QuestionI have changed tire size and brand from OEM. JEEP recommended a
235/70R-16 on this alloy 16x7" wheel (2001 Cherokee XJ; 2WD). I have
moved to another brand of tire spec'd for this vehicle type in 255/65R-16
(109 load; or, 2,271-lbs max at 51-psi)(where the tire height is published as
a half-inch or so taller, but section width & tread are increased).
The owners manual specifies 33-psi; or, for constant speeds above 65-mph,
increase pressure to 36-psi on stock sizing.
I have been unable to find load vs. pressure tables for light duty SUVs. My
two rules of thumb have been to aim for 75% of max air pressure to see at it
coincides with the OEM recommendation (I never go under that number); and
-- a bit like the chalk method -- I look to see if pressure rises more or less
from 4-psi above stone cold after 30-minutes above 60 mph.
Today, for example, (90F and high humidity at sea level), with tires only a
hair below 36-psi, a 40-mile roundtrip at just above 60 mph gave me a
consistent 3-psi rise on these new tires (about 3,000 miles). The "rule of
thumb" cited above tells me that a 3-psi rise means I am slightly high for
cold air pressure (maybe 2-3 psi).
IR thermometer temps showed 110F at rest (about a 20F rise).
The vehicle is a 2001 XJ Cherokee with a scaled weight of 3,460-lbs; where
1,810 is on the front axle and 1,650 on the rear axle (52/48 split).
Any help would be appreciated. I also run a "heavy" light duty diesel pickup
and more than one type of trailer. Am looking to improve the context of
understanding this topic, if I can, in the interest of safety first and reliability
second.
Thanks!
AnswerRoss,
First referencing the max pressure listed on the sidewall is incorrect. Here's a long discussion about the load tables:
http://www.geocities.com/barrystiretech/loadtables.html
Pay particular attention to where I discuss "Notes on Page 1-34".
But your rule of thumb is pretty close. I've always used the rule that less than 3 psi build up is good, where 3 to 5 psi is OK, and above 5 psi requires some attention - less load, less speed, more pressure, larger tire, etc.
This rule can be applied to almost any tire, except for those where the long term durability is being sacrificed for some reason. A good example might be the Space Shuttle where the tires MIGHT be a "one use, then toss" type of proposition (BTW I don't know that this is the case, but it would not surprise me that this is so.)
I looked up a 2001 Jeep Cherokee in Tire Guides and they don't indicate any versions came with P235/70R16. So please check the vehicle placard to see if this is what it indicates. The vehicle placard is usually located on the driver's doorpost or in the glove box.
What Tire Guides indicates is that most of these vehicles came with P225/70R16's inflated to 33 psi. That means a 255/65R16 would carry the same load at 26 psi. However, Tire Guides has always had a problem with placards that list 2 pressures. If we assume that the tire size is indeed P225/70R16 and the high speed pressure is 36 psi, then a 255/65R16 would need 3 psi more - or 29 psi.
This is only slightly different than what your pressure build up study indicates.
