i used to get 34 out of mine, but it wasnt the vvt, so yours should be doing better, but most of it comes down to driving style, so motorway cruising would get better consumption than driving round town
It wasn't a astra but my Vec-c, i used to get anything from 28- 47 mpg out of mine. It just depends what driving your doing. iirc the mpg figures state 28 is the usual for town driving.
So many parts can play into the factor, such as tyres, oil, filters, plugs & tracking. As for fuel treatment, DONT use redex, this will kill you cat.
Forte fuel treatmen ftw.