I have been calculating my mileage compared to the dash monitor. Over the period of ten (10) fill ups, my actual reading is on average 3mpg +/- 0.15more than my actual gas mileage. The reading are pretty consistently inaccurate. I do have stock 18" rims. No mods whatsoever. This is a pretty bad calculation the dash can be off by.

 

You guys get some good Gas mileage... I Know my IS350 gets an avg of 17 on the meter, but I'm mostly city driving with hardly any highway. I suppose you guys getting over 20 are doing a good deal of highway and city??

 

I'm at 21 just in the city. My mpg calc in the vehicle is off by .5 at the most. I average 26/28 mpg with city/highway combo.

 

According to Fuelly - I have average 26.5 mpg overall.

I have knocked out 30 a couple of times and found that the dash was very close to calculated.

205miles on 6.41gallons = 31.98 calculated - the car said 31.8mpg.

I have had a few times with mostly city driving where it has been off by several mpg.

I will say that it is far more accurate than the calculated mpg that was in my Acura TSX. I tracked that and compared to actual. It was normally in the 8% and on some occasions it was in the 10-12% off. The best accuracy it ever had was when I had 1 tank that was insanely high mpg (38mpg).