You are suffering from a classic case of the low profile tire syndrome. That's why you never see a limo or any other luxury type car wearing tires with a marked low aspect ratio (height vs. width). Also the midline of your custon wheels may be slightly different from factory. As for the tires, the low aspect ratio skins generally have less cross section and truly do put more rubber on the road. By the same token having more rubber on the road means more intimate contact with the road and all it's little irregularities. Also the tread to bead distance on these tires is much less than your standard ratio of 65, so less sidewall is available to absorb small imperfections in the road surface. This is the price you pay for that "sporty" look. BTW, have a local friendly police officer clock you vehicle for you, you might be surprised at what your speedo is telling you. In a nutshell smaller diameter wheels have to turn faster and larger have to turn slower at the same road speed.

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Jeff L
1987 300e
1989 300e
1987 BMW 325