I'm driving my 2007 X OR on the highway exclusively right now. I just put on new Michelin Latitude Touring. What should the tire pressure be for best highway handling? The Xterra placard says 35 but my mechanic says 40 for the Mich

if you have the tire pressure sensors on you X filling the tires above the recommended tire pressure on the placard could damage the sensors... If not, then run what you feel is the most comfortable/efficient for your application and tire type.
Like run with 35psi and see what kind of ride/millage you get, then the next tank of gas run 40psi and compare....

The placard is only good for STOCK tires (which we all know sucked to begin with). My BFG's I keep at 40+/-psi. You'll want to soften them up for sand, but for strictly street use, I'd lean torward listening to your mechanic. Then again, you could also look at the load ratings for your specific tires and figure it out from there...

First off, over-inflation is just a bad as under-inflation, if your running you Michelin's at 40 psi that's too much, they are overinflated.

What ever you do , do inflate them to the pressure listed on the side of the tire, that's the max pressure as recommended by the tire manufacturer.

And no, the placard in the door is not meant for the stock tires only, that is the recommended pressure the vehicle manufacturer specifies for the best overall performance and handling and fuel mileage.

36 is perfect , leave it them there and enjoy a nice ride and long tire life.

BTW, are you running nitrogen in them?

Sorry, NMT, but I have to disagree. You can't say that one pressure is the best overall for any and all tires we could put on the trucks. That just doesn't make sense. Plus, when I got my BFG's they specifically told me to keep them at 40psi. They have a max pressure of 80psi (on the sidewall as you suggested), and for the weight of the truck, 40psi is what I would need. Less than that (36 from what you say) would be too soft. Realistically can I tell the difference between 36 and 40? Maybe, maybe not. But underinflation for the weight of the vehicle will cause uneven tire wear.

So again, it depends on the tire, the ratings for your tires and where you are driving them too. For example, if you're going in sand or terain that is exceedingly soft, lower the pressure. Some go as low as 12-15psi. Then pump them back up for the road again.

And we all know that the the auto manufacturers "recommend" a pressure that would be considered underinflated so you'll have "better" ride comfort. Why else would the auto manufacturer recommend a lower pressure than the tire manufacturer?

All good points Gonzo, I will agree that one specific pressure is not the rule depending on the tire and driving surface.

I run Michelin LTX/2's on my Pick-up (It a Chevy , and that's a story for another day) and I can't tell the diff. from 36 to 40 but I run them at 36 , I don't feel 36 is under-inflated for that specific tire weather it be on a full size truck or an SUV for everyday street driving.

I got 70k plus on my last set of Michelin LTX's and ran them @ 36 with no under inflation wear issues visible on the tire, of course I rotated them every 7500 and rotated and balanced every 15k and checked the pressures every other week.

So i guess to sum things up , figure out a pressure that works for your style of driving and conditions .

Years ago on this site I learned about "The Chalk Test"
I just got done searching for it and came up empty. Basically it went something like this.Take a piece of chalk and make a stripe across the width of your tire tread.Drive the car in a straight line for (100 feet, yards I really don't remember the exact distance),then check your stripe. If the chalk wore off evenly, you have correct tire pressure. If the chalk wore off in the center only, then your tire is over inflated. If the chalk wore off on the outside edges, then your tire is under inflated. To make it easier to find the stripe I made a mark on the sidewall with the chalk.

Good call back NB! I remember the test now that you mention it and I think you've got it right.

-G

The REAL answer is actually to use all of the information together.

For example, the load range of the tire is based upon its PSI.

So, if the tire says its supports 3000 lb at 40 psi, or 3000 lb at 80 psi...you need to know that.


Its on the sidewalls.


Next, you take 35% of your GVWR, say 5,400 lb as an example (GVWR is on that placard).




35% of 5,400 lb = 1,890 lb.


So, MINIMALLY, you need at least enough psi to support 1,890 lb.


If the sidewall says its MAX is 3000 at 40 psi, you divide 1890 by 3000 and get 1890/3000 = 0.63

So, you need 63% of the max load, and, therefore ~ 63% of the MAX PSI.

If the max psi = 40, then ~ 26 psi would support the 1,890 lb.

If the max psi was 80, then ~ 51 psi would support 1,890 lb


Round up, we ARE talking about finding the MINIMUM PSI, and your GAUGE is probably only accurate to a percent or 3, etc....

(The tire that NEEDED more PSI to SUPPORT ITS MAX WEIGHT, will run at a higher PSI, because it NEEDS MORE)


The above is a great way to ball park the ranges...in real life, tires are not perfectly linear in load range/psi percents, but, are typically within a pound or so...plenty close enough.


Again, the 35% of GVWR is a MINIMUM for psi settings, many people run closer to 40% for example.


So...try the above to see what's about right...at a minimum, you'll be in the ball park, and from there, you can see how you like the ride/handling.


Too bouncy/harsh at the minimum setting...well, its probably just a bad tire match for you.

Too wishy washy for you at the minimum, add 5 psi and try again, rinse/repeat.



The chalk test can be OK, but, its a lot better for bias ply tires than for radials...much harder to tell the difference in the chalk wear with the radials.






Just like torque wrenches, etc...the tire works best at the sweet spot in its range.

Some tires work better at the lower ends of their load ranges, other at the higher ends...but MOST seem to work best between 60 - 80% of their max load, with 75% being a common target.


THAT means that when SELECTING a tire, you should consider its load range, and where ~ 35-40% of YOUR GVWR would fall relative to it.


If POSSIBLE, choose a tire with ~ 75% of its load range being about 35 - 40% of your GVWR....for the best match.



Sometimes, people choose a load range/tire that is simply wrong for their truck...its MAX load might be fine, but they run the tires at too low a PSI to SUPPORT that load....

...or they get a tire that's actually too heavy duty, such that it would be running with the sidewalls bulged out at a PSI you'd need to support the rig's weight, etc....and the handling feels like driving on greased squirrels, the rear comes out on turns, etc.

IE: If the tire is a bad match for your truck/use...there's not much you can do except try to return it...or hope it wears out quickly/try to not have an accident/not have your kidneys pounded into jelly, etc.



Food for Thought.