Grumpy Posted May 16, 2004 Share Posted May 16, 2004 I bought a set of 31 / 10.50 / 15 Goodyears for our 94 Pathfinder. The tires are branded at 50 PSI for maximum inflation at maximum load. That seems a might high, so I am guessing that 35 PSI may be a happy medium for these high flotation tires. (See image below) The vehicle load will primarily be one driver and perhaps a rider or three. I am going to be 900 miles away from this vehicle most of the time, so gaging tread depth as we go is not an option to check wear patterns vs. inflation level. Any thoughts on this issue of how much air to put in these tires? Also, this 94 Pathfinder has a button labeled E-AT, with a selection of "Auto" or "Manual" -- what is this button for? Thanks. Link to comment Share on other sites More sharing options...
big_johnson Posted May 16, 2004 Share Posted May 16, 2004 I put 35 psi in mine and it seems to be a good all around pressure, helps out the mpg a bit. As for the E-AT button, someone asked this question here before, but I forgot the answer (seeing how it doesn't really apply to my pathy). I'll try to dig it up Link to comment Share on other sites More sharing options...
somethingcool Posted May 16, 2004 Share Posted May 16, 2004 Hmm, I think 30 psi is probably more appropriate. The E-AT switch is to control the auto transmissions shift points. I thought the 2 settings were Auto and Power though. In power mode it will shift at higher rpms and kickdown easier. Link to comment Share on other sites More sharing options...
squam1 Posted May 16, 2004 Share Posted May 16, 2004 thre recommended factory tire inflation psi should be listed on the door jam sticker from the factory... should be listed as a max psi for tires when cold... Link to comment Share on other sites More sharing options...
somethingcool Posted May 16, 2004 Share Posted May 16, 2004 The Nissan factory suggested psi is 26 as per the door jamb Link to comment Share on other sites More sharing options...
Grumpy Posted May 17, 2004 Author Share Posted May 17, 2004 Hmm, I think 30 psi is probably more appropriate. The E-AT switch is to control the auto transmissions shift points. I thought the 2 settings were Auto and Power though. In power mode it will shift at higher rpms and kickdown easier. yes - that's it - Auto & Power I thought it did wait a bit long to shift. I will try the other selection to see if it shifts a little better. Link to comment Share on other sites More sharing options...
k9sar Posted May 17, 2004 Share Posted May 17, 2004 I rarely shift mine from Auto to Power. The one time that it's essential is when climbing a long hill, you may slow down enough to bog-down the engine. Having it set to power will keep it in a lower gear longer (higher RPMs) and could avoid reaching that 'should I shift or not' point in the auto trans. Link to comment Share on other sites More sharing options...
kbreitwieser Posted May 29, 2004 Share Posted May 29, 2004 26 psi is what Nissan recommends, but you will be wearing out your tires faster than you can afford, and the ride will be comfy, but sloppy around turns. I have mine at 35 psi for the past 2 years, and have had little tire wear, easier turning, and much better ride ( a little harder but handles a lot better). Basically whatever you are comfortable with between 30-40 psi. Link to comment Share on other sites More sharing options...
deej Posted May 29, 2004 Share Posted May 29, 2004 32.254 PSI and 19.662 PSI if off road. sly Link to comment Share on other sites More sharing options...
Mr. Pickles Posted May 30, 2004 Share Posted May 30, 2004 The Nissan recommended pressure would be for Nissan recommended stock tires. My BFG's say they can take up to 50psi, and I've run at 40 this whole time with no uneven wear issues at 40K miles on the set (other than caused by my bad alignment ). Link to comment Share on other sites More sharing options...
87pathy Posted May 30, 2004 Share Posted May 30, 2004 differnt tire sizes and different vehicle weights all contribute to different air pressure. it all has to do with pressure vs. area. the best way to find out how much air pressure you should put in your vehicle is as follows. Use a good chalk stick or soap stone and draw a heavy line accross your tire. do this on each and when you are finished, drive the vehicle down the road a little ways.( when the road is dry) the chalk / soap stone will wear off in certain area. if it wears off in the center, to much air pressure. if it wears off on the sides, not enough air pressure. if it wears off equally, its just right. Link to comment Share on other sites More sharing options...
somethingcool Posted May 30, 2004 Share Posted May 30, 2004 differnt tire sizes and different vehicle weights all contribute to different air pressure. Nope, vehicle weight and sidewall stiffness are basically the only factors in determining air pressure settings. But the chalk test is a very good way to go. Link to comment Share on other sites More sharing options...
88pathoffroad Posted May 30, 2004 Share Posted May 30, 2004 But if you had a Zuki weighing 2100 lbs with 31x13.5 Swampers, would you run tham all at 35 PSI as opposed to having 29x9 Swampers? I think the bigger tire should need to be run at a lower pressure. *shrug* Link to comment Share on other sites More sharing options...
87pathy Posted May 31, 2004 Share Posted May 31, 2004 bingo 88 , simple but correct remember PSI (pounds per square inch.) its surface area. if you have 8 square inches of rubber in contact with the ground, you will need less air pressure to properly suport the vehicle than you would with 4 square inches of rubber in contact. again this is a simple explanation but fairly accurate. your side walls do do alot of supporting of the vehicle but it doesn' t control how much surrface area is in contact with the road. I'm too hung over today to quote scripture or text. but.. you get the point Link to comment Share on other sites More sharing options...
somethingcool Posted June 2, 2004 Share Posted June 2, 2004 bingo 88 , simple but correct remember PSI (pounds per square inch.) its surface area. if you have 8 square inches of rubber in contact with the ground, you will need less air pressure to properly suport the vehicle than you would with 4 square inches of rubber in contact. again this is a simple explanation but fairly accurate. your side walls do do alot of supporting of the vehicle but it doesn' t control how much surrface area is in contact with the road. I'm too hung over today to quote scripture or text. but.. you get the point Totally wrong. For any given load, 2 sets of tires with the same sidewall stiffness will have the same contact patch at the same psi regardless of tire size. This is true because it is not the tire holding the truck off the ground, it is the air pressure. Someone more knowlegable than me once said: The force exerted by any fluid (air is a fluid in case you didn't know that already) under pressure is equal to the pressure times the area over which the pressure is applied. For the force exerted on the ground, it is equal to the tire pressure times the area of contact with the ground (aka the contact patch). Therefore, for a given weight and tire pressure, contact area MUST be the same. Link to comment Share on other sites More sharing options...
88pathoffroad Posted June 2, 2004 Share Posted June 2, 2004 So you're saying that a 10 inch wide tire will have the same amount of contact surface area on the ground as a 13.5 inch wide tire at the same PSI exactly? Doesn't sound right to me, but I'm not here to debate the properties of tire contact patches and pressurization theory. Oh well. Link to comment Share on other sites More sharing options...
somethingcool Posted June 3, 2004 Share Posted June 3, 2004 So you're saying that a 10 inch wide tire will have the same amount of contact surface area on the ground as a 13.5 inch wide tire at the same PSI exactly? Doesn't sound right to me, but I'm not here to debate the properties of tire contact patches and pressurization theory. Oh well. Yup, the wider tire does have a wider patch (-------) like that so you have better lateral stability, the skinnier tire will have a more square shaped patch like in this here picture. But they will have the same sized contact patch at the same pressure, just differently shaped. Link to comment Share on other sites More sharing options...
Mr. Pickles Posted June 3, 2004 Share Posted June 3, 2004 Yup, the wider tire does have a wider patch (-------) like that so you have better lateral stability, the skinnier tire will have a more square shaped patch like in this here picture. But they will have the same sized contact patch at the same pressure, just differently shaped. Not trying to step on any toes, but I'm thinking you're using the wrong thought process here. If you have, say, 2 31" tires at the same pressure, 1 at 10.5 inches wide and 1 at 13 inches, the wider one would have the bigger footprint since its length is the same and width is wider, and this would carry over if pressure is lowered or raised. Anyways, how did we all get here? The topic was preferred pressure, and I do believe I'm supposed to be the one that gets off topic easily. Link to comment Share on other sites More sharing options...
jdub Posted June 3, 2004 Share Posted June 3, 2004 I run my stock 235-75r15s at 40psi. The tire shop put them on at 30psi, was a little to sloppy around the corners for me. I will have to try that chalk theory though, brilliant idea. Now in doing the chalk thing you wouldn't want to take any corners correct? Link to comment Share on other sites More sharing options...
Mr. Pickles Posted June 3, 2004 Share Posted June 3, 2004 I'm thinking you'd do chalk and drive striaght anywhere from a few feet to a few blocks. Just enough to get it to start to wear off. Link to comment Share on other sites More sharing options...
jdub Posted June 3, 2004 Share Posted June 3, 2004 Yeah, I was just trying to think of the best place near my house to do this. Link to comment Share on other sites More sharing options...
87pathy Posted June 4, 2004 Share Posted June 4, 2004 we got on this subject because its an interesting and arguable topic, and we all want to understand why (and argue cause its human nature). the chalk idea wa given to me by a buddy of mine in North carolina, he works with custom 4x4 trucks and this is what he does to determine "road air presssure" for different rigs. Link to comment Share on other sites More sharing options...
JeepRescueService14 Posted June 4, 2004 Share Posted June 4, 2004 HAHAH i was reading this cause i was bored and i came across this.. WHOA...air is a fluid??? well that gas, liquid, solid theory... goes right out the window... The force exerted by any fluid (air is a fluid in case you didn't know that already) under pressure is equal to the pressure times the area over which the pressure is applied. For the force exerted on the ground, it is equal to the tire pressure times the area of contact with the ground (aka the contact patch). Therefore, for a given weight and tire pressure, contact area MUST be the same. Gas, compressable... Liquid, NOT. whats the difference Link to comment Share on other sites More sharing options...
88pathoffroad Posted June 4, 2004 Share Posted June 4, 2004 Some liquids are more compressable than others, but all gases are to a degree. Link to comment Share on other sites More sharing options...
JeepRescueService14 Posted June 4, 2004 Share Posted June 4, 2004 yea. VERY VERY little... Link to comment Share on other sites More sharing options...
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