Have you guys ever compared the dashboard MPG reading vs the one you calculated? For last couple of refills I have been comparing my calculated ones based on gallons purchased and odometer reading to the one shown by computer. It seems like computer is always overestimating by 1-2 mpg.

For example: If my calculation shows 24.0 MPG the dashboard shows 26+. Seems like the discrepancy is higher if I have more highway miles. .5 MPG here there is understandable but 2? What are you guys getting?

BTW took a trip to Reno over the weekend and dash is showing about 31.8 AVG for bit over 525miles. I seriously doubt I did that well. But will see in next fill up.

 

Yeah I've noticed the same thing actually. I make trips up to Pennsylvania every few weeks and I can average 31.5-8 mpg on the way until I hit the destination suburbs.

BTW..Have you ever tried setting your speedometer to say 40mph on a flat stretch of road and then reset the avg mph on the dash to see what happens? Well I did it today for the heck of it and even though I was doing 40mph the whole time not a single mph slower or faster the avg mph showed 38.

 

Recently did a work trip from Detroit to Greensburg, PA (pittsburgh area) which was 300 miles each way.

Speed: 70mph
MPG: 36.4

 

My MPG is off in the other direction. The TANK AVG is about 1-1.5 MPG lower than what I calculate.

 

i was just reading about speedo being inaccurate and reading high i wonder if that has somthing to do with it? perhaps a larger tire size will compensate for the accuracy of both?

 

Like some others have noted, the computer "Tank Average" on my 250 usually runs ~2 mpg higher than what I calculate. The computer has never reported a lower mpg than the "actual" mileage.
RJM

 

if you take your foot off the accelerator pedal, the mpg will go UP because if you switch from avg mpg to the current mpg youll see you're getting around 90 mpg (which is crazy)...so the computer takes that into consideration and figures it in the avg ... maybe? i just notice my mpg go up when i take my foot off the accelerator pedal going downhill

 

Well that sucks.... here I am all happy that I am getting so high mileage that I could save on barrels of money. So you are saying that ai'nt so? Oh well....

 

There are so many variables in this calculation that it is hard to determine which is the more accurate method. If you measure directly by how much fuel you pump into the tank you are subject to temperature variations in the fuel itself (hotter fuel means less volume), accuracy of the pump, and are you exactly filling the tank with the same amount. The inaccuracy in this method could be off by up to 1/2 gallon if not done correctly. I'm not sure of the exact calculations or algorithms the onboard computer uses but there will be inaccuracies in that system as well. I wouldn't sweat it, if both methods are within 1mpg I think you are doing pretty good. These aren't scientific instruments we are dealing with.

 

Yup... agree. Don't know which is the more accurate, just that on my car, the computer is consistently more "optomistic."
RJM

 

If we were talking about one tank of gas, I'd agree with you. But my measurements have been over dozens of tanks in the 21 months I've owned the car.

 

Yup. I track my MPG obsessively, and it has never been close... dashboard MPG is always at least .7 MPG higher than miles/gallons at the pump.

(I'm mostly freeway miles.)

 

But I would consider 0.7 mpg difference excellent correlation, that's something like 3% variation. Not bad in my book.

 

I get stressed out looking at my AVG MPG and Tank MPG so I try to avoid that display like a plague but sometimes it consumes me. But mine is opposite where the display shows like 15.5-16 when I'm actually getting about 17.

 

And mine typically reads less than calculated, but occasionally does not.