Calibration of speedo after fitting SO-2

[MATHEW]

I have just fitted a set of 225 50 ZR16 SO-2to the rear of my griff 500.
The chap fitting the new boots said they were a different size to my old 235 50 ZR16 SO-1 and said my speedo clock will now have to be re calibrated due the tyres being smaller.
Is this correct?
How much will it cost to calibrate the speedo?
Is it a long job?

I would appriciate much help as I gather if the speedo is reading high then the milage will aslo be clocking high
Ta.

[MATHEW]

My griff is a 94 500.
The tyres on the car when i got it were 50 series.
So i got the same again but was unable to get 235 width in a SO-2 style.
I am now thinking should I have got 245 45 SO-3.
Ta,

[MATHEW]

Thanks for all the help.
My fornt tyre size is 205 50 ZR15
I have no Power steering.
Can anyone tell me how much it will cost to have the clock re calib.

[MATHEW]

Thanks M@H
you have given me some good info on this.
I am not going to change the tyres as they cost me £80 each.
Ta
Matrhew