No plans to expand to Australia right now. We're using a US company to handle messaging, and they don't have any international numbers. It might be worth checking with your carrier to see how much a text message to the US costs. If it's not too expensive you might be able to use our current number.
A tip for SMS users: oftentimes, your phone will display the name of the recipient above the box where you compose your text message. At least it's done that on the last few phones I've had, including an LG enV3.
To avoid forgetting the required format, just change the contact name for Fuelly from whatever it currently is to "Fuelly Miles Price Gallons" or "Fuelly MPG", so you can easily remember it when you're filling up.
When I submitted via SMS, it went to my retired car. I didn't have a place on my profile to select active vehicle. It looks like if you have one car, and retire it, and then start a 2nd car, your retired vehicle becomes your permanent primary - unless you delete it altogether.