Yep! distance = speed x time, speed = distance/time, & time = distance/speed
I apparently look at this differently?
A speedometer indicates what speed you are traveling at any given moment. An odometer indicates how many miles have been traveled.
When driving somewhere of any distance, you typically calculate one's average speed. Total distance covered divided by the total time it took to travel that distance. This would include slowing down for traffic, passing someone, or whatever else may occur on the trip. You seldom drive anywhere of distance at constant speed. Even if you calculated one's average speed, it's not really used in a mpg calculation.
If I travel 100 miles @ 50 mph or 25 mph, I don't use the speed of my car to calculate my mpg for that trip. My mpg is calculated by taking the 100 miles driven & dividing it by how many gallons of gas were used. Traveling 25 mph may require less fuel than doing the same trip @ 50 mph, & that would translate into a higher mpg rating.
If your speedometer is off, your odometer is most likely off, too. If someone is using larger diameter wheels on a Mirage, the car may be traveling faster than the speedometer indicates, & the odometer would most likely reflect less miles than where actually traveled. I am not disputing that. I am just stating mpg is equal to distance traveled divided by gallons of gas used. I should have typed that the previous sentence first & stopped there!