I was comparing the fuel consumption of the S5/RS4 and A6 4.2, and was surprised to found out that the S5 is closer to the RS4.

A6 4.2Q combined consumption: 10.6l/100km
S5: 12.4l/100km
RS4: 13.5l

Why is that? I assume the S5 is lighter than the A6 and the performance of the S5 is closer to the A6...shouldn't the S5 be the most efficient?

While you are right the S5 is lighter than the A6 4.2. The relatively short gearing of the 6-spd manual in the S5 make it a gas hog, but a jackrabbit performer. Compared to the A6's more luxury cruise style.

The S5 is Audi rated at only 0.1s slower 0-60 than the RS4. The A6 4.2 is 0.9s slower.

If you want performance close to an S5, but the size of an A6, then the S6 is your game, but then gas mileage plummits to be equal to or worse than the S5.

The A6 4.2 is the only one of the bunch here in the states not hit with a federal gas guzzler tax.

So the S5, estimated by Audi to be 5.1 sec is even a larger performance gap than previously mentioned. Also it weighs a couple hundred pounds more and handles like it. Lastly, my A6 4.2, in heavy throttle, mixed highway and city gets 14mpg. The dealer demo S5 (also driven hard and only in the city) had an average of 12.9mpg. In perspective, my 2.7t A6 six speed averages 19.0mpg in heavy throttle city driving.

EPA has changed the way it estimates MPG.

Starting in model year 2008, estimates will reflect the effects of

* Faster Speeds & Acceleration
* Air Conditioner Use
* Colder Outside Temperatures


For example, my 99 M3 was 19/26 but according to teh new standards, its 17/24 which was much closer to what I really got.

Check out below, you can compare what your car's "new" mileage figures would be. This can give a better comparison.<ul><li><a href="http://www.fueleconomy.gov/feg/calculatorSelectYear.jsp">http://www.fueleconomy.gov/feg/calculatorSelectYear.jsp</a</li></ul>

S5 EPA 14/16/21
RS4 EPA 13/15/20
A6 EPA 16/18/23