Spreadsheet would give you the most control over how you collect/interpret your info, since you could make it do exactly what you want.
But Fuelly probably doesn't really care about that city/hwy percentage - it doesn't impact your "results" I don't believe. So you could just enter guesses with no consequence.
A lot people use
MPG and Cost Calculator and Tracker - Spritmonitor.de (especially on the other side of the pond).
And you could use our built in fuel log as well. Fuelly's graphs are prettier though.
