip through the mountains? Won't the rise in elevation decrease the pressure that this car saw at "sea level" back in Los Angeles? That's what it sounds like to me.

"Why wouldn't you set it to the manufacturer's spec that's on the driver's door jamb? My car requires 38/41. 32 would be way too low"

Because if you set the air pressure that high for a car the center of the tire will wear out alot faster and then the tire wouldn't be covered under warranty.

Generally speaking, over inflating is better for the tire than under inflating. It leads to a cooler operating tire, which is what increases its life, and fuel economy. The downsides are firm (or even abusive) ride, and center treadwear, as well as poorer traction. On previous cars that called for 35/35 (Caprice), I over inflated a few PSI. Interestingly, another car I had which was VERY similar to the other car, called for 30/30 only because it was a Cadillac (Fleetwood). It was 100% for ride and knowing the driver wouldn't be pushing it as hard. So you kind of have to know WHY the pressure requirement is what it is. Though I will say short sidewalls tend to make small changes in pressure create larger changes in contact patch.

"Because if you set the air pressure that high for a car the center of the tire will wear out alot faster and then the tire wouldn't be covered under warranty."

Then why would the manufacturer recommend a tire pressure rating that would void their own warranty? That makes absolutely no sense. 39/41 (I mistyped earlier) is recommended in my owner's manual and printed on the door jamb for under-100 mph travel. For over 100-mph it's 41/44.

Explain to me how I will be ruining my tires by following the specs the car was engineered to, and how a tire shop setting a generic pressure for cars with widely varying missions (performance, comfort, longevity), different load factors and different tire types is not negligent?

Wow there's somebody's tireshop that we should avoid.
...still arguing against manufacturer specs...

That's why I don't bring my car to independent places.
Stealerships know best, even if they take more money.

"Because if you set the air pressure that high for a car the center of the tire will wear out alot faster and then the tire wouldn't be covered under warranty."

wow... generalize much?

Yeah, tire shops should know better. OE's often have specific tires designed for each model, even if it has the same name and designation as aftermarket replacements. The OE's rating is always what it should be set at unless there is some extenuating circumstance such as racing or heavy towing. And to say that the manufacturers recommended settings will void a warranty is just moronic. Stop listening to your lazy ass high school dropout manager and start setting the pressure to what is correct.

Like adamb1 from Tennessee, South Carolina experiences 30 degree temperature swings. Right now, it's 70's in the morning, heading up to over 100. Sitting on an flat asphalt parking lot doesn't help anything in the afternoon. 35 PSI in the morning and driving home in the afternoon, the PSI could reach 41.

PV=nRT (Ideal Gas Law)
http://en.wikipedia.org/wiki/Ideal_gas_law

wow you guys probably dont know shit about cars, atleast im a mechanic, you guys are probably just a bunch of yuppies that drive your little pussy hondas and imports with automatics