50 PSI on 21's I assure you is wrong, and would be at risk of a blowout if set to that PSI cold- that's like a rolling brick, and will go up to 55PSI+ when warmed up- that's right at the maximum manufacturer limit and is not worth the tiny bit of range you will gain, and the horrible ride you will have as a result.
Tire manufacturer's maximum inflation pressure listed on the tire sidewall is maximum
cold tire pressure. The manufacturer is aware that pressure will rise above that due to heat and loading, and that is already taken into account by the maximum cold pressure inflation
value.
Safe tire pressure values for any car are within a range. The lowest pressure value in the range that is safe is limited by the tire load rating, vehicle weight, axle weights, and maximum speed. The highest safe pressure value is limited usually by the maximum inflation pressure on the tire sidewall.
Note that these pressures are considered "safe" for the tire only. The vehicle's handling requirements may dictate a tighter pressure range.
Lower pressure values give a better ride feel and quieter tire operation, but lower efficiency, less responsive handling, and possibly uneven tread wear. Higher pressure values improve efficiency and handling, but result in more tire noise and harsher ride.
Vehicle manufacturers test different combinations of pressures to get the handling and ride characteristics they want, while making sure that the pressure is also safe for any vehicle load and speed. The resulting optimal pressure (according to the vehicle manufacturer) is then posted on the B pillar. This is usually tested only for the OEM tires. If you use different tires, especially ones with a different load rating, size, profile, or manufacturer, the optimal pressure that delivers similar ride characteristics as the OEM tires may deviate from the B pillar pressure.
I have a spreadsheet that I use to calculate the safe pressure range for any tire and vehicle. For the Tesla OEM 19" tire (Michelin Primacy MXM4) on an 85D, the safe pressure range (
for the tire only) is 35.8-51.0 psi for the front tires, and 40.7-51.0 psi for the rear tires. These pressure ranges can carry full vehicle load (vehicle + 902 lbs of cargo and passengers) at 155 MPH. Tesla chose 45 psi as the recommended pressure for a balance of efficiency, handling, and noise, and has verified that the vehicle handles properly with that pressure.
A similar calculation can be carried out for the OEM 21" tires.