Tell em Professor.
It was one of the puzzler questions on Click & Clack 20 years ago before TMPS was invented.

 

Yeah.... Hard to wrap my head around it, as the physical diameter of the tire itself did not change. In my mind, for it to accept a smaller diameter, then any material not on or inside its radius would be fores to move faster. The footprint would be forced to "slip". If it isn't slipping, then the rotation point still remains at the actual tire diameter.

Ah well, sounds Like I have something cool to go study! Thanks!

 

Well I ain't no Rocket Surgeon but I believe I read our XK's (at least my model year) came with optional 18" wheels. Did someone at the factory need to do some adjusting?

 

The 18" probably have a tyre which is overall roughly the same diameter (so, larger profile than 19").

 

Black Friday special in America. ($44) Thanks to kiwi who first posted this (ht200) same thing I believe.
Much more than regular cheapo odb2 scanners.

 

This was my problem - Article from TPMS 24/7:

TPMS FREQUENCIES - 315 MHZ VS. 433MHZ - WHICH DO I NEED?

Most Direct TPMS (Tire Pressure Monitoring Systems) deploy tire pressure sensors on each wheel of a vehicle. As tire pressure data is collected for each tire, it is sent to one or more TPMS receivers, using RF (radio frequency) technology.

The majority of Direct TPMS installations transmit their data via UHF (Ultra High Frequency) radio. TPMS data is typically transmitted in one of two frequency ranges, which depends on the geographical location of the TPMS. TPMS signals are transmitted at about 433MHz in Europe, and at 315MHz in most other parts of the world.

Additional information such as the TPMS serial number, is also transmitted to the TPMS receiver(s), acting as a unique identifier. The inclusion of TPMS serial number information in the TPMS data transmission prevents a vehicle's TPMS receiver(s) from capturing tire pressure monitoring data from another TPMS equipped vehicle that is nearby.

UHF based TPMS data transmissions occur in the unlicensed ISM radio band range. The ISM radio band is primarily reserved for industrial, scientific and medical use. Nevertheless, some devices, such as non-industrial microwave ovens, also utilize the ISM radio band.

A number of other non-industrial devices such as cordless phones, Bluetooth devices, wireless computer networks and NFC (near field communication) devices now commonly transmit data in the ISM radio band range. In some cases, devices in the preceding list may cause a temporary loss of a TPMS signal, when operated in close range of a TPMS receiver.

 

There are two different types of TPMS systems. From ATEQ:
Direct TPMS systems use TPMS sensors inside the wheel to report pressure data to the vehicle’s ECU in real time. The direct systems include Asian, domestic and European vehicles. An indirect TPMS system uses an ABS system to monitor the speed of the wheel in order to record tire pressure readings correctly. Indirect systems include Asian and some European vehicles.

What Ken is referencing in his Lexus (EDIT correction - his wife's Mazda)- he has an indirect system. We have a direct system? I see no connection in the wiring diagrams to the abs, but it's also run in canbus so I could be wrong. It would be the second time I was wrong this past decade if memory serves.

 

Direct.

 

Only 2007 XKs were offered with with Venus 18" rims. But all rims and tire combination possess near identical outside diameters on the tires. This is a;so why the 18 inch provides a superior ride over 19's and 19' over 20's due to more sidewall.
20 inch tires are by far the worst performance, and yet the most expensive shoes you can put on an X150. I am unfamiliar with the Alcon brake option, as that may require 20 inch rims to fit inside.

TPMS on Jaguar utilized a sensor internal to the tire, composed of an RFID tag (Serial number), and a 3.5 volt lithium cell battery similar to a 3032, or maybe a 2016. This is why sensors fail at "about" 5 years of use.

Lexus utilized the same system as ours, only where our Jaguar can "self learn" the serial number after 20 minutes of driving, Lexus requires you have them program them to the car.

 

Close, my friend. As CleverName pointed out, the XK and Lexus both use a direct system, and my dumb Lexus has to be told the TPMS IDs. It’s the wife’s Mazda that has an indirect (via ABS) system (no batteries to wear out, but no PSI readings either).