I just put LT 285/70/17 Nitto terra grapplers on my 09 crew. It's an SLT.
The guy who mounted them said to keep @ 35 PSI which is what my door sticker says for the 20's that came on the truck. The LT's have a much higher PSI on the side walls @ 80 PSI MAX. I know it's relative to the vehicle weight what you ultimately run them at.
I felt the tires where too squishy for my taste so I went up to 40 PSI, that was better but still a little loose. Drove it a bit and decided to try 50 PSI. Seems good but a little harsh when you hit a bump or road seam. Thought I felt the truck jump a bit in the rear when I hit a small pothole. I figure somewhere between 40-50 is the right range at least for ride quality.
What is the best path to follow in this case. Digging thru the search function nets two answers. keep them at 35 PSI or go 80% of MAX, which in this case is around 64 PSI.

Second question is the tires are about a half inch wider than the 275/60/20 that came with the truck. I could fit my thumb between the top of the front tire and the suspension. Now its barely a pinky width away. How close is too close? I see there are quarter inch or so spacers available. Any pro's or cons with these slip on spacers? I figure it will put the tires right back where they need to be.
Thanks

 

I ran across this method awhile ago (note that the numbers below in no way represent your tires or truck):

(lifted from here)

 

With tires that have more sidewall common sense would say to me to keep them a few pounds harder, if you whack a curb you could break the bead and more or less get a blow out, with more air the tire should stay on and just bounce you away without damage....should now depends on tire, how thick the sidewall is. Lots to think about. All I know is I like the look of the 20's and well the ride and handling is nice as long as you keep ah HP tire on it so more or less light truck street tire, which I plan to put a set of nice sport style tires on or summer next year, not sure if I am gonna go with 17" rims for summer and keep the 20's for winter. Goes against what I said at first...maybe get rid of 20;s all together? Sure would make for a cheaper tire.

 

I plugged my numbers in below.Here's what I came up with:

tire rated at 3750 lbs @ 80 psi and a vehicle that weighs 6400 lbs.

Step 1 - Multiply the max load times 4. This is the maximum weight your four tires can safely carry.
3750lbs * 4 =15000lbs

Step 2 - Divide the weight of the vehicle (placard on driver’s door jam) by the number you got in the first step. This is the percent of the max load you are actually carrying on the tires.
6800/15000=45.33%
Step 3 - Multiply the psi stated on the tire for max load by the percentage you got in step 2. This will give you an optimal pressure for you tire on the road.
80 psi * 43.33% = 34.66 psi
Lets say where back at the original 35 PSI.

Anyone else running this size tire on a 1500? What are you doing with the tire pressure?

 

on a higher load tire (than factory) i usually run around 7 - 10 psi above factory spec.

but make sure to check tire wear fairly often to make sure they are wearing evenly. then adjust from there

 

2daks is there any way you could post pictures? I've been looking at the same tire and my questions are the same as what you are asking.. my problem is ii'm a visual kind of guy math and science weren't my strong point in school. do you plan on doing any type of lift or level kit with the new tires? when i got under my truck i noticed to how close the upper was to the tire that was my main concern to that if i went with a bigger tire would it rub. 1/4 spacer would bring the rim out but wouldn't that put more stress on the stems rather than the hub itself. what you have asked are all valid Q's hope you get feedback.

 

Edited my math in above post. I was looking at kg instaed of lbs and it thru numbers off.

 

Unless I hear different I'm thinking you're on it. 50lbs feels too harsh and the 35 was way too soft. i'm airing down to 45 and see what that does. that will be 10 above placard for the original tires.

 

You're using tires that are outside the factory load range, so the door pressure doesn't apply any more. In general a tire that has an 80 psi Max should not be inflated below 50psi. Unfortunately it doesn't look like Nitto provides any detailed specs on their tires - normally a manufacturer would call out a min pressure as well as a max.

They are going to ride harsh, they are a truck tire - not a big passenger tire. Unless you are towing heavy or hauling heavy you would have been better off with the P285/70/17

 

I'll try to get some pix. I just looked closer by crawling under the front bumper and all looks good. There is a good amount of daylight and no signs of rubbing on the sidewalls. I'm questioning the spacers as well now, because I believe the rims are hub centric. A spacer would remove it off the hub.