Thanks for the replies but I don't think changing the wheel circumference works. I tried it and there's no difference. I guess wheel circumference is just for measuring distance travelled.
Got a 2.5 kmh difference between the bike and my Hammerhead GPS meaning I reach assistance cut off too soon! The Hammerhead is very accurate on speed. Is there a fix either easy or hard? I don't want to delimit the motor as the bike is only 2 days old!