are your wheels smaller than standard?
that can chuck the speedo out, also clocking can mess with it. the wonam i work with her husband traded in his 54 plate volvo s40 sport thing, they only got 2 grand for it because it had done 90,000 miles, they went out the other day looking for a new car for her, they saw the car they traded in going for £12 with 30,000 miles on it. and it was deffinetly the car they traded as it had the same scars on the wheel from when she kirbed it. he poked about the car and it was obvious someone has been in at the ecu and stuff cos they were spotless and the enginer not, lol, needless to say the police got called,lol.