Most clusters these days can do a gauge sweep in some form of button press routine. If that doesn't work then take it out and turn it back, could just be a dodgy motor. Also it's not illegal to change clusters. I routinely change customers clusters and pre 2004 you couldn't reprogram mileage. Just write it in your service book. Many cars use an instrument cluster as a main engine module and require immobiliser reprogramming. Ring your local dealer and explain what has happened, you never know they may instantly say its a simple speed sensor or less so simple cluster fault.
OK I got too drunk last night to attempt anything and will now be too hungover to attempt much today, but maybe on Saturday I'll have a go.
Right, a quick revisit. Over the past five days I've noticed that every time I drive the car, the speedo needle returns to rest slightly lower than it did before. So on Friday it was resting at just over 20, yesterday it was down to 10, and now I'm down to just a couple of mph. As the problem is linear, that sort of means it has fixed itself. The MOT isn't due until next Spring, so I can guarantee it'll work flawlessly between now and the morning I drive it to the MOT garage, whereupon it'll go haywire again
You haven't lived till you have taken a dash apart to its core components just to get the clock cluster out. Grab a few beers get a few friends round it is a riot. Then afterwards you get a stack of free extra screws that seemingly have nowhere to go. Its like magic.
Total result matey!! I keep a box of these magical extra screws. They turn out to be useful for totally unrelated jobs!
Overcome by events. Just wait until it goes away or some other larger problem comes along to make it irrelevant! I'd have suggested getting used to subtracting 25 from everything.
Sounds like some crap spacked out the speedo sensor, I know there just a Hal sensor but it doesn't take much to piss them off and go totally derp on you.
Incidentally, is adjusting speedos something that garages can/will do, and is it even legal to do? Mine has overstated my speed by 5-8mph for as long as I've had the vehicle, I'd much rather it was accurate - as I understand it, it's normal for them to overstate by a couple of mph, but this is really noticeably off.
I think they're allowed to overstate by up to 10% or something like that, but not understate, so I'm not sure what potential there would be to recalibrate a speedo whilst ensuring that it wasn't understating.
Have you got the original wheels? Changing in wheel diameter without adjusting the speedo does cause a headache that few people can correlate. My favourite aussie modders explain a fix to the problem rather well
