2010 1.6 Astra SRI here and I get circa 47 mpg commuting 42 miles a day. Mostly single carriageway 50 mph limit, not much town driving at all. Come the summer, I expect that to go to around 50mpg.
I'm in no rush, I start early, not much traffic, accelerate smoothly, decelerate early, hardly ever use the brakes, usually driving at 50 to 60 mph. I do drive by the computer, if 50 mph is using too much, I'll either accelerate to find a speed that's more economical, or slow slightly. I can be doing say 52 mph and achieving 40 mpg, slow to around 49 mph and the consumption drops to a lovely 55 mpg ( just rough figures, but you get the gist)
Don't accelerate on hills, DO make use of downhill sections to build speed, my cruising speed can vary, but I'm constantly finding the most economical speed as I go.
You can probably deduce I've overanalysed all this, it's become a way of life. But I'm not that much slower than many drivers...I just get there in my own time...and a lot cheaper than the guy who overtook me at 75 mph...to save 10 minutes on a journey.
I do circa 250 miles a week, sometimes more. Breaks my heart when I think 2 years ago I commuted almost exclusively by bike.