I finally added this today and added information about it to the FAQ. Here's what I said:
To add a city percentage to your fuel-up, add a "c" followed directly by the percentage before your notes. If your tank included 75% city driving, you'd add "c75". So an SMS with 100 2.00 5 c75 [notes] will tell Fuelly to set this fuel-up to 75% city driving. Note that all city percentages will be rounded to the nearest 5. So if you tell Fuelly "c21", it will be rounded to 20% and "c99" will be rounded to 100%.