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OK so I have a theory on why your DIC may be more accurate than you think, and maybe more accurate than hand calculations at the pump. My basis for this is, as always, margin of error. with a car that gets such good MPG being off just a little on your calculations can skew numbers quite a bit. I am assuming most of you reset the DIC at each tankful, like I used to. I have noticed the longer you let it go the more accurate it becomes when compared to calculating by hand, which makes sense because the margin of error by hand will tend to "average out" over longer periods of time.

My 2 biggest concerns with hand calculation are that when you go to fill the tank you have no idea how much fuel is in there before you fill up, so that means at the end of that tank you really have no idea how much fuel you used. Throw in the addition of even if you use the same pump and fill to 1 click the odds are pretty good that there is a different amount of fuel in the tank than the precious fill up. So realistically, if you drained the tank completely, pumped in a known quantity of fuel, drove it, then emptied the tank again to see what was left you would be spot on. But for giggles here is an example, lets say you pump in exactly 10 gallons of gas and went 400 miles, piece of cake 40 MPG. Now if you figure that maybe between not knowing what is in the tank and margin of error at the pump that you used just 1 more gallon of fuel. That drops your MPG to 36 and change, so just one gallon of fuel off gives you a margin of error of almost 4 MPG. Now if you average that over say, 5000 miles, the variances over and under will roughly average out.

Now to the DIC, which, with the exception of variances in tire diameter due to air pressure, etc, knows exactly how much fuel it has used over what distance. Runs a very simple math equation and spits out the number. I'm willing to bet over any meaningful period of time the DIC is gonna give you closer to real world mileage than calculating by hand.

Thoughts?