Replaced 109S with 114T; pressure recommendations

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2008 Ford F150, OE is 265/60R18 109S (truck capacityy 2065 lbs). For highway passenger use only, Discount tire recommended I replace the tires that were on it, a BFG K02 in 265/65R18 with the same size in a highway tire; I installed Defender LTX M/S.

I'd like a somewhat softer ride; it's 98% empty except for me and a passenger and an aluminum toolbox with maybe 100 lbs of stuff.

As I look at the tables for the non P-metric tire, it looks like 31 psi is "equal", truck capacity 2088 lbs. If it were a P-metric, it looks like the rated load would be carried at 27 psi ... but this is not a P-metric tire.

Is 31 psi a good thought? Why are P-metric able to carry more load at the same pressure vs "E-metric"?

NOTE that I've made the 1.1 correction factor for real loads, but comparing doesn't really require that.

Separate question; what is the rolling resistance relationship to pressure? Will I use more fuel at 31 psi than at 35 psi? In the old days, I would have unquestionably said yes, but with today's technology?

George
 
The general principle is to match the load carrying capacity of the original tires at the specified inflation pressure - in this case, a P265/60R18 109T at 35 psi. The new tires - 265/65R18 114T are larger in load carrying capacity, so the next step kicks in:

If the new tires have a larger load carrying capacity at the specified pressure, then rather than use a lower pressure, it is better to match the specified pressure as that will make the tire more closely match the original spring rate.

So I recommend using 35 psi.

Oh and a nit to pick! The BFG's were LT type tires, where both the original tires and the new tires are P type tires. That means they were different *kinds* of tires and I don't think you can say they were *the same size* without leaving out some very important information. Because of this, I always include the letters (or lack of letters) as part of the *size* - and in this case, I would say the BFG's are a *different size* than the the new Michelins.
 
Originally Posted By: CapriRacer
Oh and a nit to pick! The BFG's were LT type tires, where both the original tires and the new tires are P type tires. That means they were different *kinds* of tires and I don't think you can say they were *the same size* without leaving out some very important information. Because of this, I always include the letters (or lack of letters) as part of the *size* - and in this case, I would say the BFG's are a *different size* than the the new Michelins.
I bought the truck from our son knowing it needed "some things", tires among them. He'd put the LT on for "looks" and told me they rubbed occasionally. I "bought" online OE size in the defender LTX M/S, but when I got to Discount Tire to have them installed, their guy said if it were his, he'd use the 265/65 "like you have now" because they'll look better and give slightly better fuel economy with the 3.55 axle ratio. He had both in stock and didn't add the $4 per tire different price. I mentioned the rubbing our son mentioned, and he said those LT were larger in the corners, and that the highway tires would not rub. I took his advice. I'll also take yours and reset to 35 psi.

You mention spring rate. Do you have an opinion on spring rate of a 60 vs 65 series tire (no other changes) at the same inflation pressure?

How's retirement?
 
Originally Posted By: George Bynum
....... You mentioned spring rate. Do you have an opinion on spring rate of a 60 vs 65 series tire (no other changes) at the same inflation pressure?

How's retirement?


a) The best info I have found says that the spring rate of a tire is very dependent on inflation pressure and barely affected by anything else - size, brand, even tire type (LT vs P)

b) Retirement? Once I sold the house, I found that without deadlines, I lost a lot of incentive. It's no wonder some folks don't last long after they retire. Luckily I have a short attention span and an insatiable curiosity. So I am teaching myself to cook. Every day is a new dish to conquer - which is always a challenge as I don't ever do it the same way twice.
 
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