Picking tire pressure help

[jayhaitch]

I replaced my wimpy P-series OEM tires with Pro-Comp "All Terrain" model tires, OEM size LT265/70-17. Very nice. Surprisingly quiet, much better traction, ready for hunting/fishing/winter, but more rolling resistance. My 01 Expedition slows faster letting off the gas than before. So.....

Am I inflating them too low? They're load range E. Sidewall say 80, yes EIGHTY psi max. Yikes! Isn't that semi-territory? I've got them at 45 psi all around, with good steering control, not mushy, only slightly harsher ride. The tire dealer said go up to 50 psi when towing. Ok, fine, but.....

How do I know what's the best? I could wait 10,000 km and look at tire wear, then adjust pressure. But is there any other method, test, magic spell that works?

My 94 2-door Yukon had Michelin LTX LT's, 40 psi front and 35 rear, but it was a short, light truck. Tires seemed to wear evenly with this setup.

 

[RayH]

I replaced my wimpy P-series OEM tires with Pro-Comp "All Terrain" model tires, OEM size LT265/70-17. Very nice. Surprisingly quiet, much better traction, ready for hunting/fishing/winter, but more rolling resistance. My 01 Expedition slows faster letting off the gas than before. So.....

Am I inflating them too low? They're load range E. Sidewall say 80, yes EIGHTY psi max. Yikes! Isn't that semi-territory? I've got them at 45 psi all around, with good steering control, not mushy, only slightly harsher ride. The tire dealer said go up to 50 psi when towing. Ok, fine, but.....

How do I know what's the best? I could wait 10,000 km and look at tire wear, then adjust pressure. But is there any other method, test, magic spell that works?

My 94 2-door Yukon had Michelin LTX LT's, 40 psi front and 35 rear, but it was a short, light truck. Tires seemed to wear evenly with this setup.

I would start with the pressure listed on the inside of your door jamb.
The pressure on the side of the tire is the pressure that should be in the tire with its max weight capacity on it. Your Expedition doesnt weigh anything near the max load rating of the tire.
Tire air pressure is all about how much PSI it takes to properly support your vehicles weight. When the tire manufacturer stamps the pressure on the side of the tire, they have no idea how much your Expedition weighs. Ford, on the other hand, does know, so they put the proper PSI in the door jamb. The kicker is that you went up in tire size, which means you have to reduce the PSI in proportion the increase in volume of air in each tire.
Anyway, the best thing to do is start with Fords recommendation and a piece of fat chalk. Draw a line across the tread and drive back and forth down the driveway. When the chalk wears evenly across the whole tread, your golden.

 

[schmism]

the proper way to determine correct tire pressure is to look at the contact patch.

to do this youll need to chalk a section of tire (take your kids sidwalk chalk and color the tire in a 2-4" wide strip across the entire tread.

then drive a short distance. the part of the tire that contacts the pavement will rub off the chalk. No chalk in the middle but on the edges = over inflated
no chalk on the sides but not the midddle = underinflated.

that will give the the pressure for everyday driveing. If you load up the truck with weight, youll need to add pressure.

On my toyota i run 31x10.5.... the Kumho's i have have a load range C ... max pressure 50psi... which is about 15lbs over door jam. But ive run 45-50lbs on the street for the past 5 years and have yet to see any negitive affects of it (my wear patten is fairly even), other than the ride is a bit more harsh....

 

[George2615]

I agree that you will have to look at your contact patch. I think if the tire sidewalls say 80#, you should be close to that number. At 45# the tires will wear quicker. Another factor would be how wide are the rims they are mounted on. That will make a lot of difference as to the contact patch on the road. A wide rim may require more air to get a nice print on the road.
My current 07 3/4T Dmax tires are set for 60# front and 80# rear as recommended. This thing rides like a Caddy to me compared to an 85 3/4T I owned years ago. The 85 truck would jar your teeth loose hitting a bump.

 

[mlauk]

I run 60 psi on the front and 50 psi on the back when I'm unloaded on my 2003 Dodge 2500 because of the weight of the diesel. Heavily loaded, I run 70 psi on the front and 80 psi in the back because the weight of the gooseneck. Just replaced the last 2 original stock tires at 75,000 miles. The other 2 original tires were replaced as a set when I got a screw on the corner of one of the tires that couldn't be fixed somewhere in the 60K to 70K range.

 

[RobertN]

Isn't your rig basically a 3/4 ton? If so, the E rated tires are max at 80, but you door probably says something like 65psi front/80 psi rear. That is a max load, with a lot of gear in the back and a trailer in tow.

If you look in your manual, there should be info for a lower pressure at a lighter load.

My RAM2500 with Cummins is 65 front/80 rear, but for no/light load, I can run 50psi.

Making the change to a bigger tire, the tire shop ran it through a splash of water, then drove forward and noted the mark it left(similar idea to the chalk method. My new tires are bigger, D rated, and run a lower 45psi unloaded.

In an older case, I ran my old Jeep CJ5 waaayyy low. It was an early one that only weighed about 2400lbs. I ran C rated 33x12.50 radials, but only at 18psi. The tires were rated at 35-40(I cant remember for sure). Anything more than 20, and it bounced and bucked and went all over the road.

 

[jayhaitch]

Great tips! Chaulking the contact is an excellent idea. I think my 8 year old son will think it's way cool to use his little sister's sidewalk chaulk for manly truck type things

I couldn't use Ford's psi rating, because they recommended only 35 psi on the P-series OEM tires, and the Pro Comps are LT tires. Also, 265/70-17 is the OEM size. I didn't go to a bigger size. I do notice more of a sidewall bulge now, which is another observation that prompted my posting. I didn't need load rating E tires, that's just what this model is. I was actually surprised to find out they're E's, when the tire guy was surprised at the 80 psi max rating.

I hear you about weight of vehicles and bone jarring ride. I had an 89 Jeep YJ that had off road Bilstein shocks. Great hunting/fishing/winter vehicle, tooth chipping ride when going over bumps. Our aweful potholled Manitoba roads made my vision blurry.....shaking my head so much going over rough pavement with that thing I thought I was a bobble-head.

 

[Bird]

It's been over 15 years now, so maybe some things have changed but . . . . this appears to be an old story.

Back about 15-16 years ago, some GM 3/4 ton vehicles, especially 4WD Suburbans and pickups came out with Uniroyal Load Range E tires, with 80 psi on the sidewall. But the sticker on the door post recommended as low as 45 psi. Well, a number of those tires developed "bubbles" when the rubber separated from the cords. Uniroyal did warranty them, and we replaced a few under warranty, but they also said, at that time, that it was GM's fault; that you should NEVER run highway tires lower than 80% of the maximum listed on the sidewall; i.e., if the sidewall says 80 psi, never go below 64 psi.

I still follow that rule; your mileage may vary.

 

[john_bud]

I agree with Bird!!! X2!!

You should run them as High as your kidneys will stand. Low pressure allows the sidewalls to flex. Flexing heats them up and makes them fail, usually a sidewall blowout on the freeway. Been there, done that - have the bacon strips in the underware to prove it. Didn't like it.

Either run them at 80 all around or 65/80 or 65 all around. I run mine at 80 all around and enjoy better mileage.

jb

 

[wushaw]

First you need to know what your vehicle weighs on each axle then look at the manufactures tire chart and adjust your psi accordingly.

And no you are not into a semi psi range...100 - 125 for my 22.5 lo pro on my Pete.

 

[RayH]

First you need to know what your vehicle weighs on each axle then look at the manufactures tire chart and adjust your psi accordingly.

And no you are not into a semi psi range...100 - 125 for my 22.5 lo pro on my Pete.

Exactly, the vehicle manufacturer helps you by putting a sticker in the door jamb. They have already weighed your vehicle so they know the proper PSI.

 

[jayhaitch]

100-125 psi for semis? OK, so I was little off How long does it take to fill up an empty semi tire? Me thinks my air compressor would wilt at the thought.

RayH: I understand your point: do what the factory says. But as I said, Ford spec'd P-series tires, evidently for soft on road ride and mileage. This is a half ton, not 3/4 ton. It's an F150 chassis, shorter but heavier. Curb weight is ~5400 lbs, so with me, a load, full tank, etc, I'm sure it's around 5900 lbs. But that's still only 1475 lbs per tire. At 80 psi, the sidewall says the ProComp All Terrains will hold 3195 lbs. No way should I approach 80 psi with my current loads.

Now, as an LT tire, and especially load range E, it's got a much stiffer sidewall and more tread belts. So I've got to increase the pressure from that of the P series. Thanks John Bud and Bird for reminding me about heat and side wall blowouts. I've never had a heat blow out. I've ran over nails, and had a slow leaking tire come off the rim hitting a snow rut in -30C weather, but never a high speed sidewall blow out, thank the Lord.

Wushaw: the Pro Comp web site doesn't have any chart for varying psi for varying loads. Is this something common in the OTR trucking world?

I've noticed a bit worse mileage on my trip computer since the tire changeover, plus the more rapid deceleration, so I agree 45 psi is too low. That's what got me thinking I needed to request the expertise of my TBN knowledgeable compatriots.

I'll try 60 psi, and see what happens.

I'm getting this vehicle ready for a 7.5hr trip in Sept. Four guys, a full load of gear. Driving to Thompson Manitoba (55.8 deg N latitude) then flying 3.5 hrs in a float plain to a hunting/fishing camp on the border of Nunavut and Manitoba. Caribou hunting. The trip of a lifetime!

Thanks everyone for your help.

 

[RayH]

You shouldnt be so quick to assume your mileage has decreased.
You went up in tire size, Im assuming you havent changed speedo gears, so the trucks computer doesnt know exactly how far youve gone now. Example, if you increased tire circumference by 10%, your speeoometer and odometer will be off by 10%. If it says youve traveled 100 miles, youve actually traveled 110 miles. Your computer knows your burning X amount of fuel in a given time but you are traveling further in that time than it thinks you are.

 

[jayhaitch]

I bought the same size tire as OEM. Original tire: P265/70-17 Goodyear RT/S. New tire: LT265/70-17 Pro Comp All Terrain. Speedo is reading the same.

 

[RobertN]

Bird,

Could you comment on air pressure vs rims? I seem to remember you talking about this before; putting E tires on rims rated for C tires, with rim failure from higher pressure.

I think it was a discussion on RV/5th wheel rims/tires.

It's been over 15 years now, so maybe some things have changed but . . . . this appears to be an old story.

Back about 15-16 years ago, some GM 3/4 ton vehicles, especially 4WD Suburbans and pickups came out with Uniroyal Load Range E tires, with 80 psi on the sidewall. But the sticker on the door post recommended as low as 45 psi. Well, a number of those tires developed "bubbles" when the rubber separated from the cords. Uniroyal did warranty them, and we replaced a few under warranty, but they also said, at that time, that it was GM's fault; that you should NEVER run highway tires lower than 80% of the maximum listed on the sidewall; i.e., if the sidewall says 80 psi, never go below 64 psi.

I still follow that rule; your mileage may vary.

 

[RayH]

I bought the same size tire as OEM. Original tire: P265/70-17 Goodyear RT/S. New tire: LT265/70-17 Pro Comp All Terrain. Speedo is reading the same.

My mistake, I thought you had gone bigger in size.

 

[wushaw]

Jay here is Goodyears website on tire pressures and weights. It is not just big truck tires. And it depends upon which semi tire you have, it takes me longer to fill my rear tractor tires than it does to fill my 22.5 lo pro tires.

The vehicle manufacture sticker on the door jamb is a good place to start with the weight of the vehicle but a CAT scale will tell you exactly what it is.

http://www.goodyear.com/truck/pdf/databook/5113_Lsec_V.pdf

 

[Flywheel]

Jay here is Goodyears website on tire pressures and weights. It is not just big truck tires. And it depends upon which semi tire you have, it takes me longer to fill my rear tractor tires than it does to fill my 22.5 lo pro tires.

The vehicle manufacture sticker on the door jamb is a good place to start with the weight of the vehicle but a CAT scale will tell you exactly what it is.

http://www.goodyear.com/truck/pdf/databook/5113_Lsec_V.pdf

Yep - follow Wushaws advice.

weigh your axles and check the charts - then fill accordingly - remember to add about 10psi to the number for sustained high travel at 65plus.

Start with the chart plus the 10psi - these are the minimum pressures - then adjust up for comfort and control. I tend to like mine on the tight - maybe a little rough (still talkin' tires here) side.

You can throw out the recommended tire pressure on the door jamb now - and most rims can likely take the 80psi - if you ever need that. - you'll lprolly be ok between 55 - 65ps - prolly equal all the way around - unless your going to tow heavy - then bump the rears.

 

[jayhaitch]

Here's a follow up.

I weighed my Expedition on a truck scale: 2954 lbs front, 2825 rear, 5801 lbs total with full tank, otherwise empty.

Expedition front GAWR: 3500 lbs, or 1750 lbs per tire.
Expedition rear GAWR: 4128 lbs, or 2064 lbs. per tire.

In looking at the Goodyear tire chart (excellent resource, thanks!) LT265/70-17 single axle have a load rating of 1890 lbs at 35 psi, 2075 lbs at 40 psi. If I add 10 psi for 66-75 mph driving, I'm at 45 psi front, 50 rear, to reach GAWR's.

Now this chart has no load range E LT265/70-17 tires, only load range C. I'll need to run E's at higher psi due to stiffer construction with more plys. Even with load range C, at 50 psi the load rating is 2470 lbs per tire, or more than my GAWR's. So if I bump things a bit higher than this for the E plys, I've been using 55 psi all around. So far so good, but it's only been around 2000 miles.

I go on my caribou hunting trip in 2 days. I'll put another 1000 miles on in the next week, carrying a full load of guys and gear. I'll post back when I've returned.

 

[Rara Avis]

Here's a follow up.

I weighed my Expedition on a truck scale: 2954 lbs front, 2825 rear, 5801 lbs total with full tank, otherwise empty.

Expedition front GAWR: 3500 lbs, or 1750 lbs per tire.
Expedition rear GAWR: 4128 lbs, or 2064 lbs. per tire.

In looking at the Goodyear tire chart (excellent resource, thanks!) LT265/70-17 single axle have a load rating of 1890 lbs at 35 psi, 2075 lbs at 40 psi. If I add 10 psi for 66-75 mph driving, I'm at 45 psi front, 50 rear, to reach GAWR's.

Now this chart has no load range E LT265/70-17 tires, only load range C. I'll need to run E's at higher psi due to stiffer construction with more plys. Even with load range C, at 50 psi the load rating is 2470 lbs per tire, or more than my GAWR's. So if I bump things a bit higher than this for the E plys, I've been using 55 psi all around. So far so good, but it's only been around 2000 miles.

I go on my caribou hunting trip in 2 days. I'll put another 1000 miles on in the next week, carrying a full load of guys and gear. I'll post back when I've returned.

If you will be doing alot of offroading...then read the following...
Beefing Up Older Axles: Upgrade 'Em or Break 'Em - Ford at Off-Road.com