QUOTE(akasurf14 @ Sep 16 2010, 11:59 PM)
I have taked to people that tell me that they are getting 22, 23, 24 and even 25 mpg. I ask them if they are looking at the Over Head MPG gauge? They say yes, I tell them to fill up your tank, drive it until you need fuel and divide that by the gallons you put in, that will tell the true mileage!
Akasurf14 is absolutely right!
During the first 6 months of ownership of our 2007 Dodge (early model with the 5.9 Cummins), I compared the overhead MPG gauge with the "real" fuel mileage obtained by doing the math explained above. The overhead guage always showed way more MPG than what we were actually getting.
So, if your friend is using the overhead gauge to figure fuel mileage, he's being fooled into thinking he's getting more than he really is. Have him keep track of the "real" fuel mileage for a month or more...he'll be in for a rude awakening!