Originally Posted by JPop

Actually the text book way would be to use the door placard data along with the Service Description of the OEM tire. The Service Description is a 3 number code as to how much weight a tire can support at a given inflation rate, generally often followed by a letter for the tires speed rating.

Using an ETRTO chart you can cross reference the supported weight of the OEM tires and find a corresponding inflation rate for the service description of your new tires. It's pretty simple, the worksheets and charts are on just about every tire manufacturers website and ETRTO charts are readily available if you want to download them.

While I don't believe this cross referencing is the end all, at least you know where the vehicle and tire manufacturer recommend that you are to match stock spring rates and promote your safety. It's also important to note in the case you have any warranty issues with your tires.

I use the ETRTO charts to find my initial inflation rates and then move on to the chalk test that is the favored way to determine inflation rates by so many on this forum. Generally this will tend to be lower than the ETRTO charts suggest and I use the difference as the window of where I want my tires to be, promoting ride quality and handling.

Some description on what the chalk test entails. Use a piece children's sidewalk to draw a line across the tread width. Move your vehicle far enough to get a few rotations of the tire and inspect where any chalk remains. Chalk remaining on the outside of the tread suggests you should reduce your inflation rate while chalk remaining towards the center suggests you should increase your inflation pressure.

As for off road use, this is mostly a matter of personal preference but most will find a comfort level with between 12 to 20 psi, often lower with bead locks. Different tires will respond differently and for example on my former Toyos I would drop them to around 12 psi because of the extra sidewall support and my Goodyear MTRs with Kevlar I run at 16 psi.

Hope that helps!

Originally Posted by jeepmedic

WOW !! You can always count on Jpop for a great factual response !

Originally Posted by ksk

I run mine at 40 psi...purely for fuel economy reasons...have been running this pressure for 2 years now and the wear is pretty even...jm2c

KSK

Originally Posted by geebee

i usually run mine (35s) around 28-30 psi on-road/higway. offroad i run around 10 psi stock rims. everybody running 15psi or more offroading needs to drop it down and see the difference.