jus bought a 1.6 escort today 94 model, and when im doing 30 its shows 40, when im doing 60 in reality on the speedo it says 70 so theres alwatys ppl up my arse , plus i can feel its too slow, so i left the car parked with engine on and bamn needle is on 10mph parked, so i banged top of dash went down but as soon as i start driving back to add 10mph, any theories answers???