Exactly what I was going to say.
Well, it's a 2014 Honda Civic, and he got gas at 3:30 that morning according to that MissingPersons article. Mathematics time! Get ready for some numbers...
A 2014 Honda Civic has a
13.2-gallon tank. I'm going to assume the tank was full when he left the Monticello gas station.
A 2014 Honda Civic gets up to
41 MPG highway. I'm going to assume he was getting about 36 mpg, because car companies aren't always truthful, and that number seems more reasonable based on online reviews and estimates.
A 2014 Honda Civic has either a
1.8L or a 2.4L engine, and while I lean more towards him having a 1.8L, because that's normal, I'll call it a 2.0L engine for ease of numbers.
On a cold day, an idling 2.0L engine is going to burn about
.3 gallons per hour, give or take. I ran some math on my own but someone did a nice calculation elsewhere that I'll link to at the bottom.
The distance between the Monticello gas station and the location where his car was found is about
95 miles.
So!
Starting fuel = 13.2 gallons
Driving from the gas station to Floy wash = 95miles / 36mpg = 2.6 gallons
Approx maxiumum gas in the tank upon arrival at Floy Wash = 10.6 gallons
Idling at that spot = 10.6 gallons / .3 gph = approx
35 hours maxiumum before the car would run out of fuel and the engine would die.
If I'm less generous, and we assume the car was burning more fuel (.35gph) due to it having been cold and Michael likely having the air conditioning set pretty high, that gives closer to
30 hours before the car would run out of fuel.
If I'm really pessimistic, and we assume it was extremely cold (though the images of Michael at the gas station don't seem to reflect that) and the car was burning .4gph for some ungodly reason, the car would have
26 hours to run out.
tl;dr If the car was still running when it was found, it could have been sitting in that spot for a maxiumum of
35 hours and a minimum of 26 hours. However, since the call came into the sheriff's office ON the 27th, the car probably still had a good half-tank of gas.
source for some math:
https://www.quora.com/How-much-gas-does-a-car-burn-per-hour-while-idling