I have a W203 C320, so with a 3.2 Litre V6 engine, twin spark plugs blurb!

For the US market, I would probably buy Premium, as that is closer to the UK and European octane levels.

In the UK and Europe standard (regular) is 95 RON Octane. I personally use Shell V'POWER - premium here is 97 to 99 RON.

Since switching to V'POWER im driving the same way - same journeys, and now getting 330+ Miles per tankful instead of 290 to 300.
Bear in mind my driving is usually quite heavy city mileage, and it all hills.
Motorway/Freeway driving is a credit and typically 400 to 450 miles per tank at 80mph.

In my point of view the extra cost is evenly balanced with extra mileage - about 10%+

Thats just my penny's worth.

 

octane levels are set by the compression ratio in the engine. The higher the compression the higher the octane. Even though my 94 C220 runs ok on 87. The better gas milage on 93 offsets the additional costs. We are only on the high end talking about a couple hundred dollars a year. I do over 100 miles a day mostly highway and getting 29 mpg and not disappointed at all. With the lower octane I can notice the difference at high speeds of 100 plus.

Have have my share of performance cars. 68 jag XKE 96 BMW Z3 and assorted triumphs and a SAAB sonnett. The higher the compression the slower you want the fuel to burn. The older english cars wouldn't even start with 87 octane. Just do what the manufacturer recommends.

 

I know this is an old thread but just want to add the fact(or myth) found in the owners manuals of these cars mentioning possible malfunctions in the catalytic converter if less than recommended octane levels in fuel is used.. Which is thousands of dollars to replace...

As for the savings, it is not significant enough to save you lots, you could easily spend a multiplier of your yearly savings on one single diagnosis visit at a Benz dealer to find out what happened with your car after feeding regular gasoline..

 

I've used regular in my C230 for approx. the last 20,000 miles with no issues at all. I started doing this after talking to a seasoned European shop owner that said it really wasn't necessary to run premium all the time.