[SSpeedracer]

Mileage * Tank capacity = Miles driven

Your hypermilling:
40.1MPG * 15.7G = 629 miles and out of gas
Or
40.1MPG * 13 G = 521 miles and fuel light comes on

Your 450 miles before light comes on:
450miles / 13 G = 34 MPG based on when your fuel light comes on

Use your phone GPS to check your speedometer. Typically, they design speedometer to read higher than actual speed which results in odometer reading more than actual miles driven.

So GPS speed / speedometer is your calibration offset. eg 71/75 = 0.95
1)fill up tank until nozzle auto shuts off (no more, no less)
2)reset odometer
3)drive
4)fill up tank until nozzle auto shuts off (preferably same nozzle)
5)calculate odometer miles / gallons * 0.95 = actual MPG
6)repeat 2-5 and average
Seems like you know what you're doing so above might already be known.

I consistently see 430+ miles before light goes on and get ~32mpg.
I've run my tank dry at ~500 miles and got about 32mpg mixed driving