 |
|---|
 Whether you use a nitrogen wand like this Racor TyreSaver 3.0 or not, spending the time to find a correct tire pressure for those new, bigger tires will pay off in tire wear, ride comfort and fuel economy. |
It"s almost a universal constant that when a person upgrades to a larger, or much larger, tires, they"ll scratch their head a little over new street tire pressures. With the original size tires, the OEM of your truck spent a lot of time and money figuring that out. Notable exceptions like the Ford Explorer aside, the OEM does a pretty dang good job of that. You can find those recommended pressures in your owners manual or on the tire placard of your truck, a sticker located on one of the doors, door pillars, glove box door or a few other places.
Tire pressure is partly based on the interior volume of the tire and a larger interior volume changes the equation. It"s air volume that supports the load, not pressure. Take two tires, for example, one with twice the interior volume of the other. If it takes 15 psi to support a given load with the larger tire, it will take approximately 30 psi in the smaller tire. If you measure the cubic feet of air in each tire, you"d find it"s approximately the same amount, but you have to compress that volume of air more to get it into the small tire and, by doing that, you increase the pressure.
The Starting Point
So, where do you start? The factory pressures will inevitably be too high with larger tires, but that"s a safe no-brainer place to start. Ditto for using the max pressure listed on the sidewall. You should roll out of the tire shop that way unless you"ve already figured the new pressure. The problem is that, in most cases, you"ll be way overinflated either way and it"s like riding on solid rubber. That sidewall pressure is for the maximum load. Many Load Range D or E truck tires are rated for a 3,000 load at max pressure and at the heaviest end of your pickup, you might have 1,600 pounds per tire, empty.
An overinflated tire will deliver a bone jarring ride, but will probably offer slightly better fuel economy than a correctly inflated tire. Tire life is reduced, though not as much as with an underinflated tire. Because the tire footprint is reduced with an overinflated tire, grip for traction, cornering and braking is reduced. When it comes to braking and handling issues, tire pressure is one of those “Three Bears” elements, where you have wrongs at either end of just right. Overinflated is somewhat better than underinflated, but if the right pressure means a stopping distance eight feet shorter than overinflated, and you need that eight feet to avoid the back bumper of the car in front of you during a panic stop, the question gains some clarity.
The E-Z (-est) Method
Some time back, we found a formula that allows us to convert the OEM tire placard pressure for use with larger tires. It"s an industry standard method that several companies, including Toyo and Bridgestone/Firestone, publish on the web. This method gives us a good starting point, if not a dialed-in point, for a tire pressure that is “Three Bears” just right with new, larger tires.
The first thing you need is a load inflation chart that graphs the safe load for a tire at a range of pressures. We"ve given you some of the popular sizes in the nearby sidebar, but if you search on the web, contact the manufacturer of your tire or even the tire shop, you can get the specifics on your exact tire. These are usually sourced from the Tire and Rim Association based on input from tire companies.
If your vehicle is substantially the same as stock, e.g. standard weight just with larger tires, you can compare-use the tire pressure placard on your vehicle with a load inflation chart to determine a pressure, or at least a starting point. If your rig is substantially heavier than stock, you would either use the max GVW pressures listed on the tire placard, or get weights at all four corners and base pressure on actual weights.
Start by locating your stock tires on the chart. Find the pressure on that chart that corresponds to the recommended pressure on the tire placard. Find the weight the tire is designed to carry at that pressure. Then go to the chart for the new tire and find that weight. If you can"t find it exactly, go to the next HIGHEST weight and see the pressure listed.
Let"s say you upgrade from a LT265/75R16E, Load Range E, to a 37x12.50R17LT Load Range D. Your tire placard lists 50 psi front and 80 rear. On the Load Inflation chart, the original front tire is listed as carrying 2470 lbs. at 50 psi and the rear 3415 lbs. at 80 psi. That"s for a GVW load.
Now look up the 37x12.50 (both tires are on the nearby chart) and find the weight nearest those. For the front, you"ll see 2470 lbs. is rated at 30 psi and the numbers match exactly, so there"s your equivalent pressure. In back, you"ll see the original 3415 rating falls between 45 and 50 psi for the new tire. You"re a little over at 50 psi but that"s plenty close enough. If you are going to make the “close enough” choice, go to the next higher number for safety.
When the weights fall between the numbers on the charts, you can calculate the exact amount using the formula below. Start with the closest load and pressure to the original tire (can be above or below the capacity you need) and add or subtract to get the exact pressure.
 |
|---|
Subtract the old weight rating from the new (3525 - 3415 = 110 pounds). Divide the 110 pounds of weight by the 70.5 pounds-per-psi and you get 1.56. That means you would deduct 1.56 psi from 50 psi to match the old tire weight rating exactly. Probably not worth doing in this case, but you get the idea.
The bottom line to our example is that after a switch from LT275/75R16 to 37x12.50R17LT, your new pressure of 30 psi front and 48.5 psi rear would match the original load capacity and ride quality of the truck. Likely, the handling and braking performance would be in the ballpark too, but the differences in tread width, tire profile, rubber compound, tread pattern, etc. will have changed that irrespective of tire pressures. From here, you would drive and fine tune as you like.
Bearing in mind that the tire placards in pickups often reflect their GVW (sometimes they list loaded vs unloaded pressures) you may want to run lower rear tire pressures for ride comfort. To do that scientifically, get the rear weight of the truck separately. Make sure the fuel tank(s) is full and you have all the junk aboard you normally carry... including you! Get on a truck scale and pull forward until the front tires are off the scale but pull forward more until the edge of the scale is under the driver"s seat and record the weight. Add about 25 percent back to that weight for safety and divide it by two for a per tire weight. Find that per-tire weight on the Load Inflation Chart, go to the next highest pressure if it falls between numbers, and you have a good empty truck rear tire pressure. You have to remember to air back up for loads and bear in mind that your truck will likely get better fuel economy at the higher pressures.
Finally, be aware of the differences between LT (Light Truck) and P (Passenger) rated tires. These abbreviations appear in the tire size (LT235/75R15 vs P23575R15). An LT tire is more robust and able to tolerate heat and loads better, even if the actual load capacity is the same. For that reason, the tire industry derates the load capacity of P-rated tires by about 10 percent when used on light trucks. You see P-rated tires fairly often on SUVs and half-ton trucks. The simple thing to do if you will be going to a P-rated tire on a truck is to divide its load capacity by 1.1 or 1.2.
Share
To view this entire story and photos subscribe to our virtual issue. It´s FREE!
http://www.offroadadventures.com/digital/subscription
