The XF I bought recently was factory equipped with 19" wheels, so the door placard says 36# F & R. But I had the dealer swap for 18" - what are the factory recommendations for those?

 

36 PSI for the 19inch wheels? That sounds a bit high to me - are you looking at the 'normal' driving speeds section of the placard or are you looking at the 'over 100mph' section?

I think 18inch and 19inch wheels on the XF should be 31psi front and rear for normal driving (under 100mph...). The 36psi for 19inch wheels is just for high speed driving (and would be 38 psi for the 18 inch equivalent). You should have a plate like this (attached below) on the car that shows the different wheel sizes (and corresponding pressures).

By the way - why the swap to smaller wheels?

 

Thanks for going to the trouble to take a picture of what I should see. But I can't find the placard you sent. I'll attach the pic of my driver's door. I would assume all the XF's weigh about the same and have the same inflation numbers, but your info is quite different than mine. 31 actually sounds low. ??

 

To get more rubber between me and the road. I'm a cruiser, not a racer (any more), and the choice of rear wheel drive, good handling cars is a matter of highway safety. Like seat belts. Yes, I enjoy the twisties, but I don't make a special effort to find them. I don't push the car hard enough to tell the handling difference between 18 an 19, but I can feel it in the ride.

 

Jaguar has been playing around with recommended tire pressures a bit to address complaints about ride harshness.

The sticker just has to specify "a" safe recommended pressure.

There are two recommended pressures in reality: lower for general use and significantly higher for full load and high speeds.

In North America we never need to use the high speed pressures.

"Correct" tire pressure is actually anything between the two recommended numbers.

The final authority on safe tire pressures is the tire manufacturer who specifies tire pressure to ensure the contact patch remains correctly shaped for the expected operating temperature range for the load range the tire is designed for. Tire construction affects required pressure and load rating is dependent on that and correct pressure.

Jaguar includes correct tire pressures in their owners handbook.

I believe Jaguar reduced the normal load tire pressures from those high numbers to 31/32 range for comfort.

Generally speaking tire pressure is lower for smaller wheels with the same rolling circumference since distortion of the tire carcass ( and consequently shape and size of the contact patch) under the vehicle weight is proportional to air volume. The higher the air volume the lower the pressure required to produce similar distortion of the tire as it rolls. As loading increases, more occupants, more luggage or higher speeds then higher tire pressures are required to reduce the heat generated by this distortion.

 

Ride comfort was the genesis of my question. I just had the wheels balanced and 2 new tires put on (flat spotted) and the dealer charged them to 38#. Needless to say, I noticed the difference when I drove away.

My manual says 33# at full load, so I'm sure 32# would be a good target for me in comfort and wear. When I'm off to a interstate road trip I put a couple of extra pounds in, and that should work fine.

Thanks for your very complete answer and explanation.

 

Good luck with the new car. If nothing else the 18inch will give your wheels some protection from potholes and road debris. I have the 20inch and it's a real challenge...
Happy driving!