Unfortunately the C240 is kinda underpowered as far as fuel efficiency is concerned. I find that when I get them as loaner that I need to give it more gas to be where I am used to with the C230 I have. I get horrid gas mileage in my 190E because it is power limited for getting up and going. The C240 I find has to be almost floored to get what I get with half pedal and a 6 speed tranny out of my C230. I mean the car has enough power for me and all but it gets horrid mileage compared to my C230. Of course either car of mine gets filled normally only once a week. But when I had a C240 Loaner for a week I noticed I had to fill it once and I got it with a full tank. And the C240 is not exactly rated for the best fuel economy in the C line-up. As for putting lower octane gas in, you will cause the ECU to retard the timing due to Knock. And Knock no matter how quickly it is detected and thrawted is not a good thing for your engine. Also keep in mind that there will be a power loss due to retarding the timing as well as worse fuel economy. So it is up to you.

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~Jamie
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2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....