I own a 2017 Nissan Altima 2.5 that doesn't use ethanol gas.
Its total mileage is 733. I've been on a trip for 2 hr 20 min. I've driven 135 miles for an average of 58 mph.
My mpg is 36.4 according to the screen, but I started the trip with an indicator of 300 miles left. Why does it only say 253 miles? It should say 170. I drove ten miles and it went down by one.
tl;dr: Your indicated estimate of range to empty will always lag the actual fuel burn rate since the system collects data over time to estimate future performance.
I think you've found a situation where the running estimate of the range to empty given your remaining fuel is lagging a bit behind the actual data.
Let's do the math. Your Altima has an 18 gallon fuel tank. Its MPG estimates are 27 miles to the gallon in the city and 39 on the highway. You're clearly asking about a highway trip (unless you're averaging 58 mph inside a city, which I do not advise...) so your current fuel burn rate is on the higher MPG side of things. However, it's possible that your data collection was primed by normal daily commutes which leans more towards the lower end.
So, if your car started the trip thinking "yep, averaging about 27 MPG right now and I should have about 300 miles to empty", that tells us that you had about:
300 miles / 27 MPG = 11 gallons remaining in the tank.
However, you started on your long run which, unsurprisingly, is a more efficient way to travel. At the more efficient burn rate, you have used:
135 miles / 39 MPG = 3.5 gallons expended
11 gallons - 3.5 gallons = 7.5 gallons remaining
7.5 gallons remaining * 36.4 indicated fuel efficiency = 273 miles to empty
So, not an exact match but within a 10% margin of error. I think you're seeing your estimate of range to empty updating over time to account for the fact that you're using fuel at a lower rate and covering a lot more distance.
