What is the PCM?

And why will it need changing.

The wheels have gone from 15" to 17"... but the tyres have only gone from 615mm circumference to 625mm, a total change of 1cm, or 1.6%. That equates to +1mph at 60mph, ie when the speedo "says" 60, I'll be doing 61.
(Except the speedo overreads by 1-2mph anyway, so actually it'll be pretty much bang on)