Ahh the old "Electric drill on the spedo cable trick"
Reminds me of a story:
I was working for a big company many years back and had a company car. I was out on the back roads heading to a site when I hit a big stretch of straight road. Not having had the VN Commodore long I thought I'd see what it would do. So I stood on the gas and wound it up to about 180. Common sense got the better of me (and realising that there were some big logos on the side of the car) so I slowed down again. The speedo however had a different idea and proudly displayed my transgression by sticking at 180!!?? The big company had it's own service department and I was thinking "How the hell am I going to explain this?". So using my vast aray of mechanincal talents I figured I'd fix it by banging on the top of the dash. The speedo was obviously wise to this trick and jumped further arround to 220 just to show me how it felt about my percussive repair technique. I tried a bit more banging and then moved on to swearing at it but neither technique worked and I was forced to give up. I continued my journey looking at that damn speedo and worrying. The speedo must have figured I'd been punnished enough because it decided to behave itself again about half an hour later.
Cheers