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Wednesday, February 15, 2017

Tow vehicle tire question

Dear Roger,

"Last week I purchased new E rated tires for my F150. Went with the BF Goodrich KO2 (275/65/18) and so far I'm very happy with them. These are larger than the tires that came from the factory (265/60/18). I have a quick question.

"The tire shop originally put the PSI at 35 based on the door sticker, but after reaching out to BF Goodrich they suggested 45 PSI for everyday driving. They didn't give any recommendations for tire pressure while pulling a trailer since there are so many variables like trailer size, weight and cargo. The tire shop suggested going to 50 pounds and then deflating back down after the trip. I'm just curious if others actually bump up their PSI while towing, and have you noticed a difference in doing this? Just trying to figure out if there is a benefit to doing this.

Some important information is missing, so let's identify our assumptions:

1. We are talking about the tow vehicle tires - the F150.

2. The original poster didn't identify the OE (original equipment) tires as being P type or LT type. This makes a major difference and providing that information up front will eliminate guesswork for those trying to help with the answer. With a 35 psi door jam sticker number, I suspect the OE tires are Passenger type and the replacement tires are LT type.

3. Since it is the air pressure and not the tire construction that supports the load on a tire, buying LR-E (80 psi) tires when you never run higher than 65 (LR-D) would be a waste of money. Also if you are changing from P type tires with a max inflation of 36 to 41 you need to confirm the wheels are rated for higher load and inflation of LT tires.

4. The door jam pressures are based on the car company making some estimates on how much and how often you have the vehicle empty or fully loaded. You might check the owners manual and see if they give an empty and loaded tire inflation suggestion. The inflation in the OE tires is what is needed to support the GAWR but few people run that heavy all the time with their pickup.

5. I suggest you get the F150 on some truck scales when empty and again when fully loaded with the trailer also fully loaded, and learn the real facts of the various axle loads under both conditions.

6. Knowing the real loads, you can use the tire Load & Inflation tables to learn the MINIMUM cold inflation pressure for the F150 for the two situations. I suggest you use a margin of +10% of the table inflation to establish your CIP (cold inflation pressure). See other posts on weight and inflation if you need more details.

7. Trailer tires are a completely different situation and should always be inflated to the tire sidewall inflation. You should also run no more than 85% of the trailer tire max load rating when on a scale. Cornering, sway and side wind loads have been shown to shift loads side to side by 10% or more. Also the Interply Shear on tires in trailer application needs to be considered as this is a major contributor to trailer tire belt separations.

Hope this info helps clarify.

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  1. Also note that a larger tire (bigger "rolling radius") than a stock tire can cause your speedometer to read lower than your actual speed. In some situations, this could result in a speeding ticket. Check what your actual speed vs indicated speed is using a GPS, or measuring the time between mile markers on a highway.

  2. I also note that when towing my rear drive wheel tire inflation runs 4 to 7 psi over the opposite non drive wheel side. After the drive wheel temp stabilizes (about 25 miles of travel) I adjust this pressure to be with 2 pounds of the non drive opposite side. I do this as I was seeing over inflation tire ware on the drive tire only. Because of premature center tread wear, the drive tire was replaced with new and the spare put down to start with 2 new rear tires. Also I noted with my tire laser temp gun this drive wheel temp runs about 5 to 8 degrees F above all the other (3) tires. I note too that the vehicle tires facing the sun on very hot days run higher temps than the non sunny side vehicle tires. I run OEM M+S P275/65R18 /114T Radial tires. Factory pressure recommendations are 35 PSI. I rotate tires every oil change (5000 mile). Tire wear is very balanced now with no premature drive wheel wear. I also run the TT500 tire pressure monitor on the trailer tires.


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