I read a thread on Fuelly about how Americans would record city versus highway. City was an average of 22 mph and highway an average of 49 mph.
In the UK I use the difference between urban (an average speed of 12 mph) and extra urban (an average speed of 39mph). You deduct 12 mph from whatever your average speed on the computer is, multiply that by 100, divide by 27 and that gives you your highway (or extra urban) percentage.
Oddly, in both cases, the difference between the 2 sets of figures is 27.
So, in my case, I'm saying that 65% of my driving is urban - ie very short journeys with lots of gear changes and the engine below operating temperature.