I regularly get 24-25 MPG highway with my '97 C36 (completely stock) with Temp-O-Mat set at 74-79 MPH, but only about 18-20 when burning rubber and kicking ass in the city. I run 92-93 octane premium fuel exclusively, but no additives or other snake oil crap.

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Robert Fini

'12 ML350 BlueTec, 111k
'06 E320CDI (x2)
'05 T1N Sprinter 2500/Pleasure-Way Plateau TS, 72k
'97 C36AMG, 325k (son's)
'94 E320T, 249k
'93 190E Sportline LE, 168k (daughter's)
'84 190E-2.3/5spd (Stage Rally Racer)
'66 230 W110 Sedan (Barn Find, Vintage Racer build in progress)