Your Astra weighed 1000lbs less, had no air conditioning and probably no air bags and would have probably killed you in a 40mph collision though, innit?
And that mileage was presumably measured using imperial gallons, which are 20% larger than US gallons. And it probably didn't get 40mpg anyway unless it was on the highway 100% of the time.
http://www.carfolio.com/specifications/models/car/?car=26732...