tire pressure

The factory sticker states 45lbs of pressure for front and rear. Isn't this pretty high - worried about hydroplaning in the wet weather we are currently having.

On my car with 21" tires the sticker states 42 lbs on all tires when they are cold.

For the 19" all-weather (on production model) tires that is correct, 45' front and rear.

Mine was delivered badly over-inflated - 55# or more in each tire.

Boris T
My experience indicates that at higher pressure, the tread is held open, allowing water to be pumped away from the tire, and you actually get better wet weather traction. I will follow the recommended pressures.

During transit, prevents damage from the restraining rails etc.

A great article on tire pressure and hydroplaining.

murraypetera: Excellent link - thanks for sharing!

Veeerrry interesting! And not stupid. But ... no info on overinflation and water. ???

N.B.: it's "hydroplaning".

Maybe he's referring to ordinary water.


I wonder if overinflation reduces "edge contact", and causes loss of control for that reason. Makes me go "Hmmmm...".

Mine was delivered underinflated at 40.5 PSI. will fix today!

The factory sticker states 45lbs of pressure for front and rear is appropriate for Michelin Primacies on standard 19" wheels. Just had Michelin X-Ice Xi3 snow tires mounted on 19" OZ Racing Superturismo wheels installed on my S85 for the winter. Does anyone know if these should also be inflated to 45psi?

Mostly the PSI is defined by the maximum loaded weight of the car divided by the total surface area of contact patches. If the contact patch on the Xi3s are the same, then the pressure should be the same (assuming they are rated for it).

For my track tires, I run them at 40psi because they are 265mm wide - it could probably be a bit less to support the weight, but I also need more pressure for sidewall support when cornering hard.

The 19" wheels need 45 lbs with air suspension, and 42 with coil suspension, per the manual. The Conti ProContact 21's need 42; Michelin Pilot sports are 38 front, 40 rear.

I recently bought a professional tire gauge

and it turns out my tires were WAY over-inflated.

I have a P85+ so I adjusted fronts to 38.5 and rears to 40.5.

I immediately noticed two things, a) the car drives much better at the right pressure--corners better, steering feels better. b) the car is slightly quieter at freeway speeds.

Wait till the tires are cool before adjusting pressure; those levels are set to account for heating while driving. As winter comes on, pressures will drop dangerously.

It is interesting that you can see the effect of low inflation on your Wh info.

Temperature dropped and I didn't check for a few days then noticed Wh had dropped so I checked tires. 5 lbs under was visible in Wh rate.

I think you lose one psi for each 10 degrees of temp drop or something close to that.

At 45 psi the rolling resistance is pretty low. You can easily hand push the car.

By Wh hour dropping I meant it got worse, as in the number increased. Sorry about that.

I tried inflating my tires to the sidewall pressure in an attempt to improve range. I believe sidewall pressure for 19" Primacy tires is around 50 PSI. At 50 PSI of inflation, I felt every little groove in the road. The ride became noticeably harsher and louder than at 45 PSI. I would not think that 5 PSI would make such a difference, but it did. I now keep my tires at 45 PSI and use a good quality, digital pressure gauge.

And by the way, you don't need to spend north of $100 for a good pressure gauge. You're all set for under $20:

Thanks @AmpedRealtor, I did go a little nuts when I bought mine. Though it works extremely well and seems to be extremely accurate. :)

Car lots and service departments always over inflate. I guess they don't want to have to keep servicing tires on the lot and service department know most people seldom check their tires so they over inflate.

Thanks for the advice guys. Tires now inflated to 45psi. Amazing what a few pounds difference makes to the handling...