In my last car -- a 1979 300CD--, I found that my performace and mpg increased when I switched to a high flow air filter from K&N and used mobil-1 synthetic oil. I used to keep accurate logs of how much I drove for work reasons, and both of these defintely showed a slight improvement. Also make sure that everything (like fan, wheels, and alternator) turns smoothely so that the engine's power is used as much as possible to turn the wheels and not working hard trying to turn a rusty pulley. From what I have seen, whatever you can use to keep the engine happy gets you better mpg and performance.

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Hemant

'57 190SL
'86 560SEC AMG