I am looking to get a 2008 Audi A6.

I know these cars require premium gas.

I see the fuel on there as 21/28 on some models.


is the 21/28 for premium or just unleaded.

 

Those EPA estimates on the window stickers are based off of the use of premium fuel, which is recommended by Audi. Unless you drive like, "Ms. Daisy", don't expect to see mileage that high. In fact, under normal conditions and with moderate use of the A/C and a mix of city/highway driving, you can expect real world mileage closer to the city rating of 21mpg.
Keep in mind that if you do choose to use a lower octane gasoline, it will not hurt the engine, but the lower octane gasoline will decrease your actual fuel mileage. This is because the ECU will retard the timing a small amount to minimize detonation because of the lower octane fuel being burned. Because of the retarded timing, fuel mileage and power will decrease slightly.
If you consider the savings on a fill up, the decrease in total mileage per tank doesn't save you much when you compare it to the total mileage expected burning premium fuel and add the additional cost for it. I've done this with my car in the past and found it to be true for me...I'm sure someone will que in and argue another point.

 

I agree with most of what he said. With the exception of the lower octane not hurting the engine at all. The main reason the car requires a higher octane is due to the high compression ratio of the engine.

The octane rating of gas indicates how much it can be compressed before igniting. The lower the rating, the less it has to be compressed before it spontaneously ignites. If it ignites before the spark, you're going to have engine knock. The engine retarding the timing helps a bit, but you REALLY don't want to have detonation in an engine. The uneven burn causes ALOT of stress on your internals and can blow your engine if it gets out of hand.

The .20 per gallon you save by putting in regular gas vs, premium isn't worth the damage you'll be causing to your engine in the long run. Not to mention that due to the fuel consumption, you're really not saving any money by switching in the first place.