Calculate the new tire pressure needed when changing tire sizes to match the vehicle's original tire load capacity.
Changing tire sizes typically brings a change in tire load rating, which may require the new tire's air pressure to be altered to acheive the original tire's load capacity.
One way to do this, is to reference general load inflation charts that list tire size load ratings at each psi and set the new tire's pressure to match the original tire's load capacity.
This will adjust the new tire to more correctly fit the engineered load capacity and tire wear specifications from the original manufacturer.
Adjustments may still need to be made depending on the situation, but keeping a sufficient load rating is very important.
If you are not changing the tire size from the original size that came on the vehicle, then the recommended tire pressure is listed on the vehicle door jamb or door. The maximum pressure listed on the tire sidewall is not a general recommendation for stock tires.
Reference your vehicles placard inside of your driver's side door jamb. Be sure to read it closely including any tire size prefix or suffix such as a 'P' or 'LT'. Select the correct 'Tire Type' from the drop down, then enter your OEM tire size, load rating, and pressure listed on the card.
Be sure to be accurate about your replacement tire including any prefix or suffix such as a 'P' or 'LT'. Select the correct 'Tire Type' from the drop down that represents this, then enter the tire size and load rating.
Listed inside the driver's side door jamb of your vehicle there will be the originally installed tire size, load rating, and tire pressure for the front and rear tires. The size and pressure have been engineered specifically by the manufacturer for correct load capacity and will typically render the best results for tire wear and ride quality. Tire load capacity is directly related to air pressure in the tires. As tire pressure goes up or down, so does the load capacity of the tire. Too much air can wear the tires unevenly in the center and can create a wondering effect on the vehicle's handling. Too little air can wear the tires unevenly on the outer edges and will reduce the tires load capacity below the engineered level.
Each tire is rated with a load range, load index number, and max pressure rating. Passenger tires will be rated Light Load (LL), Standard Load (SL), or Extra Load (XL). Extra Load tires of the same size will typically have the same load capacity at the same pressure as the Standard Load tire. However, they have added load capacity above certain pressures. Light and Standard Load tires have a maximum capacity pressure rating of 35-36 psi and Extra load tires have a maximum capacity pressure rating of 41-42 psi. The maximum allowed pressure listed on a tire's sidewall can be higher however. Some tires list 44 psi and many higher speed tires will list 50-51 psi. This added pressure does nothing for load capacity. Pressures above 35-36 psi for SL tires and 41-42 psi for XL tires will not increase the load capacity of the tire. Even if you put 50 psi in an SL tire the load capacity will remain what it is at 36 psi for the higher pressure levels. Adding tire pressure above maximum load capacity is used mostly for high speed situations. A tires air pressure is increased to reduce tire deflection for the increase in rotations per second compared to typically city/highway use.
Light Truck (LT) tires are rated using a tread ply system. (B) 4-ply, (C) 6-ply, (D) 8-ply, (E) 10-ply, (F) 10-ply and so on. An LT tire size can have multiple Load Rating variants. Similar to Passenger load ratings, multiple Load Ratings will carry the same weight at the same pressure up to the lower ratings max pressure. The higher rated tire will then have added load capacity up to its maximum pressure rating.
When calculating a new tire's pressure to match an OEM tire's load capacity it's important to be very accurate. With metric passenger tires there are actually two types, Metric and P-Metric. You've probably seen a 'P' in front of some of the tire sizes you've looked at. This actually designates a different size than the same tire size without the 'P' designation. They are very similar and typically interchangeable. However, they do have different load rating charts and should be assessed as different tires when calculating a new tire pressure. LT tires also come in two types, LT-Metric and LT High Flotation. Metric LT tires will look like metric sizes but will have an LT in the front of the size (LT285/70R17). High Flotation LT tires are your standard 'American' light truck tire sizes with inch measurements and the 'LT' usually at the end (35X12.50R20LT). When calculating, you should be looking at your vehicles placard in the door and finding the exact size. If it's metric, does it have a 'P'? Does it have an 'LT'?
In the case of Light Trucks, there are most commonly 1/2, 3/4, and 1 ton variations. Most 1/2 ton trucks such as Ford F150, Chevy/GMC 1500, Ram 1500, Toyota Tundra etc. actually come equipped with Passenger rated tires on them. As opposed to an LT265/70R17, it will usually be either a P265/70R17 or just 265/70R16. This is because not only are they lighter weight and provide a softer ride, but their load carrying and towing limiters such as suspension ability, don't exceed the capability of the comparable Passenger rated tire. Also despite what many might think, Passenger rated tires can actually have higher load carrying capacities at the same pressure as the LT version in the same tire size. The LT tire will generally have a higher maximum load capacity, but it takes a higher pressure to achieve the same load capacity.