QuestionQUESTION: I drive a 2001 Mazda B4000 XtraCab RWD pickup truck that I purchased used in 2002. The vehicle
came equipped with the original OEM-supplied Firestone Wilderness HT P225/70R15 all-season tires. The manufacturer's vehicle
certification sticker located on the driver-side Xtra Cab door specifies the following:
GVWR: 4800lb/2177kg
Front GAWR: 2500lb/1133kg
Rear GAWR: 2600lb/1179kg
Tires: P225/70R15 SL, 15 x 7.0J @ 207kPa/30psi (for both front and rear tires).
I've always felt that 30psi seemed rather low, particularly in light of the Firestone tire failures that were happening a few
years ago on Ford Explorers, right around the time I purchased my Mazda. At the time, I chose to inflate my tires to 35psi, eventually
dropping them down a bit to 33psi. My rationale was that it was better ("safer") to have slightly harder tires than underinflated ones.
Confusion regarding tire pressures arises from the 2001 Mazda B4000 owner's manual. The manual specifies about three or four different sizes of tires and their recommended
pressures, including size P225/70R15. The confusion is that the pressures specified in the owner's manual for both front and
rear P225/70R15 tires is 35psi, not 30psi that is specifed on the door sticker. Of course, 35psi is (was) the maximum stated pressure
molded onto the sidewalls of the OEM Firestone tires that came with the vehicle. (I notice in the Mazda owner's manuals for years 2002 to 2009
(which I downloaded from Mazda Canada's web site), that Mazda no longer specifies specific tire sizes or pressures in their vehicle owner's manuals,
instead referring the owner to the vehicle certification sticker!).
I purchased a set of 225/70R15 100Q Blizzak DMZ3 winter tires about a year ago and ran them (I think) at 35psi (or perhaps only at 33psi) all last winter.
Then, this past May, I purchased a set of four Michelin LTX M/S all-seasons (size P225/70R15 100S) to replace the Firestones (at 68,000km) to
use during the spring-summer-fall months. (I don't recall the load and speed ratings of the OEM Firestones, but I believe they were
the same as those of the new Michelin's, or rather, the load and speed ratings of the Michelins are the same as those of the old
Firestones. (I notice the speed rating of the Blizzaks is lower than that of the Michelins. I don't think this should be a problem since
I don't much travel faster than 100km/h, particularly in winter driving conditions).
I've been increasingly unsure and uneasy about what inflation pressures to use in both of these sets of tires, especially given
the discrepancy between the tire pressures specified on the vehicle certification sticker and in the owner's manual.
To further confuse things, today I just noticed for the first time that the Blizzaks are non-P metric (i.e. 225/70R15) and not P-metric
(i.e. P225/70R15), much to my surprise.
After studying a document from Toyo Tires that I found on the internet several months ago that includes some excerpts of TRA
and ETRTO tables and accompanying explanations, and after reading some of your explanations on this forum (and perhaps on the web forum
www.eng-tips.com ?), I am concerned that I need to inflate the Blizzaks substantially higher than the 30psi that is specified
on my vehicle's door sticker (i.e. 35psi? I'm guessing 35psi based on the ETRTO standard load table in the Toyo document, but
presumably, the Blizzaks were designed to JATMA standards given that they were manufactured in Japan, so maybe 35psi for the
Blizzaks isn't correct either?).
Can you please offer some clarification/recommendations concerning what are correct and/or acceptable tire pressures I should be using
in both my Michelin LTX all-season and Blizzak DMZ3 winter tires?
I see in some of your responses to queries on this forum that you prefer to run your tires 3 to 5 psi above mfr.'s placarded
recommended pressures. With respect to my Michelin LTX M/S all-season tires which specify maximum recommended pressure on the sidewall
of 35psi, would I be able to inflate them 5 psi above Mazda's specified door sticker pressure of 30psi? Should I even consider this?
I live in north/central Alberta and the roads and streets in my area suffer from major frost heaves and potholes. I'm concerned that
having my tires too hard will make my Mazda truck more vulnerable to major suspension damage. Without getting into too much detail, almost
two years ago after I moved here from the coast, I hit a pothole and bent the right-front wheel spindle and damaged the right front wheel bearing and brake
rotor/pad - something like $1100 damage by the time it was all over. At the time, I had installed on the vehicle, four Pirelli P215/70R15 Winter Ice snow tires
inflated to 35psi. I was using the narrower/smaller tire at the recommendation of my tire dealer on the coast who said I could mount
these tires (of which I already owned two from my previous vehicle) on the same rims as my Mazda. Perhaps not the best thing to do?
Anyway, in view of all the above, I would appreciate any advice you can offer concerning my optimum tire pressures for both my Blizzak winter tires and Michelin
all-season tires.
Regards,
Dave
ANSWER: Dave,
The plot thickens further!
I have a book called Tire Guides that summarizes the vehicle placards. This makes looking up the proper tire size and inflation pressure fairly easy to do. Well ........ the book says a 2001 Mazda B4000 4X2 came with P235/75R15's inflated to 30 psi front and 35 psi rear.
A couple of thoughts:
1) The book has been known to be wrong.
2) The book covers US models only.
So let's see if we can make some sense given the other information you've provided.
A P225/70R15 has a load carrying capacity of 1624# @ 30 psi and 1753# @ 35 psi.
A P235/75R15 has a load carrying capacity of 1881# @ 30 psi and 2028 @ 35 psi.
Now I need to apply the 1.1 factor because the P metric tire is used in a Light Truck application. This makes the load carrying capacities:
P225/70R15: 1476 @ 30, 1593 @ 35
P235/75R15: 1710@ 30, 1844 @ 35
You quoted the placard as saying:
Front GAWR: 2500# (1250 per tire)
Rear GAWR: 2600# (1300 per tire)
GVW: 4800# (BTW, Tire Guides says this is 4740#)
Both GAWR's are below the maximum load carrying capacity of all the combinations of size and pressure.
Let's look at the reserve load (the unused load carrying capacity of the tire) as a percentage of the GAWR:
P225/70R15: @ 30 psi: 118% front / 114% rear
@ 35 psi: 127% front / 123% rear
P235/75R15: @ 30 psi: 137% front / 132% rear
@ 35 psi: 147% front / 142% rear
What makes sense to me is that since the GAWR's are different front to rear, the pressures should be different as well. (If this were a FWD car, I wouldn't be saying that! It is common for the rear pressures to be the same as the front even though a FWD is nose heavy. I assume they specify this to make tire rotation less complex.)
So that leads me to believe that the proper pressure should be 30 psi front / 35 psi rear. Let's see if looking at the Ford Ranger helps us any.
Well ....... Tire Guides doesn't list a Super Cab in a 4X2 version and also doesn't list any Rangers with 4800# GVW. It also shows P225/70R15's on all versions of 4X2's and P235/75R15 on all versions of the 4X4's. (I'm omitting any reference to 16" tires)
That was not much help! So I'm back to the 30 psi front / 35 psi rear.
And that's my final offer!!
Other issues you raised:
1) TRA / ETRTO / JATMA:
For practical purposes the load carrying capacities are the same. I've explained this in more detail here:
http://www.geocities.com/barrystiretech/loadtables.html
2) The Ford pressure specification on the Explorer equipped with Firestone tires:
The argument here was over the reserve capacity. Ford used none, while Firestone was arguing that Ford should have used some. There was no disagreement concerning whether or not there the pressure specified was enough to carry the load specified. Firestone was arguing that the Ford specification merely contributed to the problem (They were trying to deflect as much of the criticism as possible.) While I agree that Ford should have specified a higher pressure (good engineering practice), the pressure was not the reason the tires were failing.
To put this further in perspective of this discussion. A by-state analysis of tire failures showed that far and away the problem was in the US desert Southwest - 5 states in particular: AZ, CA, NV, TX, and FL . (You'll notice that FL seems to be the "odd man", but if you look at the temperature over the course of the year, FL is much warmer than AZ in the winter, so the problem is not just temperature extreme. It's also heat history!)
Notice that these 5 states do not resemble Ontario (at least as far as weather is concerned.)
3) My 3 to 5 psi over placard recommendation:
I prefer a car that handles crisply and higher pressure gives me that. While I get a whole bunch of good things when I do that, I have to accept the increased ride harshness and the increased risk of impact failure. Impact failures are rare events, but the risk is larger if you in an area with lots of potholes (which you do.)
At the same time, I also recognize that the average person doesn't pay attention to his tire pressure. They don't connect tire pressure with safety (except for the traction issue). Anything that I can do to raise that person's awareness and get him to do a better job would be beneficial.
To further complicate matters: Vehicle manufacturers used to specify pretty minimally sized tires. As I discussed above, this was part of the Ford / Firestone situation a few years back. I have been able to document 3 increases in tire size over the years (we're talking 40 years!) - the latest being the result of the Ford / Firestone situation. I am currently reconsidering my pressure recommendation in light of this.
Whew!!!
You asked a long question and I gave a long answer - but I think I addressed everything you asked about. I hope it was helpful.
---------- FOLLOW-UP ----------
QUESTION: Thank-you Barry, for your prompt and detailed reply. I think it helped clarify a few things.
I do have some additional related questions that I've been wondering about.
1) What tolerance (if any, i.e. +/- N psi), do car manufacturers assume when they specify their recommended tire pressures?
Example: If the car mfr. specifies "30psi", do they assume a tolerance of +/- 0.5psi, 1psi, or what?
If my tire pressures are 0.5 to 1 psi below the car mfr.'s recommended pressure, is this cause for concern?
2) Regarding inflating tires above the mfr.'s recommended pressure, at what point do concerns about abnormal tire wear due to over inflation become an issue? We always read about warnings not to over inflate because the center part of the tread will wear, but is this really an issue when over inflating by only 2 to 5 psi? Presumably, total weight on the tires will determine wear patterns for a given tire pressure?
Dave
AnswerDave,
To my knowledge, nothing has ever been specified about the tolerance on tire pressure. Tires are fairly tolerant and I think within 2 psi is OK.
The issue about tire wear vs inflation pressure - especially high pressure - is hotly debated, since there isn't a lot of hard data available. What little information is available says that wear rate gets better as the pressure goes up (makes sense as the tire is getting stiffer), but the wear appearance is largely unchanged (doesn't make sense, because the center of the tread is bulged out.) This leads me to believe that a little overinflation (3 to 5 psi) is largely beneficial.
