Proper tire pressure - XL rated tire

[thr_wedge]

Recently added a 2010 Expedition EL to the Fleet. It has Continental CrossContact LX20 tires with the following size/specs.

275/55R20 117S XL

Since it is an XL rated tire, should the tire pressure be higher? The tire shop set the tires to 35psi (factory spec). I don't think they say Load Range E anywhere on the tire, but Continental's documentation says 80psi max.

Should I be running a higher pressure? Any disadvantages to running a little higher PSI? Vehicle is stock. Thanks!

 

[Kuato]

If it were mine, I'd drive it awhile at around 40psi. If the corners start wearing quickly, bump it up 5 psi and repeat; if the center wears, go down the same psi.

A heavier vehicle IMO requires more pressure to keep the tire from rolling over in the corners.

 

[Wolf359]

Tire shop did what they were supposed to do. The recommended pressure is the one specified by the car manufacturer, not the max rating of the tire. The tire maker has no idea what car the tire is going to be put on. Ford is know for using a lower tire pressure for a more comfortable ride. Increasing the tire pressure will lead to a harsher ride.

 

[thr_wedge]

Thanks for the reply that is my feeling as well. I'll go up a little and observe.

Also a slight edit To my original post, the max psi is 51 according to the sidewall but for some reason continental said 80. Tire Rack also said 51.

Edit to add I don't think max psi is the way to go but wondered if being an do rated tire if I need/should incrresse the psi to take advantage of the higher load rating. Thanks!

 

[dparm]

You should run them at the pressure the car manufacturer says to.

XL just means it has a reinforced sidewall for carrying extra loads.

 

[Johnny248]

Wrong! The tire placard is for use with a tire which is the same load rating as what the vehicle was fitted with. A P metic standard load tire and a Euro Metric XL use different load tables. It is very common to see a 2-6 psi increase in pressure when going from a standard p metric to a euro XL. This is why it is a must to check load tables when change tire size/load rating. It is not always required to increase pressure. It depends on the load tables and the original psi of the vehicle. Don't' just assume that your placard is correct for the tires on the vehicle.


I'm pretty sure you originally had a P275/55r20 111. Your new tire is a Euro metric 117XL. When comparing load tables, the P275 111 is 2403 @ 35lbs. The Replacement 275 117 XL is 2436 @ 35psi. So in this case you are good. 35 cold is what you want. If you are checking them after driving, adding 4psi when setting is a good rule of thumb.

Always a good idea to check instead of guessing.

Nice ride btw!

 

[djb]

The maximum pressure discrepancy is likely because they are two different conditions. The higher pressure, 80 psi, is the maximum when seating the tire bead during installation and may be applied only momentarily with a new tire. The sidewall maximum is the limit for continuous pressure in use.

 

[DoiInthanon]

"Any disadvantages to running a little higher PSI?"

Say the gross vehicle weight is 6,000 lbs, each axle has 3,000 lbs, and each tire has 1,500 lbs. 1,500 lbs divided by 35 lbs/square inch = ~43 square inches of pavement contact at each tire.

Inflated to 40 psi - 1,500 lbs / 40 psi = ~37.5 square inches of pavement contact at each tire.

37.5/43 = 0.87 or about 13% less friction force between the tire and pavement for tires at 40 psi vs tires at 35 psi.

 

[thr_wedge]

Originally Posted By: DoiInthanon
"Any disadvantages to running a little higher PSI?"

Say the gross vehicle weight is 6,000 lbs, each axle has 3,000 lbs, and each tire has 1,500 lbs. 1,500 lbs divided by 35 lbs/square inch = ~43 square inches of pavement contact at each tire.

Inflated to 40 psi - 1,500 lbs / 40 psi = ~37.5 square inches of pavement contact at each tire.

37.5/43 = 0.87 or about 13% less friction force between the tire and pavement for tires at 40 psi vs tires at 35 psi.


You are calculating the pressure that the vehicle is exerting on the tire. The only way that PSI would change is if I added weight to the vehicle.

 

[thr_wedge]

Originally Posted By: Johnny248
Wrong! The tire placard is for use with a tire which is the same load rating as what the vehicle was fitted with. A P metic standard load tire and a Euro Metric XL use different load tables. It is very common to see a 2-6 psi increase in pressure when going from a standard p metric to a euro XL. This is why it is a must to check load tables when change tire size/load rating. It is not always required to increase pressure. It depends on the load tables and the original psi of the vehicle. Don't' just assume that your placard is correct for the tires on the vehicle.


I'm pretty sure you originally had a P275/55r20 111. Your new tire is a Euro metric 117XL. When comparing load tables, the P275 111 is 2403 @ 35lbs. The Replacement 275 117 XL is 2436 @ 35psi. So in this case you are good. 35 cold is what you want. If you are checking them after driving, adding 4psi when setting is a good rule of thumb.

Always a good idea to check instead of guessing.

Nice ride btw!




Thanks Johnny, that was what I was looking for. At what psi (in this case 35) is the load rating on the new tire (117XL) correct.

The Expy is awesome, previous owner took great care of it.

 

[DoiInthanon]

Originally Posted By: thr_wedge
Originally Posted By: DoiInthanon
"Any disadvantages to running a little higher PSI?"

Say the gross vehicle weight is 6,000 lbs, each axle has 3,000 lbs, and each tire has 1,500 lbs. 1,500 lbs divided by 35 lbs/square inch = ~43 square inches of pavement contact at each tire.

Inflated to 40 psi - 1,500 lbs / 40 psi = ~37.5 square inches of pavement contact at each tire.

37.5/43 = 0.87 or about 13% less friction force between the tire and pavement for tires at 40 psi vs tires at 35 psi.


You are calculating the pressure that the vehicle is exerting on the tire. The only way that PSI would change is if I added weight to the vehicle.
I personally over-inflate my own tires about five psi. I am willing to accept a lower friction force.

"The only way that PSI would change is if I added weight to the vehicle." If your spare is inflated to 35 psi in the trunk and you mount it on the vehicle it would still read 35 psi. But the load carried by the tire has changed from ~zero pounds to 1500 pounds. Changing the load on an inflated tire only changes the contact area. The tire automatically adjusts the contact area to keep the sum of the forces in the vertical direction equal to zero.

When I overload my pickup with fresh cut firewood, the tires squish down on hard pavement or sink deeper into the soil in the woods. Psi in the tires stays the same, just the contact area changes.

It is too early in the morning for my old brain. I have already used up my quota of thinking for today. Good luck with your new ride.

 

[CapriRacer]

Boy do I just love all the opinions being expressed - even the contradictory ones.


First: While it may be true that there is a difference in the load table between P metric and Euro-metric (and even Japan-metric if you want to be more encompassing), FOR PRACTICAL PURPOSES, the differences are trivial.

The load table is an estimate of the relationship between load carrying capacity and inflation pressure. But tires behave the way they behave and the differences in the load tables are more about the differences in the way each group did their calculations - and much, much less about differences in tires.

FOR PRACTICAL PURPOSES, if the tire size is the same as original - and in this case it is - then the pressure listed on the vehicle tire placard is appropriate.

Now some folks think adding a few psi (and we're talking 3 to 5 psi) is advantageous, and I used to be in that camp. But there have been changes in the way vehicle manufacturers determine the pressure listed on their placards, so I also think I have to change my recommended practice as well. I now use the placard pressure for any vehicle after about 2006. (yes, it varies)

The fact that the tire in question is an XL version doesn't change the load vs inflation pressure relationship.

And one last thought, but one that doesn't apply here: Be careful when replacing P type tires with LT type tires. LT type types require 15 psi more (for the same dimensions).

 

[rslifkin]

If you're not sure, chalk it and see if it's got an even contact patch. If not, adjust the pressure accordingly to get one.

 

[thr_wedge]

Originally Posted By: DoiInthanon
Originally Posted By: thr_wedge
Originally Posted By: DoiInthanon
"Any disadvantages to running a little higher PSI?"

Say the gross vehicle weight is 6,000 lbs, each axle has 3,000 lbs, and each tire has 1,500 lbs. 1,500 lbs divided by 35 lbs/square inch = ~43 square inches of pavement contact at each tire.

Inflated to 40 psi - 1,500 lbs / 40 psi = ~37.5 square inches of pavement contact at each tire.

37.5/43 = 0.87 or about 13% less friction force between the tire and pavement for tires at 40 psi vs tires at 35 psi.


You are calculating the pressure that the vehicle is exerting on the tire. The only way that PSI would change is if I added weight to the vehicle.
I personally over-inflate my own tires about five psi. I am willing to accept a lower friction force.

"The only way that PSI would change is if I added weight to the vehicle." If your spare is inflated to 35 psi in the trunk and you mount it on the vehicle it would still read 35 psi. But the load carried by the tire has changed from ~zero pounds to 1500 pounds. Changing the load on an inflated tire only changes the contact area. The tire automatically adjusts the contact area to keep the sum of the forces in the vertical direction equal to zero.

When I overload my pickup with fresh cut firewood, the tires squish down on hard pavement or sink deeper into the soil in the woods. Psi in the tires stays the same, just the contact area changes.

It is too early in the morning for my old brain. I have already used up my quota of thinking for today. Good luck with your new ride.


I understand where you are coming from. I think that you are neglecting the forces from the tire sidewall, especially when going to a higher load rated tire of the same size. In any case, with a tire this size and load rating, and a vehicle of this weight, a 3-5psi change in tire pressure most likely has a trivial but not necessarily a non-zero impact on contact patch. Empirically determining a change in contact patch with a change in tire pressure is the only way to account for the sidewall strength part of the equation.

I appreciate everyone's comments on this thread. My biggest concern was not getting the full use of the XL rating. The vehicle handles fine at 35psi. I had other vehicles that required 40psi to not wander like a drunken pig (and these were 3000lb. sedans, not 6000lb. SUVs).

 

[Kuato]

I can't agree 100% with the factory recommendations on all vehicles. Cases in point:

I had a 2004 Dodge Ram pickup purchased new. Outer edges of the fronts wore very quickly at factory pressure; once the PSI was increased a bit, even wear resulted.

On our GC, the placarded PSI causes insanely fast wear on the outside edges of the tire. When it was purchased with just over 18k on the clock, three tires needed to be replaced. I've since looked at other GCs on the road, and some have 2" or so of smooth rubber on the inside and outside edges.

I'm running the current Michelins at 42psi, and have very even wear at 60k miles.

I'm not saying the factory is wrong for every vehicle; but rather that it may not be right for every vehicle with every tire combination. Blindly following manufacturer's recommendations (hello, OCI intervals, anyone?) might be fine for the uncaring masses of drivers, and for your warranty, but is a only a good ballpark figure for us BITOGers.

 

[thr_wedge]

Quick update: I aired them up to 39PSI, will keep an eye on handling and wear. Since they are XL rated, the load rating is based on a 41PSI standard (the load rating on SL rated is based on 35PSI). I always run my sedans between 38-40 psi and get even wear, I've found that to be a good compromise between handling and ride quality.