speedo accuracy

[nic83px]

got a mk1 px that had a mk2 head set when i got it. now fitted the correct head and a kph speedo but..... tested it and it reads 35 kph when im doing 30 mph!! get worse the faster i go. i know they aint hugely accurate but this is rediculous! go fast enough and the reading gets to true mph speed. any ideas?

[joey]

Ignore it and get a GPS!!
Also I think it's illegal to have a KPH speedo unless it also has MPH markings, you'll also have the fun of explaining at your MOT that the mileage is in fact in KMs