300cc is a big difference, but with all due respect, theres not THAT big a difference between the consumption of the mentioned1800cc and 1600/1500cc.
As for being specific - that's quite a debateable issue. There are so many factors you need to consider if you are being specific-as consumption depends upon all of the below mentioned factors. You asked the first question yourself, the next questions will be
2- How many people are sitting in the car
3- What are the tyre specifications
4- Was he going uphill or downhill
5- Was the road tarmac or soil
6- When was the last time you tuned your car
and the list goes on.
And you know one thing for sure that it depends ALOT on the driving style as well.
That is the reason why I quoted the margins, and not the exact figure. I'm pretty confident the majority of 1.5-1.8 civics give nearly that average.