Tires: Tire Pressure, law of diminishing returns, inflation pressure

Question
Thanks Barry!

You are correct on all counts:

(1)I misspoke when I stated "40psi for highway." I mean to say, "35psi for highway."
(2) The max pressure is indeed 51psi.

By the way, the tires are: Dunlop Sp 20 FE.

One last question:

Is there any reason --except for the rougher ride-- why I should not inflate the tires a little more.. e.g., to 45psi? In other words, if it were you, what tire preassure would you use?

Thanks again!

Pat


-------------------------
Followup To
Question -
Greetings and Thanks Barry!

We bought a new Honda Civic 2004. (We live in Dallas, Tx.) The car manual states to inflate the tires to 30psi for city driving and 40psi for highway. We still have the original, factory installed tires --no, unfortunately, I don't know the brand, etc.-- The tire legend indicates Max tire pressure: 55.

For a while I was following the car manual recommendations. I have now inflated the tires to 40psi. The car seems to "roll" better.

Is it safe/advisable to inflated them to 55?

Thanks again!

Pat
Answer -
Pat,

If you'll look again, I think you'll find the pressure on the sidewall says 51 psi.

No, I don't think 51 (or 55) psi is appropriate (unless you are racing on a closed road course and even then....)

Do they really say use 40 for highway driving?  Cool!

And I assume by "roll better" you mean less resistance to forward motion.  This is true.  It is one of the real ironies that one of the easiest ways to improve fuel economy is to use higher inflation pressures, and all these folks complain about the price of fuel and don't check their pressures.


It's also safer to use a higher pressure - more load capacity.

Hope this helps.  

Answer
Patrick,

I use 3 to 5 psi above the placard.  However, I temper that when I'm dealing with a Japanese vehicle - They just seem to do a better job of specifiying enough inflation pressure.

The problem with increasing the inflation pressure is the "Law of Diminishing Returns.>  The big issue with increasing inflation pressure is traction.  There is usually plenty of dry traction, but wet traction (no hydroplaning involved) is another story.  Wet traction needs either contact area or contact pressure.  As you increase the inflation pressure, the contact area goes down fairly rapidly, but that increases the contact pressure.  But there's a point you reach where there isn't much increase in contact pressure compared to area and wet traction starts to decrease.

In your case, I wouldn't use more than 38 psi.

Hope this helps.