Originally Posted By: SHOZ
And I use tires rated at 51 lbs max. Unless I bump the pressure up 10 lbs over the OE the things handles like a wallowing pig.
Other than a rather specific anecdote as to the handling characteristics of that particular tire at the manufacturer-specified pressure for that particular vehicle it has no impact on the facts posted by CapriRacer who is an authority on this topic. You've chosen to deviate from the manufacturer-specified pressure based on "feel", which I'm sure is a far less extensive process than that employed by the manufacturer in order to arrive at the pressure specified on the placard.
Now, once you've deviated from the manufacturer-specified size, then there will be some adjustment necessary from the OEM recommendation, which I believe is also covered on Barry's/CapriRacer's site, but that's not the general gist of the discussion and is why I included the qualifiers I did.
My BMW had multiple pressures listed on the placard depending on how the car was loaded. These pressure were significantly lower, even for maximum load, than what the maximum inflation pressure on the tire was, which, if used, would result in a much smaller contact patch. In fact, if you followed the "American" pressure guidelines (which were equivalent to the max-loaded European pressures) you made the car "twitchy" and had a reduced footprint on the rear (which were 275's). Now, despite BMW going to all the trouble of specifying various inflation pressures relative to load, they did not mention the maximum sidewall pressure anywhere. There was no recommendation to modify their specified pressures if the replacement tire had a higher maximum inflation pressure and the reason for that was that it didn't matter. If you used a P275/30/18, then you used the pressures they specified. The PSS's I put on the car to replace the Toyo's had different maximum inflation pressures. The Pilot A/S3's I put on my wife's car have different maximum inflation pressure than the tires they replaced. This is not uncommon and if it were something that was supposed to be factored into the inflation process relative to the rating on the placard it would be mentioned. But it isn't, because it doesn't.
People running improper tire pressures is a safety issue. The OP running his rear tires on a Saturn S-series, which is an extremely light vehicle, at like 20psi higher than the placard because he's developed some bizarre formula is an example of that safety issue.
