Guys, I am new on site, I imagine this has been asked many times, but I may have a slightly different question. As we know, the owners manual in the 2009 F150 FX4 states multiple times not to switch from P metric to LT metric tires (or the reverse, if you have LTs form the factory) . We also know many people do it routinely with zero problems. My question is, does Ford calibrate the electronic stability control, traction control, ABS etc differently at the factory depending on whether the given truck has P metric or LT metric tires s it rolls off the assembly line? Is that why the book says not to switch?

Thanks!
Paul

 

Welcome to FTE Paul. Thats a good question... I dont know the answers. What I DO know, is that the LT version of the tires are superior in all respects (in my oh-so-humble opinion). I fail to see any appreciable difference between the tires... maybe a couple of pounds. Im not sure if we have any techs in this section yet... so you might have to wait a bit to hear from a professional. Gotta deal with the rest of us who just play pros on tv for now

 

Thanks Rich

 

The weight and towing capabilities are calculated for your truck with either P or LT tires. So if yours came with LT and you put on P you can't in theory tow or haul as much safely. I don't see a problem going from P to LT other than ride comfort goes down IMO. I wouldn't go from LT to P though unless you truck doesn't tow or haul.

 

Berry, agree on all counts, but the question was whether the factory calibrates the electronic safety stuff (stability control etc) to match the given vehicles tire type. I agree this seems excessive, but why else woudl Ford repeatedly warn against going from P to LT metric tires (doing the reverse is obviously a very bad idea, as you say). Maybe I am chasing ghosts with this question, but it is worth exploring, as I am sure a ton of readers are interested in doing the same when the wretched OEM tires wear out, which mercifully shouldn't take too long.

 

Ah, forgot that. I doubt the stability or each trucks computer is programed based on whether the factory installed P or LT, but maybe the safety tests, etc. was done with either tire. Somehow Ford does not want to be liable for a lesser tire being installed.

 

Im thinking the lawyers forced them to put it in there... kind of like the threatening tag on all mattresses

 

l would think it has to do with the TPMS. A LT tire holds a lot more pressure than a P. You could in theory be way underinflated for a LT tire and still not have the TPMS alarm because it is calibrated for a P tire, leading to an accident (for someone that doesn't know how to use a tire gauge or visually inspect a tire).

 

By golly, I bet 89LX is exactly right here. If you put P rated tires on a truck with LT calibrated TPMS sensors, it might read low pressure all the time, and if you do the reverse, you could have one LT tire really low and the TPMS would never tell you.

But moving from P rated tires to LT rated tires will trash the ride, and increase squeaks and rattles both short and long term because the truck undergoes more vibration. I would not change to LT tires unless I was going to do some stiff off-roading that puts the sidewalls into jeopardy, or was towing and really wanted to minimize the side to side movement in the tires. But the heavier, higher pressure LT tires will also spend more time in the air when you are bouncing over a bumpy road, and skitter much worse around bumpy corners. The shock valving on a car-like F150 is probably really light right off zero to make the truck glide over tiny bumps, and putting hard, heavy tires on it is gonna cause it to ride more like an old beater truck.

George

 

I switched my '04 Expy from P to LT 3 years ago and the stupid TPMS light does come on but no more than it did when I ran the OE tires. I'm actually switching back to the P rated tires as the LT's are a lot stiffer in ride.

There are '09 owners who have switched from the OE tires to 33's and 35's and they haven't reported any difference.

Tim

 

You can run a P-rated tire at 35 PSI and a load range "E" LT tire at 35 PSI with the same effect. One has a maximum pressure of 44, the other 80.

You are not underinflated in once case and correct in another. For a give tire size it takes a certain amount of pressure to hold up the vehicle and avoid excessive tire flex, which would lead to tire failure. Just because there are more plies and thicker sidewalls does NOT make an "LT" tire need more pressure to do the same job.

LT tires have stiffer sidewalls which enable them to hold more pressure safely, which increases the amount of weight they can carry. That same 35 PSI would work just fine in whichever tire you use, so long as the size is the same. Don't get fooled by the maximum inflation pressure!

 

Your basic thoughts are correct, but I believe that running a heavy LT tire at 35 lbs is going to make the sidewall and tread flex a lot with the heavier rubber and cord and build up a lot more heat. A P-metric tire has a more supple sidewall that is designed to work with the lower pressures.

My E150 van came with Michelin LTX P-rated XL tires, with a max (and door sticker recommended pressure) of 41 lbs. These tires ride great for a given pressure. I had LT tires on it for a short while, and with 50 lbs in the LT tires, it rode rough and rattled like a school bus. With the LT's aired down to 41 lbs, it still rode much harder (and bounced around more) than the original Michelins because the sidewalls had no give, and notably lost gas mileage due to the heavier tread. I could literally feel the LT tires being very hot after a drive.

So there is still a big difference in ride quality despite the pressure. I'm also a bicyclist, and have had the experience of 100-120 lbs in bike tires--and if you buy the expensive, supple tires, it's amazing how 120 lbs in a tire can still give you a supple ride, whereas 100 lbs in a cheaper, heavier built tire will ride like a rock.

George

 

I doubt the trucks have two different pressure sensors. I assume it's one, set to go off at 25psi, or whatever it is.

 

Good post, George. I agree with most of what you said, LT tires will NOT give you a cushy ride!

I've tried to find inflation tables for LT rated tires, but it's not easy. Michelin publishes the tables for their heavy truck tires on their website, but I can't find anything on LT-rated tires.

There just seems to be lots of folks who insist that the maximum pressure on the sidewall is really what they are supposed to be at, which is only true if you're operating right at the maximum capacity for the tire.

On an F-150 you'd almost never be operating that heavy, therefore the pressure would be significantly under the maximum pressure. Anyone have LT-rated tires from the factory? Care to post your tire inflation and loading sticker?

 

haven't messed with my 09, but on my 04 FX4, it originally had 18" P-rated tires with recommended 35 psi sticker. Later I changed to the 18" LT-rated tires, and other FX4 owners with the same tire that originally came on their truck told me their recommended psi sticker said 40 psi. So I ran them at 40 psi. Sure, the LT tires felt stiffer but by no means did it completely ruin the ride or make my truck rattle. I liked that it handled much better, especially when going around curves at a decent speed and they did not squeal or the sidewalls did not flex like the P tires.