Well... that was pretty pointless, there aren't realmpg figures for most of the cars on my list
OK. Quick guide (bearing in mind your mileage may vary):
On average most cars get about 86% of the combined figure in mixed use.
Cars to about 2005 generally get about 85-95 of the combined EU mpg.
NA Petrols still get about 85-95% of the combined EU mpg.
Post 2005 larger engine diesels still get about 85-95% of the combined EU mpg.
Post 2005 Small engine diesels (1.6 ish down) usually get about 75-85% of the combined EU mpg.
Hybrids usually get about 75-85% of the combined EU mpg.
Very recently it all goes pear shaped. Some RealMPG figures are terrible (Ford Ecoboost 1.0 variants 60-70% ish) others are fantastic (Honda 1.6 diesel 90% ish). But they are probably too new for you to worry about. There's a lot of 'gaming' the official testing system and stuff like stop-start systems have a disproportionate impact on official figures.
A good starting point is what % of your car's official figure are you currently getting? Don't expect to hit the official figures with a different car if you're only getting 70% now.
A big difference is that recent small engine cars with a turbo (diesel or petrol, there are exceptions) fail to hit their combined figure on a long motorway run. The older ones would. This is because the NEDC test on which the figures are based includes practically no steady state high speed driving. It only very briefly hits 70mph and above. Therefore the manufacturers, knowing they'll only have to publish official figures, don't need to aim to get good fuel economy at higher steady speeds. These smaller engines get fantastic results at a steady 40-50mph. At 70mph +, they don't do so well.