speedo problems in an NG

[Sammyboy]

Hi all. My speedo does not give me an accurate reading, especially at speeds of over 80km/h. In an 80 zone, my speedo varies between 70 and 90km/h, just like the suspension bounces when I go over a bump! I am familiar with the revs and gears at particular speeds so I can get away without using the speedo too much, but this speedo is just getting worse and worse... I have taken my tacho up to 3000rpm in 5th gear on the freeway and had the speedo sitting on 150km/h (impossible in a turbo diesel Paj of this era)!! Another thing too- I was on a day trip using both my speedo and my GPS unit to measure distances, and at the end of the day, the trip metre had a significantly lower reading than the GPS unit, yet the speedo has a significantly higher reading than the speeds I am travelling at... The stock standard tyres on this vehicle are 205 r16 (i think) and I run 31" Cooper STT's. This was never a problem in my old NA (with exactly the same engine and tyres), but this one is just getting worse and worse. I was thinking about talking to Marks 4x4 Adaptors in Dandenong and aksing them about a speedo correction kit. Do you think that this would fix my problem, or do I need to first address the problem with the speedo (eg speedo cable)?? Any help would be apprieciated, Thanks, Sam

[Sammyboy]

Thanks for all your help guys...........................