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It is pretty straight forward to calculate MPG. When the manual calculation matches the car's numbers to the tenth of the MPG, I think one can pretty safely assume the "MPG Thing" is working properly. If you're serious about calculating MPG, I suggest you check your trip computer several times to see if it is consistently accurate. I check every tank of gas against the trip meter in all my cars. Keep in mind that if you don't always get your tank completely full, only a long-term average is going to get you a particularly accurate answer. If it works correctly, the built-in calculation is right all the time. The built-in calculations can also help you learn your most efficient speed for a given setup. For example, I get about 27.1 or 27.2 MPG in the 65-70 MPH range. As I approach 75 to 80 MPH, the MPG drops down to about 26.

Now if you REALLY want to get serious, you need to compare your speedometer / odometer to a known accurate reference like a portable GPS that calculates based on satellite position (and reports the MPG or miles traveled directly), because not only are many cars' odometers not completely accurate, but tires sizes vary from set to set (even with the same nominal size). In fact, I wonder what built in GPS systems do as far as reporting speed on cars. This Honda is my first car with a built in GPS, though I have owned and used at least 6 portable units. As far as I can tell, there's no place this car reports the satellite-based speed, but it has to calculate that info and I doubt it overrides a mechanical speedo that would have to be built in for the odometer.

Tire diameters also change (all of them) over the life of a set of tires as the tread wears off them. They can actually vary by several percent over the life of a set of tires on some cars. Just about all the computer or a person can do without something like a GPS based number for speedometer / odometer accuracy is assume the speedometer is correct all the time.

If you've never compared your speedo to a GPS, you may be surprised what you find, BTW. I've sometimes found as much as a 10 percent discrepancy between the two.

That's the way all of this strikes me.

As "they" say, YOUR mileage may vary...
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