Tire Pressure Question

[jtaz]

Sorry for the caveman question, but:

I have a 2004 G35. Infiniti owners manual says 30 PSI for all 4. Whenit came back from the dealer after routine maintenance it had approx40 in the fronts, 35 in the rears (after sitting overnight). I sent amessage to Infiniti (company website) inquiring whether to follow themanual or the dealer and never heard back. FWIW, the tires areBridgestone P215 55R17.

Both the manual recommendation and how the dealer actually filled themsound equally unusual to me. Any comments?

Also: I have always wondered if the "tire pressure when cold" needs tobe qualified a little for different climates. I'm in Southern AZ.Outside air temperature is 95 degrees today, and I'm at 4000 feet(altitude). Do either of those things matter?

Thanks for any education.

Jim

(P.S.- posted this in the yahooGroup but can't stir interest-- thanks again for any help.)

[Rex]

There should be a recommended range, the lower value for softest ride, the higher for best MPG.

What's the tire manufacturer's recommended psi?

[DAEDALUS]

Safety and tire longevity are other factors to consider. 40 psi does sound high though.Absolute temperature is what corresponds to pressure differences, so even a 30*F temp change means roughly 3psi pressure differences. The cooler in temp you go, the more the tire pressure (gauge pressure) drops, so if you're shooting for pressure at the high end, doing it at 95* is safer than at 60*. Same for altitude, the higher you are when you fill up, the more the tire pressure will drop as you descend. Absolute pressure doesn't change, but the gauge pressure does, and that's what matters.

[szh]

I'd keep it higher than the recommended minimum for one obvious reason. The pressure does drop slightly over time in even the best tire and valve and other conditions.

Being over is safer than being under. Load index is increased and tire flex (with corresponding sidewall heating) is reduced.

So, while my M45 manual recommends 32 psi in all 4 tires, I keep it at around 36 psi. I check every other week and it tends to drop about 1 psi or so.

My tire pressure measurements are always taken inside the garage, in the morning, before driving, and before the sun has had a chance to warm them up (black absorbs heat.)

Z

[Q45tech]

35 cold [for 44 psi tires]is the reference lab pressure where all tire parameters are analysed by tire manufacturers.

The is ongoing debate at "THE TIRE SOCIETY" [engineering association for tire engineers] as what should be the new standard for 51 psi tires.

80% of 44= 35.2 psi...............80% of 51psi=40.8 psi

To yield the same safety reserve.

Must know static weight on each tire to zero in on the correct psi.

For example a 4300 Q with 55% on front would have 1183 pounds per front tire that is 77.7% of full load rating [1521] not counting any turning or braking load increases..........that alone would say 34.22 pounds [psi]assuming the load index is linear in decrementing down with lower pressure.

The oem 29 [for non aggressive driving] increases 4-6 psi from rolling resistance so 33-35 psi hot..........note the Q has a 35 psi COLD dual tire sticker for speed/aggressive driving.

Calcute the G or any car the same way!

With 51 psi tires consider 41 psi cold to be a safe number based on tire reserve, not smooth riding.

[szh]

assuming the load index is linear in decrementing down with lower pressure.

This is a good assumption, although I expect that the extremes (of pressure) might show some non-linearity when the sidewall gets too curved (low pressure) or too stiff (high pressure).

Attached is my Excel chart of the load index reduction for the original OEM tire size for the 90-96 Q45, based on the Tire and Rubber Association tire tables that I have. "View" the image in your browser to get the best resolution and see the axis lines.

The oem 29 [for non aggressive driving] increases 4-6 psi from rolling resistance so 33-35 psi hot..........note the Q has a 35 psi COLD dual tire sticker for speed/aggressive driving.

Note that the load index at 29 psi for the 215/65-15 OEM tire is low enough that I would be concerned about the static weight (including passengers, luggage and gas) of the Q45 almost being too much for most tire vendors ... Michelin being an exception, for all the reasons you talk about so well!

Z
Modified by szhosain at 4:51 PM 7/7/2005

[Q45tech]

Because most don't brake for more than 3.0 seconds or max thru curves it works but the repeated actions over 15,000 miles take their toll!

Why upgrading to 98 load index [the extra 100-130 pounds of reserve at same inflation]..........extends tire life significantly.

[szh]

Why upgrading to 98 load index [the extra 100-130 pounds of reserve at same inflation]..........extends tire life significantly.

Yes, agreed!

Here is the comparison charts for the original 215/65-15 95 and a 235/60-15 98 for example.

Z

[jtaz]

Thanks to all for the education -- Jim

[maxnix]

You know, this thread once agains begs the question why on the sidewall, maximum load carrying capacity is quoted at maximum allowed inflation pressure?

[Q45tech]

For the same reason that hydroplane speed is almost purely a function of inflation pressure...........a fact that marketing sometimes uses to confuse tire buyers into thinking that agressive directional tires are much better in the rain...............or that 20" tires handle better than 15" tires.

[szh]

You know, this thread once agains begs the question why on the sidewall, maximum load carrying capacity is quoted at maximum allowed inflation pressure?

I have no idea!

The Tire and Rim Association tables are pretty clear in that the max load carrying capacity is supposed to be measured and stated at 35 psi.

Z