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Your engine would run rougher, and your mileage would decrease. Because "regular" sometimes has fewer cleaning additives, you may see gunk build up sooner.
 

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what would happen if i started filling up with regular instead of premium?
If you did this and noticed a decreased performance, sluggishness, knocking and pinging, MPG decrease you can get yourself a can of Octane Boost and that will convert your entire tank of fuel into higher octane fuel.

MB owners manual has a lot to say if regular unleaded is introduced into
their system. It's worth while reading.
 

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I have from time to time with no noticeable change. I think the ECU compensates for the lower octane. Nothing bad will happen if you don't make a habit of it.

The reason I don't anymore is pure economics. At roughly a 10 cent per gallon differential between the grades around here, at worst I am $3.00 poorer running what is recommended in each tankful (15 gallons times 20 cents). When I realized that I stopped using cheaper gas.
 

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Modern engines have knock sensors. When the ECU determines the engine is knocking, it retards the timing. Engines with retarded timing produce lower fuel economy numbers than engines with the factory recommended timing specs.

As for saving money, lets do the math. Assuming regular gas is $3.30 a gallon, premium gas is $3.50 a gallon, and the car in question gets 25 miles per gallon. This means, premium fuel costs 14 cents per mile. If 2 miles per gallon are lost due to the timing adjustment for substandard gas, regular then costs 14.35 cents per mile. It's a losing proposition, and if the pinging (yes, even with knock sensors the engine will ping) happens to cause something expensive to give way, the couple of bucks saved may cover bus fare.
 

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This has been discussed many times. As already mentioned, the money you save is minimal. The owners manual warns against using anything other than premium. I believe it says that using regular could damage the catalytic converter. Look up the cost of a new cat and decide if you want to risk it!
 

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Knock sensors--despite wishful thinking--are NOT designed to compensate for use of low octane fuel. They are fine--not rough--tuning feature.

Mercedes gets essentially the same horsepower from a little bitty 180 cubic inch engine as GM does from a big old iron 350 cubic inch engine via (ta da) high compression! Wanna burn regular, get a Chevy.
 

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Knock sensors--despite wishful thinking--are NOT designed to compensate for use of low octane fuel. They are fine--not rough--tuning feature.

Mercedes gets essentially the same horsepower from a little bitty 180 cubic inch engine as GM does from a big old iron 350 cubic inch engine via (ta da) high compression! Wanna burn regular, get a Chevy.
EXACTLY. If the price difference between regular and premium is an issue, perhaps a different vehicle would better suit the needs of the OP.
 

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Knock sensors--despite wishful thinking--are NOT designed to compensate for use of low octane fuel. They are fine--not rough--tuning feature.

Mercedes gets essentially the same horsepower from a little bitty 180 cubic inch engine as GM does from a big old iron 350 cubic inch engine via (ta da) high compression! Wanna burn regular, get a Chevy.

Or do what lkchris does and get a diesel. No choices or decisions required. :)
 
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