I think the 300SEL has plenty of power for around town driving, but getting on the freeway can be scary sometimes...lol. I think that a 560 rear end would save you a lot of money in gas if you do a lot of highway driving... of course that would be at the cost of some low end power...

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles