Here is the document from Bridgestone/Firestone, it lists all kinds of sizes and how much pressure you have to run if you go with LT tires...just look through to what size you are using and compare to the OE size at OE pressure....
just for insight the TJ rubicon came with load E MTRs, i ran those on my TJ. air them down to around 30-32 for a daily driver and they wore like iron. hit the trails, aried them down to 18lbs and the sidewall flexed nicely for what it was.
a load E tire on a WK is probably alot better of a ride than one on a TJ.
Yep that is the more correct way to do it, since a P tire is supposed to be de-rated when used on an SUV or truck.
FWIW, as an experiment I inflated my stock p245/65-17s to 40 psi and there was not much difference in stiffness. And with an LT 245/70-17 you have a taller sidewall which normally gives a softer ride than a shorter sidewall. So it really depends on what you need. If you need the puncture resistance that an LT tire can give you that is what you need. If not then you can go with a P rated All Terrain like the Grabber AT2. I just wanted people to know, that running LT tires on a vehicle that came with P tires at the psi specified for the P tires could be dangerous. Unless you monitor your tire pressures after highway driving in hot weather you'd never know that though.
When I was investigating this on the ZJ, I used an IR temp gun to take readings of the tread temps after driving at highway speeds for an hour in 90* weather. I found that 40 psi gave me the same temps I had on P rated tires at 36 psi.
The reason why this is true, is that the LT tire has thicker tread and sidewall (which is what you want for off road use) but that requires more air pressure for it to keep its shape and support the weight than for a P tire. Just look a the difference in weight. Using a 245/70-17 size, a P rated Grabber AT2 weights 40 lbs, a Duratrac weighs 46 lbs and a three ply sidewall BFG AT weights 48 lbs. That's a big difference.
Well I have not tried LTs yet on the WK but on my ZJ, I went from a P235/75-15 @ 36 psi to an LT235/75-15 (C rated) @40-42 psi and there is a slight increase in harshness (keep in mind the ZJ has the live axle up front and it also has Bilstein HD high pressure gas monotube shocks) but nothing that you'd find terribly objectionable. Its a Jeep, its ok for it to ride stiffer than a car.
A lot depends on the tires, if you read enough reviews you will come to the conclusion that the Duratrac sidewalls are not as stiff as the 3 ply sidewalls of the BFG AT, so of the two it will probably ride better. Do you really need the heavier construction of an LT tire? If not, then you can stick to OE sizes in P rating or, if you want more clearance, get the General Grabber AT2 in the same size (245/70-17). In P sizes they are pretty soft riding tire. It all depends on what you are doing with it.
We used the Grabber AT2 in the LT 235/75-15 size on the ZJ and aired down on the beach they do great. But on the beach you don't really need super strong sidewalls as you might with rocks, tree roots etc depends on what you do. If you are really going off road and there will be hazards like sharp rocks, roots etc, then you need the stiffer sidewall.
Keep in mind that the size change is a bit more than 3%, if you have QD-II (electronic locking diffs) you want your spare to match.
Looks like I'm gonna bump my 245/70r17 E load Duratracs up to 40 psi for the perfect tire pressure. I've had it at that before and they really didn't feel THAT much stiffer than at 35 psi or my stock Goodyears at 35 psi.