aretardedorange

I always use 93. I have noticed a big difference in MPG if I use regular 87 vs 93. Haven't tried it on the A6 yet, always use 93 in that car but in my daily driver, a 08 TL I put 87 two times and I was getting 19 MPG on the highway. With 93 I get around 30.

With that said, the cost difference was $3, I rather run 93 and avoid poor MPG. My car felt really sluggish with 87 (of course that could be my imagination) but the MPG was no lie. With a mixture of city/highway my full tank lasted 200 miles. Usually with 93 and same mixture of city/highway I at least get 320 out of the tank.

newmoon

I know I'm bumping this old thread.

Did anyone consider the significance of the 3.0T engine being supercharged? When a supercharger compresses the intake air, it increases the temperature as well. Not all of the heat can be reduced by the intercooler so the intake air will always be hotter than the ambient temp, or what you get in a normally aspirated engine. Hot intake air increases the risk of knock because when it gets compressed again in the cylinder, its temperature increases again and may get hot enough to ignite the gasoline mixture on its own before the spark plug fires, which is knock. Therefore, in a supercharged engine, it is even more important to use premium 91+ high octane gas, which resists knock.

I would not risk running regular 87 or mid-grade 89 octane in the 3.0T supercharged engine or the 2.0T turbocharged engine.

The engines do however have some design features which reduce the risk of knock such as using direct fuel injection which reduces the intake air and cylinder temps as the gasoline evaporates, reducing the compression ratio (but it's still pretty high at 10.6:1), and the knock sensor system to retard ignition timing which might make it possible to adapt to lower octane fuel, but then the engine will produce less power.