Let's let math do the talking for us. According to the EPA, the combined average fuel economy on premium fuel is 20mpg for your car in automatic form without 4-matic.

Using local pricing, regular gas is $2.55/gal and premium is $2.85/gal

Let's assume you take a 15% fuel economy penalty for switching to regular fuel (retarded timing, greater quantity injected to combat knock, less power, etc). That drops your average fuel economy to 17mpg.

So averaging 20mpg on $2.85 premium fuel gives you 14.25˘ in fuel per mile.

Averaging 17mpg on $2.55 regular fuel gives you 15˘ in fuel per mile.

That 3mpg fuel economy difference obviously offsets any "savings" you made at the pump filling up.

In reality, the difference can be much greater than 15% based on the quality of fuel. My Honda (which isn't even designed for premium) will do ~25mpg in city driving, but struggles to even do 20mpg on regular. On the highway it's even bigger of a difference - somewhere around 36mpg on premium vs 28-29mpg on regular.

Sticking regular gas in the tank of a car designed for premium fuel is false economy, it will end up costing you money in lost economy.

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