Firstly this is UNRELATED to any diff/wheel size changes, as anything like that would effect both equally.

Heres the problem. The LED diagnostic mode, and my actual speedo read about 5km difference (with the speedo being 5km faster)

If anyone wants to try theres I'll repeat the sequence to enable the mode, as it looks like the document for it is down.

I wouldnt be too worried except recently ive got the diff upped to 3.23 from 3.08 which adds another 5% of error in the same direction. lol if only the original error was the other way around :P

ugh yes mine did...until i changed the % on it and now it reads about exact. I judged it by adjusting it...driving at 90..then settings my speed warning to 90 n if it didnt beep when i hit 90...obviously it was still out. Works bril now, its exactly the same as when the dash is in diag mode too. I just hope the car goes the speed it says it is else ill get booked lol...i still seem to always be going slower than everyone else on the road tho haha.

But how did you adjust it?

I assume the beeping speed meter is some funky fairmont/lane gadget as my futura doesnt have the trip computer and other fancys

Yeah I actually printed out that doco on the diagnostic mode about a year ago and I still have it tucked it away in my glovebox for when I need it. But I also found the Diagnostic speed seems a bit faster than actual. I also know the sequence on how to activate it. So if anyone wants to know it PM me.