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I've been religously using DK since day 1 and have been getting 15.4 mpg via the lie-o-meter.
As an experirment (actually, it was raining at the time of fill-up) I decided to not go to the trouble of adding DK to the tank.
I saw an immediate decrease in mileage to a reading of14.8 mpg and it has remained there during this 2nd tank w/o additive.
I have not reset the meter in 5000 miles so this is a long-term reading. Averaging says that I am probably getting a lot less than the indicated 14.8 actual.
After this tank empties, I will do the calcs by hand to be more accurate.