no, you didn't. good try though. the speedo goes by the rolling diameter. If you increase/decrease the sidewall, increasing/decreasint the width wont offset the change. What I think you meant to say was if the height of the rims instead of the width.
At any rate, I think the speedo can reprogrammed but I am not sure how. On OBDII there is a computer than can be plugged in to change it, but I don't know about OBDI.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>