Ok, I'll bite. How small is the fuel tank in this car? Gas here is in the neighborhood of $2.15/gal for regular, and I just dumped 16 gallons in my Honda this afternoon. Right at $35. The SL usually takes somewhere around 22 gallons. Unless your gas is stupidly cheap, or your tank is stupidly small, I'm guessing you're only doing a half tank or something here.

Keep an eye on your MPG's. With my Honda, I get ~30mpg on the highway with regular gas. With Premium gas I get ~35mpg highway. Regular is $2.15, Premium is $2.49 (as of this morning when I filled up). That makes the cost per mile 7.17˘/mile for Regular, and 7.11˘/mile for Premium. An insignificant difference in cost per mile. I do use Premium when I'm on the highway and travelling since the car idles smoother and feels like it has slightly more power. Cost is negligible between the two, as always Your Mileage May Vary.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)