Tires: tire pressure, law of diminishing returns, chrysler intrepid

Question
Believe it or not the owners manual for my 1994 Chrysler intrepid says that if driving above 75 mph for  lengthy periods of time(who does that? I am usually hauled over by the boys in blue)one should raise tire pressure to 35 psi.I have increased my tire pressure to upside and the 3.3 lire v-6 engine and I both love it. At 60 mph the rpm is down almost 200 and starting from a stop snaps peoples head back.I will have to develop a lighter touch. My tires,which came with the car when bought by me in march of 07 are new(bought in 2/07) They are bottom of the line all season Canadian Tire(the biggest chain in Canada) radials which specify max. pressure of 40psi. These are very ordinary tires-the least expensive the seller could find.As the psi is increased I lose some traction. Can you put a # on the traction lost at 35,40,45,50psi and is it safe to run them permanently at 35 and could I go to 45 or 50. Is a tire available which I could run a 50 psi all the time. The ride at 35psi is comfortable and the suspension handles the change very well.Do you think the suspension could handle 45 or 50 psi and not be overstressed and yet still give a comfortable and rather soft ride. THANK you for your time and for the answers.

Answer
John,

No matter how you try to slice it, increasing tire pressures 1)  decreases ride quality,  2)  improves rolling resistance,  3)  reduces the size of the footprint, which in turn reduces dry traction, 4)  increases the responsiveness to steering input,  5)  transmits more input to the suspension, and a whole lot more.

I think a few psi above the placard inflation is OK - 3 to 5 psi.  But anything beyond that starts to get into "the law of diminishing returns" - meaning you get a lot at the beginning and less the more you do it.

And for practical purposes, the tire size and pressure are tied together (for a given vehicle) and that's reflected in the placard.

So no, you shouldn't use pressures much above what the placard says.  And there aren't tires that would allow you to use a higher pressure safely - it doesn't work like that.