I just wrote a program to convert fill ups put on a Garmin device to a CSV file that Fuelly will recognize.
I am not sure if a program like this exists but I needed a weekend project and this is sort of what it turned into. I like Fuelly a lot and use it for all my fill-ups. I recently bought a Garmin Nuvi 1350 and it is convenient to fill out my fuel up information right there while it is with me. I saw that it saved these fuel ups in a CSV file. I remembered that Fuelly also accepts CSV files and saw that making an extremely simple, minimalistic, conversion program wouldn't be all that difficult. If anyone out there has a Garmin and would like to test this out that would be great. I only have the Nuvi 1350 so I can not say if it will work for all fill up apps on the Garmin. The program is free to download and free to use, all I ask is that if any issues are found to please let me know so that I can continue to improve the program.
A search revealed only this old thread with a broken link, so attached are two Perl scripts I whipped up to convert the files generated by my Garmin Nuvi (At_the_pump.csv) and the MyCars Android app. Rename from .txt to .pl and redirect output to the file you wish to upload.