Just reading the review of the Audi S6 Avant in Autoweek.
They started out getting 0-60 of 6.8 ( I won't mention the ridiculous hundreths place decimal they mention), with a full tank of gas. The Manufacturer claims it to be 6.5 seconds. When they got the car back after having a "check engine" light problem fixed they put in five gallons of 100 octane "racing fuel" in an almost empty tank. New 0-60 was now 6.3, a full half second faster! Then when they went back to a filled tank with 91 octane, they then matched the manufacturers claims of 6.5.