CT05Legacy Posted February 9, 2007 Share Posted February 9, 2007 In the manual (11-50 of my '05 manual)...it says. Standard tire pressures: Front: 32 psi (220 kPa, 2.2 kgf/cm2) Rear: 30 psi (210 kPa, 2.1 kgf/cm2) Garage temperature: 60°F (15.6°C) Outside temperature Adjusted pressure [psi (kPa, kgf/cm2)] front rear 30°F (-1°C) 35 (240, 2.4) 33 (230, 2.3) 10°F (-12°C) 37 (255, 2.55) 35 (245, 2.45) -10°F (-23°C) 39 (270, 2.7) 37 (260, 2.6) Placard on door says 35/33. Whats the deal? Which to believe? Link to comment Share on other sites More sharing options...
CT05Legacy Posted February 12, 2007 Author Share Posted February 12, 2007 bump Link to comment Share on other sites More sharing options...
Vimy101 Posted February 12, 2007 Share Posted February 12, 2007 Go with the manual. As long as you stay under the max cold inflation pressure, you're good to go. Link to comment Share on other sites More sharing options...
rc0032 Posted February 12, 2007 Share Posted February 12, 2007 Taking into account global warming I run 39/37 Link to comment Share on other sites More sharing options...
Vimy101 Posted February 12, 2007 Share Posted February 12, 2007 ^ If one doesn't believe in GW, what would be your recommendation? Link to comment Share on other sites More sharing options...
rc0032 Posted February 12, 2007 Share Posted February 12, 2007 Yes since GW is a bunch of BS Link to comment Share on other sites More sharing options...
Vimy101 Posted February 12, 2007 Share Posted February 12, 2007 Hey, it influenced my choice for winter rubber. Winters just aren't what they used to be despite the recent chill. More snow in the forecast. Link to comment Share on other sites More sharing options...
Xenonk Posted February 12, 2007 Share Posted February 12, 2007 I just follow my own set of pressures. Keefe Link to comment Share on other sites More sharing options...
TRS Posted February 12, 2007 Share Posted February 12, 2007 In the manual (11-50 of my '05 manual)...it says. Standard tire pressures: Front: 32 psi (220 kPa, 2.2 kgf/cm2) Rear: 30 psi (210 kPa, 2.1 kgf/cm2) Garage temperature: 60°F (15.6°C) Outside temperature Adjusted pressure [psi (kPa, kgf/cm2)] front rear 30°F (-1°C) 35 (240, 2.4) 33 (230, 2.3) 10°F (-12°C) 37 (255, 2.55) 35 (245, 2.45) -10°F (-23°C) 39 (270, 2.7) 37 (260, 2.6) Placard on door says 35/33. Whats the deal? Which to believe? What is shown in the Owners Manual is just a generic example of how to adjust the cold inflation pressure values posted on your door jamb for when the temperature of the air in the (cold) tires at the time you are measuring the pressures is significantly different from the prevailing ambient temperature conditions the tires will be operating in. It's showing the rule of thumb of adjusting pressure by 1 psi for every 10 deg F difference between temp at time of measurement and actual temp of the environment the tires will be exposed to. Here's an example: Before leaving for work you check tire pressures in your garage which is heated to 60F. They are spot on at 35/33 according to your sticker. You drive to work and park out in the lot. It's been about 20F outside. Before leaving to head back home you decide to check your pressures again and they're all about 4 psi lower than they were this morning in the garage. "WTF!! Do I have a slow leak in all 4 of my tires???" No, the difference is that your original spot-on pressures were only good if the tires were going to be operating in temperatures that were in the neighborhood of those when you first measured. In this case that was the 60 deg in your garage, but since the tires have now equalized with the much colder outside, the air inside them has contracted resulting in the pressure drop. So, when you measured in your garage before heading off to work you should have compensated for the fact that the tires would be operating in a real world temp of 20F and not the nice cozy 60F of your garage. You do this by applying the rule of thumb. Placard says 35/33. This is what you measured in the heated garage. But wait, it's only going to be around 20F outside. That's 40 deg colder than the temperature you measured at, so to compensate for the cold outside temps you'd add 4 psi to the sticker numbers and inflate to 39/37 in the garage. Now when the car is outside your cold inflation pressure will drop back to the correct 35/33 values. Now lets look at it the other way. You live in Phoenix and have your garage air conditioned at 75F. Outside temperatures are around 105F. Because of this difference if you measure cold inflation pressure in the garage then you would have to compensate for the much hotter ambient conditions outside. If you want to end up at the factory recommended cold inflation pressure and since there's a 30 degree temperature difference, you'd want them to be at 32/31 in the cool garage so after equalizing with the hot outside and the air in the tires expanded you'd end up at your goal of 35/33 cold inflation pressure. Do this: Tire temperature when measuring cold inflation pressure (deg F) - Average outside air temperature tires will be exposed to when driving (deg F) = Temperature difference Temperature difference / 10 = psi to adjust target cold inflation value by More examples: 60F (in garage) - -10F (outside) = 70 70/10 = 7 If your target is 35/33 then inflate to 42/40 in the garage and they'll fall to 35/33 when exposed to the extreme cold. 75F (garage) - 115F (outside) = -40 -40/10 = -4 If your target is 38/36 then inflate to 34/32 in the garage and they'll rise by 4 psi when exposed to the hot outside conditions. Link to comment Share on other sites More sharing options...
pillboy Posted February 12, 2007 Share Posted February 12, 2007 Option one is buy a Hummer H1 with the onboard inflator system and change the pressures on the fly based on ambient temps. Option two is don't worry about it because you will cut or bruise a sidewall before you wear them out and will have to buy a whole new set of 4 tires. It is still ugly. Link to comment Share on other sites More sharing options...
Fast05 Posted February 12, 2007 Share Posted February 12, 2007 I just follow my own set of pressures. Thats great thanks for sharing dick Link to comment Share on other sites More sharing options...
Xenonk Posted February 12, 2007 Share Posted February 12, 2007 ^ in all honesty, getting a pyrometer is the best way to get things done right. Even an infrared reader is good enough if anyone wants to be that precise. All you are looking for is even heat across the tread. Keep adjusting pressures until it's all even. You simply can't sit there and calculate the tire pressures just based on load capacity. If the alignment isn't at par, you will see that you end up having a hotter spot on the tread than the other, which will eventually yield to heat cycling and chunking of the tires at certain areas over longer periods of time and use. btw, I end up adjusting my tires cold for daily driving because I dont warm up the tires hot enough to get a good reading.. the most I have gained from a long distance trip on a typical 225/45/17 tire was no more than 4 psi up front and 3 psi in the rear.. on the track and autocross, I can easily gain as much as 14 psi on a hot day up front after a few laps (probably no more than a couple of miles of driving) and about 10 psi in the rear. So in this case, I measure my tires when they are hot. Different ways to read based on the different ways of use. We can start off another thread about car handling balance just on changing the tire pressures alone if there's a need to go into further detail. Keefe Link to comment Share on other sites More sharing options...
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