245/40R18 on 18x8 wheels (2.4L TSX)- What pressure to use?
10-16-2016, 05:18 PM
#1
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Thread Starter
245/40R18 on 18x8 wheels (2.4L TSX)- What pressure to use?
Like the title says. I have been using 36psi but wanted to see if there's a chart or formula to use when using non-standard wheel/tire sizes.
10-16-2016, 10:55 PM
#2
Figure out the volume of both sizes.
Technically, you should adjust the PSI by the percentage change in the volume.
I know PSI refers to pressure over AREA, not volume. But volume is derived from area.
Figuring out volume takes less steps. But area is easy too...assuming that you assume a uniform barrel shape for the tyre.
Or
Since a 245/40/18 is lower profile...but wider than a 225/50/17, their surface areas are likely fairly similar.
Just use the PSI listed on the door. Adjust it if you need to, based on tyre construction and personal preference...or if you see uneven tyre wear.
Technically, you should adjust the PSI by the percentage change in the volume.
I know PSI refers to pressure over AREA, not volume. But volume is derived from area.
Figuring out volume takes less steps. But area is easy too...assuming that you assume a uniform barrel shape for the tyre.
Or
Since a 245/40/18 is lower profile...but wider than a 225/50/17, their surface areas are likely fairly similar.
Just use the PSI listed on the door. Adjust it if you need to, based on tyre construction and personal preference...or if you see uneven tyre wear.
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Guanacoracing (11-04-2016)
10-17-2016, 04:45 PM
#3
Advanced
Figure out the volume of both sizes.
Technically, you should adjust the PSI by the percentage change in the volume.
I know PSI refers to pressure over AREA, not volume. But volume is derived from area.
Figuring out volume takes less steps. But area is easy too...assuming that you assume a uniform barrel shape for the tyre.
Or
Since a 245/40/18 is lower profile...but wider than a 225/50/17, their surface areas are likely fairly similar.
Just use the PSI listed on the door. Adjust it if you need to, based on tyre construction and personal preference...or if you see uneven tyre wear.
Technically, you should adjust the PSI by the percentage change in the volume.
I know PSI refers to pressure over AREA, not volume. But volume is derived from area.
Figuring out volume takes less steps. But area is easy too...assuming that you assume a uniform barrel shape for the tyre.
Or
Since a 245/40/18 is lower profile...but wider than a 225/50/17, their surface areas are likely fairly similar.
Just use the PSI listed on the door. Adjust it if you need to, based on tyre construction and personal preference...or if you see uneven tyre wear.
Yeah what he said. pressure is pressure.
10-18-2016, 10:47 AM
#4
lol. what I'm saying is that if the car weighs the same (it does), and the tyres have the same surface area (measured in square inches to keep it simple) , then the pounds per square inch (PSI) that the tyre needs to in order to support the car is the same. The end goal is to keep an "ideal" contact patch for grip...and reduce rolling resistance so the tyre doesn't overheat at the very least...and so the tyre steals as little energy as possible from the fuel.
Yes? Yes.
The manufacturer recommends the PSI written on the door jamb to be used for typical weighting of the car AND for use with the same tyre as spec'd from the factory...or some acceptable replacement tyre based on engineering approval.
Nobody takes it that far. Usually, you buy a tyre that is the same size...and if you're prudent enough, you buy a tyre with the same basic construction. This is good enough for most applications.
BUT some tyres are softer sidewalled. Some have different type of reaction to pressure or cornering. Each tyre for each application may require slight changes in PSI to be optimal.
TL;DR. AMIRITE?!!!
Cliff's notezzzz: Use the pressure printed on the door jamb. Adjust it as needed if you see uneven wear or reduced mpg or some other factor.
Yes? Yes.
The manufacturer recommends the PSI written on the door jamb to be used for typical weighting of the car AND for use with the same tyre as spec'd from the factory...or some acceptable replacement tyre based on engineering approval.
Nobody takes it that far. Usually, you buy a tyre that is the same size...and if you're prudent enough, you buy a tyre with the same basic construction. This is good enough for most applications.
BUT some tyres are softer sidewalled. Some have different type of reaction to pressure or cornering. Each tyre for each application may require slight changes in PSI to be optimal.
TL;DR. AMIRITE?!!!
Cliff's notezzzz: Use the pressure printed on the door jamb. Adjust it as needed if you see uneven wear or reduced mpg or some other factor.