Can I double up to account for missed fuel-up?
 

I forgot to note the odometer reading last fuel-up (and the iPhone app was being squirelly). I have the fuel amount and cost, if I simply add that to my next fuel-up with the latest odometer reading, will that work?

I figure it will be roughly double the fuel and the distance, so it should average out the two fuel-ups.

I'd rather not start re-tracking.

It just depends on how accurate you want things to be. If you're ok with estimating things, you could just estimate your last odometer reading. You can also wait until your next fuel-up and check "I missed a fuel-up". That will tell Fuelly that your odometer readings aren't back-to-back and it'll start tracking again from there.

You could try doubling up the fuel-ups, but the system isn't designed with that in mind. If it works for you though, that's fine.

It seesm to have worked. The gas price obviously became averaged, but the mileage seems to be in line with what I was getting.

There's no reason it wouldn't work as long as you had the total amount of fuel and the total miles/kilometers driven. It would just give you an average mpg over 2 tanks instead of 1.