My 500Es dif is OE at 150K miles and I have been known to track and auto-x the car a few times a year.

I don't baby the car by any means and still get a combined town/highway mpg of between 13-16 in the 8 cylinder... don't use A/C much tho. Can't imagine it making your mpg any worse than it already is.

The difs seem to be solid, have not heard of anyone with a E500E really needing to replace a worn out one. In fact they last long time in the cars with over 400hp too(factory E60s, RennTechs 6 liters, etc).

So, SL600 dif will work more appropiate for your car maybe, not as sporty as 500E? How often and quick do you need to get up to 155 mph?

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Jim

1992 500E
2007 CLK63
2003 Audi RS6
2001 Audi allroad