Manufacturers' figures may be faintly helpful as a comparison, but don’t bother trying to achieve them. It will be a real boon to be able to get a realistic idea of what your fuel costs will be.
Back in the day, car magazines used to run fuel cards, where anyone filling a car with fuel had to note the mileage, how much they put in, who they were (so the person writing the test knew if a real lead-footer had been in the car) and where (which helped you remember if you’d been on lots of motorways, over the Alps or stuck on the M25). What Car? used to send testers out on an economy run, but stopped while I was there a few years ago due to lack of times and bodies.
Now they’ve been collaborating with boffins and have started strapping kit to the cars, that looks suspiciously as if it could make them jump through time at 88mph. It actually measures the emissions coming out of the exhaust pipe and calculates fuel consumption from that.
Have a look at http://www.whatcar.com/truempg/my-true-mpg to find what your next car will really do to the gallon.
See http://www.whatcar.com/car-news/what-car-true-mpg-the-best-so-far/262495 for their winners and losers. Glad to see the MX-5 is a winner again.