Fuelio. End of discussion. no really its absolutely brilliant.
http://fuel.io/" onclick="window.open(this.href);return false;
Feature list
Easy and clean design (no ads!)
Mileage log (track your fill-ups, costs, fuel economy, partial fillups, gps location)
Multiple vehicles
Useful statistics (total statistics, fill-ups, costs, average, fuel economy stats ...)
Distance unit: kilometres, miles
Fuel unit: litres, us gallons, imperial gallons
Import/Export database to SD
Import/Export to CSV file (Excel, LibreOffice, OpenOffice, Google Docs spreadsheet...)
Show your fill-ups on Google Map
Languages: 15+ translations
Charts: Fuel comsumption, Fuel price, Fill-up costs, Total Odo, Monthly costs
Create report for your vehicle
Bi-Fuel vehicles support (cars with 2 tanks - for example Gasoline + LPG)
Flex fuel type support
And more...
PRO feature list (now FREE!)
Dropbox Sync (official API)
Auto-sync with Dropbox (while adding fill-ups or costs)
Google Drive Sync (official API)
Auto-sync with Google Drive (while adding fill-ups or costs)
Shortcut (widget) for faster fill-ups adding
Costs module you can track other expenses of your car (not only fuel)
Cost stats - you can define your own category (like service, maintenance, insurance, wash, parking...)
Summary and each category stats
Cost charts (fuel vs other costs, categories, total montly costs)
Donation to author :)
I bought the pro version many moons back, I see its free now. Been using it for more than 2 years.
