Please tell me these folks went to grammar school. Not rocket science
Start stuffed full. Drive whatever distance makes you have happy. Refill till stuffed again. Take miles driven and divide by gallons replaced (filled) whatever. = mpg. Most dash readings are a ballpark at best. My Range Rover is always ~2.3 mpg too high. My Stelvio is actually within a tenth or two every time. My 11S Evora is pathetically off, My 13 Evora decent with in 1 -2mpg. My 4C is VERY accurate also. My GTS is usually a bit genorous ,but usually with 1-1.5 mpg. Aroun\d town , not city, I am in mid teens. Highway 21 at 80++ 22-23 at 65-70.
If you live in that nasty a city environment, you have bigger issues than shi!!y mileage.