Look on the rear edge of the RF door for a label showing the stock size.

That's where my 240's have it, but since my newest is a 1988 244GL, yours may differ.

The speedo system is set up to be accurate with the stock size mounted on the rears. Actually the factory set-up may be to read a tad faster than reality. Better to err on the safe side, I suppose.

If you are running stock tires, and are still getting a speedo reading 3%-4% higher than reality, maybe a re-check of that actual speed is in order??

If there is a way to calibrate the speedo, I've not come across it.

Good Luck,

Bob

:>)