Unless you've got the actual receipt from when the speedo was replaced, you can't tell exactly what it was reading at the time, so you can't work out what it has really done.

All you can do is tell the prospective new owner that this has been done, and let them judge for themselves what the actual mileage is.

It will affect the price. To what extent is debateable. Maybe 20% of the price, maybe a bit less. However, you will do far better by being honest from the outset. If I was selling a car like that, I'd probably try selling through the evilbay, auction, £1 start, as you've got plenty of space to fully detail the situation to prospective buyers.

Sounds more like it's been clocked!!

A new speedo would have zero miles on it.

Of course it could have been a secondhand speedo. Do you have the receipt in the file?

When you advertise, just be honest. Your car is 12 years old, and not worth too much. Any potential purchaser should judge it just on its condition and your honest desciption. The fact it had a second hand part fitted a couple of years ago really shouldnt matter too much if the rest of the car is still good.