The mpg modes are normally optomistic, but the "remaining range" mode is pretty spot on (based on 2.5 years of brim to brim tests).

My trip computer (on an 07 vRS) is consistently 20% out. There is some fudge factor that you can set using VAG-COM, but only for earlier models. There's a thread about it somewhere on here but I can't find it at the moment.

Yeah, it looks like it won't work on my 54-plate...

The TC on my GPS is very over optimistic, normally 5mpg out brim to brim and the speedo reads well over. Have to do an indicated 77mph to actually be doing 70 (according to TomTom) i know 10% is legal, but this is far worse than any of my other cars.

I tried adjusting the accuracy of my trip computer using VAG COM. My VRS is an '06 but the option (channel 3 I think) was invalid. I found another value for adjusting which I have tinkered with and going to see out of this tank if its made any difference.