Commuting - Tire inflation on commute...

So, kind of a commuter question. My PSI max is said to be about 65 on my hybrid tires...now when I have my rack on with panniers and my ass all on this back tire at maximum inflation it seems that it still sinks down a bit. At times it even looks flat to me....is it possible that the inflation standard on the outside tire doesn't match the standard for the inside tire? Please help.

http://www.sheldonbrown.com/tires.html#pressure
"Pressure Recommendations
Most tires have a "maximum" pressure, or a recommended pressure range marked on the side of the tire. These pressure ratings are established by the tire manufacturers after consultation with the legal and marketing departments.

The legal department wants the number kept conservatively low, in case the tire gets mounted on a defective or otherwise loose fitting rim. They commonly shoot for half of the real blow-off pressure.

The marketing department wants the number high, because many tire purchasers make the (unreliable) assumption that the higher the pressure rating, the better the quality of the tire.

Newbies often take these arbitrary ratings as if they had some scientific basis. While you'll rarely get in trouble with this approach, you will usually not be getting the best possible performance with this rote approach.

Savvy cyclists experiment with different pressures, and often even vary the pressure for different surface conditions.

Optimal pressure for any given tire will depend on the load it is being asked to support. Thus, a heavier rider needs a higher pressure than a lighter rider, for identical tires. "

FWIW, an employee/racer from my LBS said he runs his 120 max tires at 130 front and 160 rear when he races. I run my 120 max tires on my tandem at 120 front and 135 rear. (we have 380 lbs of rider)

There is also this part on Sheldon Brown's website. I mention this because of the reference in the original post to the tire sinking down a bit:


A tire is supposed to deflect a bit under load. This deflection the whole purpose of pneumatic tires. When you sit on your bike, your tires should visibly bulge out at least a bit under your weight. If they don't, they're overinflated.

There is also this part on Sheldon Brown's website. I mention this because of the reference in the original post to the tire sinking down a bit:

Does this mean that I, who weights 125 lbs should inflate the tire a little less than someone who is 150 lbs and a lot less than someone who is 200 lbs?

The packaging on the tires for my road bike gave a varied pressure recommendation depending on the "load" of the rider. These were folding Michelin slicks but the premiss doesn't really change that much. The more you put on the tire the more pressure it will require to give optimal performance and prevent snakebite conditions.

Incidentally, the bulge of your tire is going to be illusory. I was once concerned about the pressure on my tire with a load and wasn't happy with the assurances of my family. To check for myself I got my brother, who is a lardy car drive to sit on the bike and from the rear view it didn't seem even half as bad.

Now I no longer ask "does my tire look fat in this?"

my conti top tourer 2000s say max 85 psi but for my 230lbs of rolling fury to feel comfortable on them i like em at around 110 (at 85 when i took a hard turn the bead would start to fold over a little, not a reassuring feeling). no ill effects so far...

max a mill- i'm going to use your line of x lbs of rolling fury. hysterical.