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Go to the dealer or online and get the pressure/load carrying chart for the tire. Then you can see how low you can go for highway. When I ran 37 KO2 E range tires, the Jeep was only about 4,200 pounds, and I could go as low as 16 and still be rated for the road. I normally ran 24 psi most of the time.
Once you find how low you can go, you can experiment with different pressures until you find the best one. My GUESS is around 28.
With all due respect, 20-25 for off road is way too high. I do a ton of off road in Jeeps and RZRs and now the Raptor. Even with my C range 35s on the Raptor, I go down to 14-15 off road and they are fine. The reason to deflate is two fold, one a more comfortable ride as the sidewalls flex more and absorb the bumps better and two to increase the tire patch on the ground (for climbing obstacles). If you do research on tire patch, you will find out it is a geometric curve. Very little contact patch change for a long time (like from 38 to 25 PSI) and then it starts to increase geometrically. The only problem with going lower is the possibility of "blowing" a bead. That is why rock crawlers use bead lock wheels. I have never seen anyone blow a bead unless they had less than 10 pounds in their tires. I can't go below 14 or so in the Raptor because the tires are C rated (very flexible side walls) and the truck is heavy. When you come down a steep rock face and the truck is pointing down, all the weight is on the front tires and if you go too low they can wrinkle up to the wheel, which is not good. Stiffer sidewall tires, like load range E tires, need to go down lower in PSI to increase the tire patch area, and can handle the weight better. The only other thing you need to be aware of is that at higher speeds, the tires flex faster that can build up heat and cause damage. The rule of thumb with rock crawlers, is when the tires are deflated they generally don't exceed 45 mph for any sustained time. So the pressure you can use for road and off road is determined by the load range of the tire, and the weight of the vehicle. I run my RZR at 6.5 pounds with out bead locks and it does fine off road, 14 on road. The Raptor 34 on road and 14 -15 off road. My 6,000 pound Jeep with C range 40" tires and double bead lock wheels, 28 on road (which is does not do much of) and 11-12 off road.28 seems excessively low. I deflate my 37's to 20-25 when off-road. For regular highway use, I put them to about 48 so they are just over 50 when hot.