To add a little confusion here I'm wondering if the 500X (with SAT NAV) self calibrates.
To expand my 500X Cross Plus speedo exactly matches my Garmin SAT Nav. Now this, compared to older Fiat models which usually read 5% to 10% high, is rather unexpected.
I only found the speedo/sat nav correlation AFTER being caught by a mobile speed camera doing 79MPH on a dual carriage way. I also know I was knowingly driving at 80MPH (thinking the Fiat speedo was traditionally high).
This and other "not so sure" experiences lead me to thing that periodically the vehicles speedo self calibrates to a stable GPS (0.1MPH accurate) value.
Technically from a manufacturing point of view the vehicle is programmed with a default ABS wheel speed and tyre/rim size. Given an integrated SAT NAV then this default value could? be refined over a period of time. This would also allow the system to accommodate tyre wear (smaller rolling distance & increased speed).
Why is this relevant to fitting a different profile tyre on a rim? Well *IF* there is a system wide correlation then changing a tyres profile *may* self correct and adapt values to the new situation.
Also the 500X has TPMS. This is a passive system as there are NO pressure sensors in each wheel. Instead the ABS system monitors wheel rotation (in a straight line determined by steering angle) and can detect an underinflated tyre. So yes there is tight integration regarding wheel size, speed, pressure, etc.
When my tyres hit the minimum tread I'll try to remember to do a 3 way test.
Speedo at 60mph on motorway, Sat Nav speed and to cross check a stop watched timed 1.6km motorway marker timed period.
Without access to detailed manufacturer information, much of which is never disclosed to dealer level master technician level, we will be guessing, and as I have done, trying to understand actually what really happens, etc.