A cow-orker just bought a new (to him) 1998 Dodge Durango. Not my kind of car, but suitable to his needs -- wife, three kids, and a travel trailer.

It has a feature I have heard of but not actually seen myself -- a gauge that tells him how many miles per gallon he is getting. His Durango is equipped with the 5.2 liter V-8 with automatic transmission and four wheel drive, and the mileage is appalling. According to his gauge, he is getting less than 10 miles per gallon around town. Since this vehicle will be driven fewer than 5,000 miles per year, he can stand it.

Here's the question: How does that mileage gauge work? I am guessing that it is metering the fuel delivered to the injectors, and comparing that to distance travelled. But is the readout an instantaneous calculation, or an average over some pre-determined time (30 seconds? A minute? An hour?), or an average over some pre-determined distance, or is it a cumulative figure since the most recent time the engine was started, or is it a check on all the fuel that has passed through it since the car came out of the factory? What happens when you stop the car and leave the engine running? Does your fuel mileage display then drop to zero for the duration of the stop?

Who knows how this works and can explain it to me?

tanstaafl.