Last year, I needed to replace the tires on my LC, so I decided to upgrade. The OEM 31 inch tires (P275/60R18) got swapped out for 32” Michelen Defenders (275/65R18). I remember the suy at the shop telling me the inflation psi the new tires needed, but I didn’t write it down. Some number in the low 30s stuck in my mind. So I’ve been inflating front and back to about 32 psi since then.
A recent thread in a 100-series FB forum made me rethink this practice. It pointed out that different sized tires require different target psi fills for the same load, and that if front and back psi measures are different on a rig it’s because the loads on those axles are different.
Here’s a pic of the sticker on the DS door frame. Notice two things. First, the OEM tire was a P275/60R18. Second, those front and rear tires have different target psi targets.
What should my target psi be for my new tires? One commenter pointed to this document from Toyo Tires to help explain how to calculate the pressure a set of tires needs. After finding the document hard to read (I could figure out how to calculate the pressure for my new tires), I Googled and found this Replacement Tire Pressure Calculator tool.
Using my tire information as input, it calculated that my new tires should be inflated to 27 psi in the front and 29 psi in the rear. I was (and am) surprised that the pressure is less! There’s probably some basic physics principle that I’m forgetting that will make sense of this.