QuestionQUESTION: I recently upgraded my tires to nitto terra grappler 325/60r18 on my 2009 chevy 4x4 ext cab.Nitto says run 35 psi as in the door sticker and this seems ok on the front but..on the rear tires the entire tread is not touching the road.all except about an inch on the outer and inner edges. the tire load range is d abd i never haul anything and the tire max psi is 50 psi.it seems that i would have to deflate the rear tires to about 25 psi to run the full footprint buit i dont wanna underinflate and cause a failure.I just want to get good even wear.What do you recommend for front and rear tire pressures for my truck?
ANSWER: Mike,
Let's start with the basics:
On every vehicle sold in the US, there is a sticker – commonly called the tire placard - that lists the original tire size and the proper pressure for that size. The placard is usually located on a doorpost or in the glove box – but sometimes it is located in the trunk or on the fuel filler door.
Please note: this is from the vehicle manufacturer, not the tire manufacturer.
BTW, it doesn't matter who makes the tire or what pressure is listed on the tire's sidewall, if the tire size is the same as the placard, then the pressure listed on placard is also appropriate.
If you are using a tire size that is different than what is listed on the placard, then the pressure has to be recalculated. The calculation is not difficult, but it requires tire load tables which are not allowed to be published in the Internet because of copyright laws. However, I have a copy and would be glad to do the calculation, but the starting point is the vehicle placard: Tire size and inflation pressure.
I have a book called Tire Guides that summarizes vehicle tire placards. Unfortunately, there are so many different versions of your truck, it is difficult to determine what the original tire size was.
Assuming that the 18" wheel was original to the truck - and the vehicle tire placard says P265/65R18 at 35 psi, then a LT325/60R18 (notice the "LT" in front of the tire size) needs to carry ..... well .... uh ..... the calculation says it should be less that 35 psi, but I am uncomfortable with specifying anything less that 35 psi for an LT tire because it requires a certain amount of pressure to hold the tire onto the rim when side forces are applied - and for LT tires, that limit is 35 psi.
But there may be another problem.
If you are using stock 18" rims, then Tire Guides says they are 8" wide - and an LT325/60R18 requires a minimum of a 9" rim. If a tire is mounted on a rim that is too narrow, it will arch across the tread face and may make the tire wear more in the center, as well as make the vehicle tend to follow the ruts and grooves in the pavement and be sensitive to side winds.
So you need to check your vehicle tire placard for the original tire size - AND hopefully you have aftermarket rims that are at least 9" wide.
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QUESTION: Im sorry I didnt leave enough info on my truck but it originally came with 265/70r17 tires...the dealer added a 6' lift.These nitto tires are considerably wider and maybe thats the issue.The rim is an 18x 9.5.This 325/60r18 is about 12.8 wide.I saw that you said you dont recommend less than 35 psi so what can i do to get the full footprint of the rear tire on the ground? I don't wanna have them cupping out and it seems if i stay at 35 when im empty that it might.Im guessing that the narrower stock tire is definitely flatter running at 35psi?.But like I was saying when I leave work and the powderey dirt is on the tires you can clearly see after just a short distance only the middle of the rear tire is wearing.about an inch of the inside and outside lugs never touch the pavement.Im so confused!
AnswerMike,
I think your largest problem is going to be alignment. A 6" lift and a wider tire and rim with more offset is more than enough to cause bigger problems than the simple air pressure could.
Put another way, any wear issues you get will more likely come from the alignment and the suspension modifications and not the inflation pressure. The fact the tire tires don't appear to be in full contact with the road suface is just the tip of the iceberg compared to the problems these other things could cause.
But if you want to alter the profile of the tire, I would suggest you use a wider rim. Something like an 11". The 9 1/2 is a little on the narrow side and may be part of the problem.
