Yep 10mph at all speed when I the sat nav reads 30 the dash is reading 40mph and so on. When I first got the car just over a week the speedo cut out completly along with the fuel gauge so may be the cluster is faulty. But as I paid £310 for the car with 10 months mot and a 4 months tax custom exhaust from powerflow, pipercross induction kit st220 lights and a perfect set of momos I expected a few issues.
OK, well that £310 does set expectations...
Presumably, it is reasonable to expect that, with the price and the various 'trick' bits that have been fitted that the previous owner may have done some weird things, either as ways to avoid spending more money on the car, or because his definition of 'fun' stuff was a bit off.
I didn't really take the 10 mph as a constant offset seriously, at first (but I should have...you did say that, and I assumed that it was just shorthand). Anyway, with an 'old technology' speedo, the positive force actuating the needle acts against a spring, and if that spring is set to give too little restraining force (that is, the initial position is set wrong), then you will have a constant offset.
Any sign that the dash has been out a lot? For example, retaining screws with rounded screwdriver slots, some screws missing, that kind of thing?I just wonder whether the previous owner has had reason to mess with this, perhaps because the original speedo broke, or something, or he was trying to compensate for odd sized wheels/tyres, or something.
I'm not sure how 'old tech' your speedo in fact is; by then, it could be driven by, for example, the ECU or another electronic module, but I can't see any good reason to think that the electronics is the problem, but maybe I'm overlooking something. Most of the other things that I can think of would give a percentage change, and not an offset, so after that, I think I'm out of ideas.