No dwarfs living under my '85 Carrera. It's firm but that's about it. I had a shimmy with mine a while back too ....wheel balancing and alignment sorted that one out though. As for the speedo error, what tires and rims are you using, if they are not stock size the rolling circumference may have changed which will cause your speedo to over/under read depending whether they're bigger or smaller.
Here's a prety cool site to calculate tire sizes
http://www.powerdog.com/tiresize.cgi
Ralph