Tires: Tire Pressure, 2005 volvo s60r, load curves

Question
Hello Barry,

I just replaced the summer tires (235/40 ZR18 95Y) on my 2005 Volvo S60R with some all-season performance tires (235/40 ZR18 91W) for the coming winter months... my car has a est. curb weight of 3571 lbs and what I was wondering, the OEM tires have the higher load rating of 95, and the recommended cold pressure is 35psi front and rear... how do I figure out what the pressure in the new tires should be, should it be more, or less, and what's the method to calculate it?


Thanks,
Josh

Answer
Correct me if I'm wrong, but your vehicle placard may say something about using 39 psi, maybe for a fully loaded vehicle - at least that's what my book says, but it only lists that maximum value and some European vehicles have different values for different situations.

ASSUMING you are correct - the original tires on your vehicle are "Extra Load" or "Reinforced", which means they have more load capacity than "Standard Load" and they get this by using more inflation pressure.  Normally 35 psi is within the range of a "Standard Load" tire - another reason I think the placard will say something higher than 35 psi.

The net effect is that the all season tires have a lower load capacity than the summer tires that came off - not a good situation - and you can't get more capacity by using more inflation pressure because of the limitations of the tire.

Normally to figure out what the inflation pressure should be, you need the load curves for both tires and compare what inflation pressure on the new tires gives the same load capacity as the old tires.  In this case the load curves are the same, but the "Extra Load" tire's curve is extended. (BTW, there are so many load curves, it's impractical to published this info on the web.)

You've referred to the curb weight of the vehicle, so I'm guessing you think that enters into the equation.  It does, but it's the starting point the vehicle manufacturer uses to decide what load capacity he should specify and that's what results in the pressure (and tire size) listed on the placard.  There's a lot more to what load capacity is required than just the static curb weight.  and of course, you want to have MUCH more load capacity than you need for safety reasons.

So they make it easy on everyone by displaying this information on the placard.

Here's my suggestion:

1)  Go back to where you bought the tires and point out this problem.  Refer to the vehicle placard - it's what the vehicle manufacturer says should be there.  Unfortunately, US law doesn't prohibit putting smaller tires on a vehicle, but the legal liability is clearly on the guy who allowed this to happen.

2)  If YOU happen to be the guy on the hook (so to speak),  and you can't arrange a situation to get the proper tires on there - Do one of these:

  a) Use the inflation pressure as listed on the placard even though it is higher.  While this will highly stress the tire's reinforcing materials (the plies and the steel belts), this would be better than an overload failure generated by fatigue.  

OR

b) Use the 35 psi value (assuming there is a factual basis to use it) and promise not to speed or load the vehicle excessively.

In either case there is an elevated risk, and even though the risk is still low, you need to be aware of it and compensate.

Hope this helps.