Pretty much all cars these days use electronic speedometers, even if they have an analog display. While I’m not exactly sure Tesla has set theirs up, the general principle is there are magnets attached to the drive shaft with a sensor that picks up when the magnet goes past. The more rotations the shaft makes, the more times this is picked up and the faster you are going. I’m guessing on the Tesla this will be on the rear motor so they don’t have to change anything for single and dual motor cars.
in the case of wheel size the car knows the circumference for a standard wheel and tyre, so knows how far you’ve travelled with one rotation. If the winter wheels (with tyres) are smaller than 20” performance wheels/tyres then the speedometer will show a higher speed than you are actually doing, as you’ll travel a shorter distance for each wheel rotation. If the winter wheels are bigger then the speedometer will show a lower speed than you are actually doing (I’m guessing this is less likely unless you’ve gone for high profile winter tyres). The standard 20” wheels use 235/35 tyres while the 18” use 235/45, which are close in circumference (less than 1% difference). There is a setting for wheel size in the Service menu, which may make the small adjustment for the slightly different circumference but I don’t think it allows non-standard size adjustments. If you aren’t intending on using the standard 18” wheel tyre size then your best bet is to speak to the tyre fitter as they’ll be able to tell you how much difference there is in circumference.
Finally if you are going to a tyre fitter for the tyres and they are going to swap the wheels at the same time, make sure they’ve worked on a Tesla before as you don’t want them trying too jack the car up where the battery is!
