I spoke with a MB rep about getting 18" wheels on my car and he said my ride would go away with them, that I would feel every bump in the road.
The wheels I want come on the 2003 S600 with 245/45/18 tires.
He said the people that own these cars have to put up with that, (but they should handle better and look better).
If I was to put this setup on my car, would it screw up the speedo reading, do to a different tire diameter?
Thanks

How would this change affect my ride?

Actually the ride on 18" tires isn't that bad at all. The airmatic adapts well. I say go for the 18" upgrade

Riding on 20"s definitely affects the quality of the ride.

Any other comments on effects of 18 inch wheels on this car are appreciated.

I have the 18" AMG staggered wheels on my '01 S500 and I don't have a problem with the ride. They came on the car so I don't have anything to compare with. You don't feel every bump, just the larger ones, but the wheels look so good I'll be keeping mine!