With the release Hot Wheels in 1968, Matchbox was under new pressure to compete.  Their cars had less "play value" when compared to the fast cars from Mattel.  The thin wheels used created a lot of friction and were simply not fast.  Matchbox responded with their Superfast line of cars, many featuring a metal base.  The early cars from 1969 and 1970 featured thin wheels, but over time the new wheels were widened as new castings were created.  I had a few of these cars collected when I was little, but have recently purchased a number of the known faster ones.