The first days with new tires are a honeymoon all right but, as with every long-term relationship, there are practical issues to be addressed if you want it to last. With a tire upgrade your most immediate need is finding the correct street tire pressures. We addressed this topic back in 2002 and found that it was one of our most requested back issue stories, so it's long overdue for a fresh look.
When you want to know the best tire pressures for your stock rig, there's no better place to look than the owner's manual or the tire inflation placard. The problem is that these pressures are only useful with the stock tires, or another of very similar size and construction. When you upgrade to a larger tire, you are essentially starting from scratch.
There are just a couple of basic concepts to grasp here. First, it's the volume of air in the tire that supports the weight of your vehicle. You need a given number of cubic feet in the tire to support a given weight. That's a constant. The only difference is the size of the vessel in which you are going to carry that volume of air. Putting that necessary volume into a small vessel creates more pressure than when it's placed into a large one. The bottom line is that at a given vehicle weight, the larger tire needs less pressure to carry it... but how much less? There are several ways to make this determination.
Method One: Read the Sidewall
Well, this isn't actually a method, just the no-brainer way to deal with the choice. Simply look at the maximum pressure listed on the sidewall and use that as your street pressure. Until you determine a more appropriate pressure, that's how you should leave the tire shop. In most cases, this setup will mean your tires are overinflated. In the case of an unloaded pickup, they may be grossly overinflated. Remember this: Maximum pressure is for a maximum load. While looking at the maximum pressure on the sidewall, note the load capacity of the tire at that pressure. That capacity may be enough to support more than half the weight of your rig.
Upsides: It's the safe, no-guesses move. It's often the most fuel efficient pressure because it reduces the rolling resistance to the bare minimum.
Downsides: Can you withstand the pummeling you'll get from the reduction in ride quality? Handling and braking performance are both reduced because only the center of the tread is in contact with the road. The tire usually wears out the center of the tread first.
Method Two: Get Weighed and Read the Chart
Tire manufacturers have load/inflation charts for every tire they manufacture. This shows how much air pressure is necessary for various loads. You can request these charts from the tire manufacture, find them a tire shops or online. There are also generic charts from the Tire and Rim Association that cover many popular sizes, but it's always better to get the specific info specific for your particular tire.
Second, you need to get your vehicle's front and rear weights. Don't just guess or use the manufacturer's listed specs, weigh it yourself. Load the vehicle with the stuff you normally carry, full fuel tanks and some simulated weight for passengers in their normal places. You want to know how much weight is on the front and rear tires separately.
Finding a scale in your area could be as simple as a truck stop, recycling center, scrapyard or landfill. Race shops often have portable scales that go under each wheel. At a normal truck scales, pull the front wheels on, record the weight, then pull fully on, record the weight, then pull off until just your rear wheel wheels are on the scale and get that weight. The front and rear weights should be approximately equal to the total.
If you divide the front and rear weights by two, you get the approximate weight on each tire. Don't be surprised to find the front is carrying much more weight than the rear, especially with pickups. Compare the weight on each tire with the load/inflation chart and round UP to the nearest pressure. Add about 10 percent to that.
Finally, as a test, measure the profiles (from the ground to the rim edge along the centerline of the hub, obviously on flat ground, of the tires front to rear. If needed, equalize them in pairs at each end by ADDING pressure to the low tires to bring them up. That covers you for differences in side to side weight. By always going up a little, you reduce the risk of erring on the underinflated side. If everything works out perfectly, you will have nearly equal profiles front and rear. From there, go on a test drive as outlined in "Test Drive" farther on.
Upsides: If accurately done, this method gives you no more and no less pressure than you need for a combination of optimal fuel economy, treadwear, ride and performance.
Downsides: Subject to human error. Time consuming.
Method Three: The E-Z Chart Method
If your vehicle is substantially the same as stock, e.g. standard weight just with larger tires, you can compare using the tire pressure placard on your vehicle with a load inflation chart to determine a pressure, or at least a starting point.
Start by locating the chart for your stock tires. 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.
Doing this with an '05 F-150HD, the tire placard listed 50 PSI front and 60 rear for a maximum load on its 24570R17D Load Range D tires. We look up that tire on the Load/Inflation chart and find at those pressures, the tires are rated for 2205 and 2469 pounds respectively. Contemplating an upgrade to a larger 285/70R17D Mickey Thompson MTX Load Range D, we look at the chart for that tire. By substituting the numbers, we get 40 psi front (2340 lbs) and 45 PSI (2540 lbs) rear for the bigger tires to carry the same load.
When the pressure or weight fall between the numbers on the charts, you can calculate the exact amount using the formula below. Use the closest load and pressure to the original tire and add or subtract to get the correct new pressure.
Tire Weight
____________ = Load Capacity Pounds per PSI
Tire Pressure
Using our numbers
2205
____ = 44.1 pounds/PSI for original tire
50
2130
____ = 60.8 pound/PSI for new tire
35
So to bring the new tire up to the same capacity as the old, we'd add 1.23 PSI to bring the front tires to 2205 pounds capacity. We get that number by first subtracting the new tire weight at pressure from the old tire (2205-2130= 75). Then divide the weight difference by the pounds-per-psi of the new tire (75/60.8= 1.23) and that gives you the amount to add or subtract from pressure. Because ours was a little low, we added 1.23 psi to bring it up to the same capacity as the old tires, which would be 36.23 psi, which we round up to 37 psi. In back it worked the opposite. The new tire was at a higher capacity than the old. Not by much, so you'd have the option to just run with 45 or air down a little, about 1.4 pounds, to 43.6 psi
Method 4: Chalk It Up
Another time-tested way of determining tire pressure is to chalk up a section of the tread to determine whether or not it has full tread contact with the ground at street pressure. You start at the tire's maximum sidewall pressure on the smoothest piece of level concrete you can find. You need to be able to drive forwards and back in a straight line at least 50 feet. Very smooth asphalt works OK but don't use a really rough surface. Concrete is ideal.
Using very soft chalk, make a mark across the tread of the tire. Drive forward and back slowly a few times over the marked area, going absolutely straight, and then look at the chalk mark. You will see that the chalk has been rubbed off the portions of the tire that have touched the ground. When you start at a high inflation, most likely the center of the tread will be worn off. If that's the case, you can reduce the pressure a few pounds and retest until you see the chalk is worn off all the way across the tread. You absolutely need to do this gradually so you can catch that moment when the tire just reaches the full contact point.
Do this individually for all four tires (yeah, it will take a while) and then add 10 percent to the pressures. As with section two above, measure the tire profiles from the ground to the rim along the centerline of the hub and equalize the profiles of the front tires with each other and again with the rear tires, bringing the low tire UP by adding pressure.
Upsides: If accurately done, this method gives you no more and no less than you need for a combination of optimal fuel economy, wear, ride and performance.
Downsides: Very time consuming. Probably subject to more potential errors than method two.
Test Drive
You can double check whatever street inflation you select using a heat test. A fast heat rise going down the road is an indication of excessive rolling resistance and friction from under-inflation. In the manufacturer's tests, a tread pyrometer is used, but there is a way to do it by measuring pressure rise. This check is mainly to avoid accidental under-inflation.
This test is best done on a warm day, but not excessively hot: Say the mid seventies to low eighties. You want to find a stretch of freeway you can drive 10-15 minutes at the legal freeway speed (55 mph minimum), with a minimum of turns. You need to start with the tires close to ambient temperature, so if you had a long drive to your test area, find an off-ramp fast food place for a break to let the tires cool, in the shade if possible.
With the tires cool, measure and record the pressure. Get out on the freeway and run at the max legal limit for 10-15 minutes. Find a safe place to stop, pull over to re-measure and record the tire pressures as quickly as you can. You want the tire pressure to rise no more than about 10 percent, or 3-4 psi. Tire pressure rises with temperature... about one PSI per ten degrees of temp, Fahrenheit. A five psi rise means that tire temperature jumped 50 degrees. If the pressure rises excessively, add a few pounds of air (yes, you'll need a compressor, air tank or CO2 tank), let the tires cool and try again until you are in that 3-4 degree area. You may find that only a couple of tires need more air, perhaps just the front or rear.
In performing this test on similar sized tires on similar vehicles of similar weights, it has been noted that an aggressive tire will generate a little more heat than a milder tread tire. We don't have any exact figures to offer, but a mudder may run a little hotter at the correct pressure than a street tire.
Most people have a subjective "feel" for how their rig is supposed to drive. As the final test, use that to evaluate how your rig performs. It shouldn't wallow or slither around corners. You can add or reduce pressure to suit the feel but take care when you start reducing pressures.
Carrying a Load
All the above works with your rig at any state of load, but is accurate for only the state of load that you tested. If you add a bunch of accessory weight, or remove some, you will need to reevaluate. If you have a pickup and tow or haul, in theory you could do the tests to determine what pressures to run, but in practice, most people just pump the tires up to their max load inflation for the duration of the loaded trip and air back down for a smooth ride after.
Time and Temperature
The average tire looses about 1psi per month. Also, the seasons will take a bite out of pressure at the 1 PSI per 10 degree rate mentioned earlier, e.g. a 35 psi tire at 70 degrees will read 30-31 psi at 30 degrees. The bottom line to check tire pressures regularly throughout the year, which will help save fuel and keep them wearing good year-round.
Finally
Overall, how well these procedures work for you will depend on how patient you are. It you are impatient, lazy or a self-acknowledged corner-cutter, you are best off just pumping in the sidewall pressure.
(Sidebar SB1)
Sample Radial Tire Load(pounds) vs Inflation (psi) Chart
Tire Size 25psi 30psi 35psi 40psi 45psi 50psi
30/9.50-15LT 1240 1410 1570 1715 1855 1990
33/9.50-15LT 1565 1780 1980 2170 2345 2510
31/10.50-15LT 1400 1595 1775 1945 2100 2250
32/11.50-15LT 1575 1795 1995 2185 2360 2530
33/12.50-15LT 1765 2000 2225
35/12.50-15LT 2015 2295 2555
