New to the site which is great by the way. This may have been mentioned before but I have adapted the City/Highway formula to relate to the European system as used in the UK. The reason I did this was my average speed of 20 mph indicated no highway driving whereas I had done at least 40 miles on the M6.
All you do is change the values in the formula. For Urban assume 12 mph rather than 21 mph as per the US EPA City figure and for Extra Urban assume 39 mph.
Only done 2 fuel ups so far but this method gave me a more accurate picture of what I was doing on our congested roads.