There are several ways to determine the fuel consumption rate for a car.
Lie.
Make it sound like you did good calculations, lie.
Estimate mileage and try to remember how many tanks of gas it took. Estimate the resulting fuel mileage, honest effort. Useless.
Record the odometer mileage, Carefully fill the tank to the top of the filler line. Repeat on next fill-up or if possible several fill-ups. Calculate the result.
Lie.
My 2000 4-cyl Camry calculated from the log book I maintain 22.1 to 23.1 MPG locally depending on the current formula for the fuel sold in this area. On a long trip using the cruise control and probably buying fuel with more gasoline and fewer EPA additives, 24.5 MPG.
Recent business trip, rental cars, calculated from about a half tank each: Dodge Durango 16.7. That was a convention at Lake Tahoe. With permission I was able to use the Durango to explore around the lake and down into Reno (not enough time to stop, but I did get to see the real estate of the area for the first time and put some miles on the car). Big V-8, full time all-wheel drive, pushed it hard, never maxed the pedal in the mountains or ran out of power. That vehicle did not know it was going straight up at 65 MPH above 8000 feet, push down the pedal some and burn rubber! Or on those impossible turns barely clinging to the sides of the mountains, 5, 10, or on the wider roads 20 MPH above the yellow sign suggested speed. No real problem with the Durango at all. My right hand did miss the placement of the shift lever. It prefers the Camry. All of these, by the way, are automatic transmissions. Durango was Great!!!!!!!!
Another city, airport field elevation 510 feet. Jeep Laredo, V-6,
15.6 MPG. Feels just like the Durango except there is noticeably less room inside and visibility is fairly poor. If I was buying I would not hesitate to pick the Durango. I do like the shift lever and transmission for the Jeep better, a lot better. Easy to drop gears to slow down once you learn how to make it happen.Came home, stopped by the Toyota dealer. The Dodge and the Jeep make the offerings on the Toyota lot look subminiaturized. I think I will wait for the next major body design change of the Camry, too. And keep my fingers crossed.