Austin- 225/40R18's only have a 8.9-inch wide section-tread. If you want it to stop as well as 500E with 245's on it, you'll need to get the front tire closer to 9.5-10 inches wide.

BTW: I run 245/50R16's which are 9.7-inches wide on a 8-inch wide wheel.

:-) neil
1988 360TE AMG
1993 500E