City - highway?? Wouldn't MPH be better to use
 

I was comparing my Outback MPG on fuelly and something came to mind. It is very difficult to determine what percent is city verses highway driving. Plus city driving in a small city is not the same city driving in say, Boston.

The small city will most likely have a higher average mile per hour than Boston. SO if people started giving their average MPH we could compare what our MPG to others that have some of the same average mile per hours.

It is always a guess when I put the percentage of city verses highway on fuelly.

Yep, estimating city v highway miles for each tank is just that, an estimation, i.e. a guess, but at least it gives you some inkling of the environment in which the miles are being driven.

In my case, I stipulate a standard 10% city for each tank unless my driving conditions are significantly different for any given trip, in which case I stipulate a different proportion. I generally use 10% city miles driven as my standard split because that's very roughly the proportion I drive in one small and two large cities, and the rest is on the highway. That's the best estimate I can give for each tank and still keep my sanity.

I've found the largest influence on my fuel economy is the speed I maintain on the highway, since 90% of my miles are spent there. So, my stipulating that I spend 90% of my miles on the highway gives a reason for my higher MPGs as compared with someone who spends 90% of their miles in the city.

If I were to stipulate average speed it would further refine reasons for the MPGs I achieve, but since my car does not give me that information, I'd have to keep notes on elapsed time during trips, which would be quite the headache for me.

So even though I think you're right that listing the average speed would be worthwhile in comparing fuel economy between cars, it would cause a lot of complexity for those of us with cars that don't provide that information, and especially for those who spend a lot of time in the city with multiple short trips for each tank.

With my small engined Honda Jazz, designed for city use, I actually get better Urban mpg then I do highway. Lots of out towns and cities have large areas with a 20 mph limit, and I find I can sit in 3rd and 4th and get great mpg figures.

Somewhere in these forums is a post from a member who uses a formula based on the US EPA fuel consumption tests. In those tests city mileage averages out at 22 mpg and highway at 49(stand to be corrected here) and he used those to establish the percentage of city and highway. Coming from the UK I've been using the European system (soon to be changed - it was unrealistic anyway) where "urban" driving averages at 12 mph and extra urban at 39 so my city figures are really based on the urban cycle. It shows I do 75% city and, to be fair, that's a pretty fair reflection of what I do.