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."
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