At least I'm not the only one who was thinking those things.
I actually emailed the author right after installing it, and I said all that to him (actually, I cut-'n-pasted all my comments above from my email to the author), but he never responded.
I didn't think about the gas maxing out at 20 gallons (I only have a 12.5 gallon tank), but yeah, for large trucks/SUVs, they have much bigger tanks. Heck, there might even be a trucker somewhere with an iPhone who would find this useful, and they have double tanks that hold 50+ gallons each. It should allow a LOT more.
Come to think of it...there should be an option for the person to specify how large their gas tank is, so the app can track how much is left in the tank and more accurately determine what the MPG/fuel cost is. Most gas tracking apps I've seen (and truth be told, I've only looked at a few and didn't like any of them) have that option.
edit: heck, while I'm at it, I'd like to see an entry for "oil change", so the driver can track how many miles it's been since the last oil change (and conversely, how many until the next one is needed.) Would like the ability to specify how many miles between oil changes the user wants (some people like me do it at 3k/3 months, others at 5k, others use that extended 10k oil...)