Initial fillup on a new car
 

Is there STILL no way to show the mileage until your SECOND fuelup with a brand new car, in the year 2018??

Any model of car can get a wide range of mpg figures depending on use and driver input. If a new car can get between 20 and 50 mpg how does software GUESS what you will get?
It is 2018 in the real world, not a fantasy land.

yo what?

the gas mileage of your first tank of gas in a new car is the first odometer reading divided by the amount of gas you put in the tank, not rocket science

the problem is that fuelly won't let you put in an initial odometer reading at all before your first tank refill

How do you know what fuel the dealer put in before you got the car? Did they fill it? Did they just stick a load of fuel in and SAY they filled it?
If you wish to assume the tank was full and that the odometer read zero on the new car (maybe in the US, definitely not the case on the UK), then there is nothing to stop you entering two fill ups on the same day.
Personally, I never calculate the mpg until my second fill up. When I get a new car, I fill the tank to the first click off, record the mileage reading, then at the next fill up I do the same, plus record the amount of fuel, and calculate my mpg. That is exactly what Fuelly does.

You shake hands and take the keys, drive to the nearest station and fill the car, 2 ounces or 2 gallons, whatever. Now you have the baseline to build on with only a few miles driven.

Ahhhh the first thing you do is plug in a scanguage and it will tell you your mileage as you start to drive. You can't measure gas mileage until you burn some fuel so fill it and start driving unless you want to start with an empty tank and run it out with only a gallon ... doesn't work too well that way unless it is a lawnmower or something of like getting stuck or carry a gas can in the trunk. Most new cars I would think will have some form of trip computer with gas mileage so check the owners manual. You know that thick booklet or cd in the glove box that came with your car. Engine and a few other drive train components will be tight at first so don't expect too high at first.

I just got a new car and entered the odometer reading, 16 miles, and zeroes for the price, gallons, and total cost. Is that the way to do the first record?

That's one way. My way is in post 5. I did that with my new car so only the first 4-5 miles didn't get counted in my results here.

It's just odd that Fuelly does not provide a clear-cut first-log-entry way of doing things. I'm looking at what other apps are out there.

I see what you mean about topping off the tank first. Too late for me to do that. ;-)