If you're in the US, you can now add a fuel-up at Fuelly via text-message.
It does require a bit of setup first, here's what you need to do:
1.) Set your primary vehicle and your mobile number in your profile.
Make sure you're logged into Fuelly and then go here to edit your profile. Look for the "Primary Vehicle" option and select the vehicle you want to fuel-up via SMS. Then at the bottom of the page under "SMS Preferences", set your ten-digit mobile phone number.
2.) Learn the text message format.
To fuel-up via SMS you need to format your text message in a special way. The basic format is: Miles Price Gallons (MPG). So if you drove 300 miles, paid $2.00, and put in 10 gallons the text should be: 300 2.00 10. If you're using odometer tracking, swap out odometer for miles. If you're tracking with liters or kilometers, you can think of the format as [distance] [price] [fuel volume]. (Though that's not as catchy as MPG.)
Anything extra after the first three numbers will be added as the note for that fuel-up. So if you text 300 2.00 10 chevron off the freeway the last bit of text, "chevron off the freeway" will be the note for that fuel-up.
If you want to mark your fuel-up as partial, add a "p" to your fuel volume without a space. So 100 2.00 5p [notes] will tell Fuelly that this is a partial fuel-up.
3.) Send your fuel-up text message to 503-512-9929.
Fuelly will send back your MPG for that tank, your average MPG for the vehicle, and a link to your vehicle profile page at Fuelly. If the fuel-up didn't work for some reason, we'll let you know that too.
This service is only available to our members in the US, and your text message rates will apply.
Updated (2/19): Added info about fuel-up notes.
Updated (4/27): Added info about partial fuel-ups.
Updated (2/2/2011): Any changes to SMS will documented in the FAQ, here.