First I see this is your first post. Welcome to mercedesshop, if you like to learn about Mercedes, you have definetly come to the right place. As far as fuel prices go. You're right gasoline prices are going up, and I would imagine diesel may follow. For gas, it's a seasonal thing. With more people gearing up to travel in their cars because of the nicer weather, the gas stations start to jack up the prices. Diesels do get better fuel economy. My car is 21 years old, weights a little over 4,000 lbs and I am averaging 25-27 mpg, and that's in combination driving.
Diesel used to be really cheap compared to gasoline, but in recent years that has changed. Around here in NJ, at most places diesel is priced somewhere between regular unleaded and premium. At my last fill up I paid $1.35 a gal and at the same station, supper unleaded was $1.47. Another thing that makes diesels cheaper is there are no sparkplugs, sparkplug wires, dist. cap, or rotor to change. Basicly all a diesel needs is clean diesel, clean filters, a strong battery and periodic valve adjustments(every 15K). Now don't let the words valve adjustment scare you, did you know that all Hondas/Acuras require valve adjustments too. Actually to own a diesel, the operating cost would be LESS than a run of the mill Honda. Are you looking into buying a MB diesel?

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member