It's a little more complicated than that. To add data to the system we need to verify identity. To verify identity we use a username/password system here at the site. Your app would need to ask for the Fuelly username/password to be able to POST data to our site.
I'm sure you're a nice, trustworthy guy but we don't know you personally and we can't know everyone who wants to integrate with Fuelly personally. So we need another way to verify identify that doesn't involve sharing personal site credentials. That's a severe security problem. Typically apps use
oAuth, and it's something we're interested in doing at some point when we're ready to build an API.
We'd also need to create/test/document all of those POST methods and make it available to everyone. Then we'd need to support that API via this forum and email as people run into problems. Again, something we'd like to do someday, but it involves making a commitment to build the API and take on the extra work of helping developers who want to use it. We're not there yet.