QuestionRecently I noted that our tire pressure recommendations on our 2009 Suburban for P265 65R18 tires is for 35 PSI (door panel placard). But the tires are stamped for 44 PSI cold. Discount tire said to run them at 35 PSI, but I see on the internet where gas mileage can be affected AND underinflation is associated with rollovers. (course the latter is on environmental websites--hard to find objective advice). We tow boat & travel trailer quite a bit (several times per month). Can you give some objective advice? Is the lower pressure just for the ride? Should I run at higher pressure when towing? Thank you very much for your help.
AnswerGuy,
When vehicles are designed, they have a designed load carrying capacity, commonly called payload. This is the weight of the stuff you can put into the vehicle.
Tires are selected so that the load carrying capacity is larger than the weight of the vehicle, commonly called curb weight, plus the payload do not exceed the load carrying capacity of the tire - for each corner of the vehicle. In other words, each tire is not supposed to be overloaded at the pressure listed provided the payload rating is not exceeded.
On every vehicle sold in the US - and I am told this is the practice worldwide - there is a sticker, commonly referred to as the vehicle tire placard, that will list the original tire size and the proper inflation pressure for that tire size (on that vehicle). For GM vehicles this is usually on the driver's doorpost or on the driver's door. Go find that placard. When you find it, you should also see some other figures.
GAWR = Gross Axle Weight Rating = the maximum design load for the axle indicated.
GVW or GVWR = Gross vehicle Weight (Rating) = the maximum weight the vehicle was designed to weigh.
Notice that the GVW(R) is a little less than the sum of the GAWR's. That's because the maximum weight on each axle should be designed to be larger to accommodate off center loading.
Taking the GAWR's and dividing them by 2 should give a value that is LESS than the maximum load carrying capacity of the tire. For a P265/65R18, the maximum load carrying capacity at 35 psi is 2469# - and this should be listed on the sidewall near where the maximum pressure is listed.
Because the "P" means the tire is designed to passenger car tires standards - and you have essentially a light truck, the rating is decreased by a factor of 1.1 to 2245#. This value should be less than half the GAWR for either the front or the rear.
Just doing the math, the GAWR can not be larger than 4490# and the GVW can not be larger than 8980#.
According to Tire Guides, a book that summarizes vehicle tire placards, the GVW of a 2009 Suburban is 7400# (for a 1500 LT 4X4, which is what Tire Guides says came with P265/65R18's).
Hopefully all this has lined up.
That means that so long as you don't exceed the payload limits and the towing limits as published, the pressure listed for the tire size is adequate for towing or hauling.
Now let's turn to the way tire load carrying capacities are determined. I've covered this in more detail here:
http://www.barrystiretech.com/loadtables.html
Short version: The MAXIMUM pressure listed on the sidewall is usually different than the pressure where the maximum load carrying capacity occurs - and the sidewall will indicate this by listing both as separate items. And that load carrying vs pressure relationship is independent of the tire manufacturer.
Would there be any harm in using a slightly higher pressure when towing? Probably not, but I don't like being too far off the vehicle manufacturer's specification - say 3 to 5 psi. I figure they spend a lot of time - both engineering and testing time - to get this right, so I'm in no position to argue.
What about fuel economy and rollovers?
Well, I cover that on my web site, too.
