Tire Pressure: What's the Proper Inflation?

[TRT87]

Is there a formula to find the proper tire pressure for aftermarket tires? Over the years, I've been given different advice.

For example, I owned a '95 Chevy with 35-inch Mud Terrains years ago. One tire shop advised me to maintain the factory pressure listed on the door, even with aftermarket tires. This seemed off, as it was a larger tire than stock, so it would imply that the pressure should be adjusted. Another tire shop recommended that I inflate the tires to the max cold pressure listed by the tire manufacturer since I should follow the guidance of the aftermarket tire manufacturer over the vehicle recommendations.

Years later, I've learned that neither is sound advice when dealing with aftermarket tires. Doing searches on the forum, many people discuss how much to deflate on the trail for different terrain, but what is the proper tire pressure for the road when you go from a 265 to a 285 or 305 tire size?

I understand it is based on the tire load (vehicle weight + cargo) but is there a formula to find the proper tire pressure for aftermarket tires?

 

[slomatt]

Some tire manufacturers provide load charts you can use to look up tire pressure for a given amount of weight on the tire, but that requires you to know how much weight is on each axle of your truck.

A potentially simpler solution is to do a "chalk test" (Google for details) to determine the air pressure that gives you the most even wear on the tires.

 

[Smileyshaun]

Chalk test is the only real way to get your correct numbers , the same size tire between manufacturers will actually be different sizes so they will all require diffent pressures to run true.
For example on my xterra on 35s I had to run 26psi to get them to run properly.

 

[TRT87]

Some tire manufacturers provide load charts you can use to look up tire pressure for a given amount of weight on the tire, but that requires you to know how much weight is on each axle of your truck.

A potentially simpler solution is to do a "chalk test" (Google for details) to determine the air pressure that gives you the most even wear on the tires.

I looked around for the load charts and found these two calculators, which should be useful for changing tire sizes/ratings on a stock vehicle.

For example, my truck's stock tires are P275/70/R18 and the previous owner installed LT275/70/R18 tires. Going from a passenger-rated tire at 36 psi. to light truck tire adjusted the proper inflation.

The first calculator from tirepressure.com states that my new tire pressure with the LT tires is 50psi.

The second calculator from tiresize.com states that my new pressure with the LT tires is 46psi.

It appears that each calculator is doing the math differently, but in either case they would be underinflated by 10-14psi. if I ran the LT tires at the door jam recommended pressure of 36.

I think the load charts will be necessary for a modified vehicle. Once you add armor, racks, jerry cans, etc., knowing the weight on each tire/axel would be more important. Finding the weight on each axle could be done at a truck stop's CAT scale.

And as you & @Smileyshaun both mentioned, a chalk test could be used to verify the new pressure.

Thanks for the help.

 

[madcratebuilder]

I looked around for the load charts and found these two calculators, which should be useful for changing tire sizes/ratings on a stock vehicle.

For example, my truck's stock tires are P275/70/R18 and the previous owner installed LT275/70/R18 tires. Going from a passenger-rated tire at 36 psi. to light truck tire adjusted the proper inflation.

The first calculator from tirepressure.com states that my new tire pressure with the LT tires is 50psi.

The second calculator from tiresize.com states that my new pressure with the LT tires is 46psi.

It appears that each calculator is doing the math differently, but in either case they would be underinflated by 10-14psi. if I ran the LT tires at the door jam recommended pressure of 36.

I think the load charts will be necessary for a modified vehicle. Once you add armor, racks, jerry cans, etc., knowing the weight on each tire/axel would be more important. Finding the weight on each axle could be done at a truck stop's CAT scale.

And as you & @Smileyshaun both mentioned, a chalk test could be used to verify the new pressure.

Thanks for the help.

A tire chalk test is going to give you much lower TP than your on-line calculators. Almost ever rig I've done the chalk test on came up 4-6 psi under oem recommeended.

 

[Red_Leaf_Overland]

I've always just eventually figured it out over time.... my compass ( witch very rarely sees trails but does from ime to time) I run at 32psi front and 35 psi rear which is what it says on the door... my TJ runs best at 28 psi on road and 6psi offroad. I ran a higher pressure with my last set of tires because they were a heavier tire... it really depends on a lot of things.

 

[Dilldog]

A tire chalk test is going to give you much lower TP than your on-line calculators. Almost ever rig I've done the chalk test on came up 4-6 psi under oem recommeended.

Not surprising, the OEMs probably give a slightly higher PSI spec to improve fuel millage and handling.

 

[the_great_white]

do a "chalk test"

^^^^^ this

 

[DintDobbs]

Chalk test is generally regarded as the best method.

I don't care that much, and run 35 PSI in all my trucks on-road, and air down to anywhere between about 10 and 25 for the extreme off-road fun.

I tackled loose sand in my 2nd-gen Explorer on highway tires at 35 PSI and didn't get stuck or bogged down anywhere, even when better outfitted vehicles with superior tires sank. So consider that the ideal pressure changes depending on not only tire size and weight on the relevant axle, but ultimately on where you are driving the vehicle.

And please, once you've decided on your on-road pressure... don't forget to rotate your tires at least every 5,000 miles.

 

[rtexpeditions]

A lot of those pressures seem a little low for on-road. Low pressures can cause heat buildup in the tyre walls, which causes damage and blowouts.

You could use the temperature pressure test, which compensates for loads. Note this is about safety and non necessarily optimising tyre wear.

Take your fully-loaded vehicle.
Have tyres inflated to near recommended pressure whilst cold (not used for several hours).
Drive on the highway for 1 hour.
Measure the pressure with a gauge.
If the pressure has increased by more than 10%, your pressure is too low.
If the pressure has increased by less than 10%, your pressure is too high.

Driving with deflated tyres off-road is usually done slowly, so the heat doesn't build up as much.