20x9 wheels fit tire widths 245 and 255 ideally. A 235 tire is the minimum and a 265 tire is the maximum for the wheel. 245 is maximum for the Tesla front wheels to prevent rubbing while turning in reverse.
Original fitment tires are 245/45R19, with a diameter of 27.7". Acceptable tire diameters will be within 2.5% of 27.7", but ideally you should stay within 1% to minimize speedometer error and vehicle navigation calibration.
245/40R20 tires are an exact match for diameter - 27.7", +0.0%.
Other acceptable tires for this wheel are:
255/35R20 (rear only) - 27.0" diameter, -2.5%.
265/35R20 (rear only) - 27.3" diameter, -1.4%
255/40R20 (rear only) - 28.0" diameter, +1.1%.
235/45R20, 28.3" diameter, +2.2%
265/40R20 (rear only), 28.3" diameter, +2.2%
So, my two recommended options:
Square setup: 245/40R20 on all wheels, exact match for factory tires.
Staggered setup 1: 245/40R20 front, 265/35R20 rear for a -1.4% difference from factory, and -1.4% difference from the front tires.
Staggered setup 2: 245/40R20 front, 265/40R20 rear for a +2.2% difference from factory, and +2.2% difference from the front tires.
Another option is to change to 20x10 wheels for the rear. This wheel fits 265 and 275 tires ideally, giving the following option:
275/35R20 (rear only) - 27.6" diameter, -0.4%
Staggered setup 3: 245/40R20 front, 275/35R20 rear for a -0.4% difference from factory and -0.4% difference from the front tires. (Requires 20x10" wheels on rear axle).