Take a look at our FAQ entry about SMS: Fuel-up via SMS. Specifically, this part:
"If you want to add a fuel-up to more than one vehicle at Fuelly, you'll need to set a SMS shortcut letter for each vehicle and add that letter to the beginning of your message. For example, you could set the letter "p" to represent your Prius and "c" to represent your Civic. When you want to fuel-up the Prius you would send p [miles] [price] [gallons]. And for the Civic you would send c [miles] [price] [gallons]. Without a letter, the fuel-up will be applied to your primary vehicle."
You set the letter in your vehicle preferences. Go to your vehicle profile page and look for the link: Edit This Car. (Or Edit This Motorcycle.) Click that, and look for "SMS shortcut letter" on the page. Click "Update Car" and the letter will be set.
I just added "...in your vehicle preferences..." to the FAQ to make that more clear. You also have the option of setting the letter when you create a vehicle, so I think people who are new to the site will discover it that way much of the time.