I find the MPG can vary wildly depending on the way my 2.0tdi DSG S3 estate is driven or what is in/on it - is that the same for everyone? In Eco mode I can get high 50's mpg, in sport mode on a particularly spirited drive it only managed 42mpg, then on a recent trip to Snowdonia with wife, kid, dog and a roof box even in eco mode it only managed mid 40's even with very careful economical driving. Is this normal? To be fair I only have 2500 miles on the car so its still bedding in. My last car was a volvo XC90 which being such a big heavy beast didn't seem to notice if it was just me in it, or fully loaded with camping gear, people, roof box, etc the MPG was always about the same so maybe I'm just not used to a lighter more efficient car being more susceptible?

To be expected. A roof box is an air brake and fully loaded car is heavy. 42mpg sounds about right.

Snowdonia is quite a hilly place so your MPG will suffer, the extra weight in the car won't do you any favours too.

As above there are two main losses that have the greatest impact.

Air resistance, exponential impact at high speed or by increasing drag with roof box

Second braking is conversion of the energy you paid for into waste heat which you the pay for again. The brake pedal converts fuel directly to heat. Minimise brake usage for best economy

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I used to own an xc90 as well, fully loaded or empty, cruising or hammering it was always the same 26mpg. I find my s3 likes long slow cruises and super diesel. Anything under 25 miles on a motorway and it'll always return about 45 mpg. Fully loaded or not. Longer journeys and it'll do about 55 mpg. Stuck a tank of super in it and no difference in my shorter journeys but on the longer ones it jumped to around 60mpg