The previous owner installed Amalthea 17 inch wheels with 235 45 R17 tires. My gas cap flap doesn't address the tire pressure for these wheels. It gives the pressure for the stock 205/55 R16 wheels (29 PSI front and back).

I would assume that the larger tires and the lower sidewalls need more pressure. The question is how much? Any ideas?

Weight of car divided by total tire foot print equals tire pressure.

Example - 3500 lb car, 100 square inch total foot print, 35 psi.

Tires with low air volume are sensitive to air pressure changes.

Use care and watch tread wear patterns.

A great equation but what is the the 100 sq inch foot print? Is it the foot print of the tires on the road or the total sq inch of the car?

The total footprint is the sum of the area of each tire's contact patch with the road.

I have read that stock car racers on a budget use this formula to gauge the weight on each corner of their car - it did not mention how they measured each contact patch.

So I went searching for the equation and did find stock cars mentioning tuning their tires but the calcs called for more measurmens than I cared to try.

I found this on a search --


"we received the following rule of thumb from Oscar Pereda, an engineer for BFGoodrich. He calls it a "realistic starting point," saying it has never been just right, but is a good place to start. The rule is:

(Vehicle Weight in lb/100) + 2 psi at heavier end + 2 psi all around if suspension and alignment are stock.

Example: Stock 911, 3,000 lb.
(3000/100) = 30 psi
Add 2 psi all around = 32 psi
Add 2 psi to heavy end = 34 psi at rear
With modified suspension, the result is 30 psi front, 32 psi rear.

Read more: http://www.modified.com/tech/0208scc_tire_pressure_guide/#ixzz2yrONKp8s

Not sure about thecurb weight of my car with the 17 inch wheels but on a 15 inch wheel a 2002 Volvo S60 Curb Weight 3146 lbs. Tires / Wheel Size P195/65HR15

So if my four 17 inch wheels take that weight up to 3200 that would mean
3200/100 = 32
Add 2 PSI all around = 34
Add 2 PSI to heavy end = 36


But wait now --- what about adding passengers? If the normal daily drive contains two passengers it could add another 350 pounds to the curb weight. That would mean a curb weight of 3550

3550 / 100 = 35.5
Add 2 PSI all around = 37.5
Add 2 PSI to the heavy rear = 39.5

I also ran across articles expressing that even if your tires heat up to max pressure that is quoted on the sidewall, experiencing a burst tire is very unlikely. One quote said that even though tire manufactures don't give out their burst strngth he is confident that it is easily at double the sidewall max pressure quote.

Until you find out otherwise, I would use 29-30psi as a guide. These wider tires probably have the same total air capacity as the 205 tires, but the tread width is wider and spreads the load better.

You might want to ask tirerack.com or some other outfit, but my guess is that unless you are going over 120 mph regularly the pressure will be the same.
--
My back feels better when I sit in a Volvo seat

Solved the problem...it was a question of balance. Yep -- removed the tires balanced all four, installed 38 PSI on each tire and at 75 MPH it is smooth. My wife is happy with it.

Yesterday I took my neighbour / mechanic / car sales guy to pick up a BMW to run through the auction. There was a car there, a dead car with a set of 95% perfect 225 45 17 tires. I was tempted to get them and use them on the Volvo. We aren't high performance drivers and the lower total height would help solve the turn the wheel far and it hits the inside of the wheel well.

The 225/45/17 tire equals
Width - 8.85825
Wall height - 3.986
Total height - 24.9724

The 235/45/17
Width - 9.25195
Wall height 4.1633775
Total height - 25.3267

There is minimal difference in the foot print between the two tires.

Thanks

I went to Tire.rack and found some interesting info and their take is that size doesn't trump manufacturer recommendations.

"As many of us know, one of the easiest ways to improve the looks of your vehicle is to select a new tire and wheel package. "Inching up" your wheel diameter provides a lower profile tire with a larger contact patch affording greater traction and steering response. But what about the tire air pressure?

One of the most common questions I hear is "what is the proper air pressure for my low profile tires?"

Your vehicle manufacturer recommends an optimum tire pressure found in your owner's manual or a sticker in your door jamb or gas filler door.

What is surprising is that even if the tire size changes, the recommended air pressure remains the same. In other words, the air volume from one tire size to another may be different but the recommended air pressure is unaffected. -

See more at: http://blog.tirerack.com/blog/tire-talk/air-pressure-vs-tire-size#sthash.i9rfT7gB.dpuf"