Sub platter diameter

As a newbie, I am increaingly baffled by platter speed influences.

The first hurdle is the diameter. Is it 4" (exactly), a nominal 4", 101mm or what? Even my cloth ears will tell the difference at 1kHz.

Then the drive belt. If the motor pulley is grooved, surely the effective diameter depends on the belt tension.

Before I plunge into the upgrade whirlpool, I would welcome a few facts.

Anybody please?
AFAIK, there is no standard for subplatter or platter diameter.

Yes, tension matters.

Graham, It sounds like you may be inquiring specifically about the Rega subplatter. If you are, the diameter varies slightly in samples that I have measured between 101mm and 101.2mm
Obviously the other pieces to the puzzle are the pulley diameter and motor RPM. Add the belt to the equation and you have a nice mathematical formula.
Following all of the calculations you must machine everything perfectly to deliver the final result, not a simple task and this is why there are so many turntables out there running at incorrect speed.