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MPG Anomoly?

My MPG was 21.9 and 21.4 the 2nd and 3rd tank back, after averaging 13.5 for four years. My last tank was 12.7. I have a 2000 Dodge Dakota 4.7 liter V-8, 145K miles, automatic AWD. I checked my log, and believe it was correct. I spoke with the station where I always purchase fuel, no changes there. I also asked my dealer, he couldn’t give me any answers. How could this occur? I only drive it around town.

There must be a calculation error or mistake in the numbers used. A 60% increase in gas mileage is just too incredible to be believed. Maybe someone added gas to the tank without your knowledge and that entry wasn’t recorded. That would throw off the mileage big time.

For comparison, EPA estimate for your vehicle is 13 mpg city, 17 mpg hwy. And here’s what some other owners are reporting:

If you’re only driving around town, as you said, your previous averages of around 13 mpg is exactly what EPA predicts. To suddenly exceed 20 mpg in the same type of city driving would seem very hard to explain. Either someone is adding gas to your tank on the sly, or there’s some error in the measurement of gallons pumped/miles driven, or some unexplained miracle is occurring by which you’re exceeding EPA estimates by almost 10 mpg (city).

Calculating MPG on each tank is notoriously unreliable. Over not many miles and not many gallons, small differences, usually in measuring the gasoline volume, can get to look pretty big. The issue is basically that after filling and driving you have to refill to see how much fuel you used. To be perfect, you’d have to fill the gas tank to exactly the same level at every fill up. But you can’t. You can’t see the level and can only rely on the auto shut off on the pumps. That operates via an interaction between the pump and your filler neck and there is plenty of room for variation each time you fill up. So compared to an initial fill level, at one fill up you end up with an extra 1/2 gallon, but then at the next you end up short 1/2 gallon. Suddenly your calculations are a mess. So calculating over many tanks for the errors to cancel themselves out and become statistical noise is about the only way to do it.

I think Busted Knuckles has the answer. I’ve kept a log on my 2005 Dakota since I bought it in October 2004. Average for 9 years is 14.7 (4.7 V8, automatic, 4WD, 3.92 axle). Worst tank was 12.0, best tank was 20.6 (filled up at gas station next to freeway at both ends of a long trip with cruise set at 65). Average is worse in winter, 90-95% of my driving is around town as well.