I have the Perelli Scorpion 20 inch tires that came with my 2011 FX4. The tire reads 44 p.s.i. max, but inside the door reads 35 p.s.i. I checked my tires and they had 29 p.s.i. in them. What is the correct tire pressure for this tire ?? Thanks

 

I would go with what the sticker says unless you're loaded to the max payload.

 

I have the Wrangler SRA's 18" and I asked my Service Guy what he thought. He said run the sticker pressures around town (35), but increase to 39 when towing my Travel Trailer.

Seems to work for me.

 

44 is the max psi suggested my the manufacture, IMO to get the best wear, of the last 5 or so trucks I've owned, I run 35 to 38 on the front and 32 to 35 on the rear that's with the truck empty...

 

Always use the recommended tire preassure (the one on the door jamb). If the vehicle is going to be used under extreme load conditions inflate to a preassure higher than the recommended but lower than the max tire pressure on the tire sidewall. All of a vehicles's capacities load, speed etc are determined at the pressure on the door jamb (recommended tire pressure). Never drive a vehicle with a lower pressure than the recommended one (a couple of PSI doesnt matter). Ive only had one car that had 2 recommended pressures one for normal driving and one for extended high speed driving (100mph plus) a manual tranny 2004 TL, the optional pressure was 5 PSI higher than the normal recommended one.

 

With LT-metric tires, the answer would be easy: Find the load/inflation table for your tires and go by that.

But only the F-150s with the HD Payload Pkg (7-lug wheels) have LT-metric (light truck) tires. The other 95% of us have P-metric, or "passenger car" tires. If you dig hard enough you might find a load/inflation table for your car tires, but the easiest is to just go by a rule of thumb. When unloaded or lightly loaded, go by the tire inflation sticker on the door frame. When loaded for bear or overloaded, go by the max PSI on the sidewall - in our case 44 PSI.

If you do find a load/inflation table for your size P-Metric tire, remember that P-Metric tires come in three different load ranges = light load LL, standard load SL, and extra load XL. If the max inflation pressure is 44 PSI, then you have XL tires.

The following link is to a load/inflation table for Toyo tires. Tire brand doesn't mater, because the load/inflation tables apply to all tires sold in the USA that are the same size and load range. Note the Toyo tables include size P275/55R20, but the table goes up to only 35 PSI or standard load for that tire. That's probably because Toyo does not offer a tire in size P-275/55R20 with load range XL.
http://toyotires.com/tire-care-safet...flation-tables

But thanks to Toyo for that table, because most other manufactirers do not publish a load/inflation table that includes P-Series tires. Because of legal ramifications, they want you to follow the vehicle manufacturer's rules, regardless of how vague those rules are.

 

I ran 40 psi in mine till I got sick of them and replaced them with some good tires.

 

I water check them that never lies. Place some water on the ground and drive though it. If the entire tread shows that is a correct AP. if just the outside shows you need more air if only the center shows then you need less. This has always worked for me. I write down the AP empty and then full so I can switch pretty easly. Okie dokie

 

I guess you learned this rule from grandpa!!! This was true when you used bias ply tires, for radials this "rule" would lead to severely under or over inflated tires since the tread wont "fold in the middle" till its almost flat or blown up like a balloon.

 

Ford made that sticker based on the tires they sold with the truck. What everyone should have learned from the Expy tire blow outs is that the tire manufacturer not the vehicle manufacturer knows what is best for that tire. I run an E rated tire. At 35 psi, they are under inflated. I know Ford doesn't recommend 75psi. The bottom line is in 99 or 2000 when they built my truck they had no clue that 12 years later I would be running these tires, and as I adjust the kind of tires I use, I also have to adjust the pressure I run them on. You wouldn't run the recommended 87 octane with 6 pounds of boost if you added a turbo, air pressure works the same way.

The water test mentioned works well if and only if your tires aren't pooched from something else. If you have scalloped insides from a bad camber for example, it can cause false readings on the water test. If you did that test with brand new tires and recorded the results, you would be laughing to the bank. If you tried to do it with crazy mileage and messed up tires, you may never find the right pressure and if you do that may still be the wrong one.