^ DIS isn't accurate at all. I log my fuel ups on an app called Mileage. Also shows lots of info/statistics, so over time you can analyse/compare, if you want.
iOS App? Is it just called mileage or something else? obviously searching for just mileage has given me loads of result. Interestingly enough a Genuine Audi Mileage Tracker!
The one I'm using is for Android. I'm not sure about the others. It's, 'Mileage' by Evan Charlton. https://play.google.com/store/apps/details?id=com.evancharlton.mileage
Edit: Only thing you have to watch out for is US and UK gallons (imperial) when tweaking the units for mpg.
Also a genuine one from Audi? Hmm...