How does on-board MPG work?

[albuquerqueith]

Does it calculate since last reset? Last 100 miles? Other?

I reset at beginning of my last road trip. 400 miles later it read a whopping 47.3. However when I filled up, the actual MPG was quite a bit lower, though still impressive 45.2.

 

[Swordsmith]

It calculates since last reset, however it is notoriously optimistic about your MPG by a couple miles per gallon. You're getting the expected result.... some get a smaller variance and some larger, but 2 is about average.

Also note that hand calculation is also inaccurate, since the shutoff point can vary pump to pump and even on the same pump can still be different. If you put 10 gallons in, does that mean you started with 12.2 gallons and used 10? Maybe, but maybe you started with 12.0 gallons, because last time you filled up shutoff was a little early, and this time you got all the way full, so you really only used 9.8 gallons. Or perhaps you did start with 12.2 gallons but this time the shutoff was early and you only filled up to 12 gallon total... while you really used 10.2 gallons.

450 miles on 10 gallons is 45mpg but 450 miles on 9.8 gallons is 45.9 mpg... and my worst case scenario 450 miles on 10.2 gallons is only 44.12mpg

Because of this, to get a real picture you need to try to keep filling up at the same exact pump, and also use the miles and gallons for several fillups together.

 

[DoctorFan]

It calculates since last reset, however it is notoriously optimistic about your MPG by a couple miles per gallon. You're getting the expected result.... some get a smaller variance and some larger, but 2 is about average.

Also note that hand calculation is also inaccurate, since the shutoff point can vary pump to pump and even on the same pump can still be different. If you put 10 gallons in, does that mean you started with 12.2 gallons and used 10? Maybe, but maybe you started with 12.0 gallons, because last time you filled up shutoff was a little early, and this time you got all the way full, so you really only used 9.8 gallons. Or perhaps you did start with 12.2 gallons but this time the shutoff was early and you only filled up to 12 gallon total... while you really used 10.2 gallons.

450 miles on 10 gallons is 45mpg but 450 miles on 9.8 gallons is 45.9 mpg... and my worst case scenario 450 miles on 10.2 gallons is only 44.12mpg

Because of this, to get a real picture you need to try to keep filling up at the same exact pump, and also use the miles and gallons for several fillups together.

Agreed.
My records show about a 5% difference between DIC-indicated and calculated mpg.
I keep a spreadsheet (yeah, I'm a geek, and it's on a flash drive attached to my keychain [emoji16] ) of my mileage, and rely on a 5-tank floating average for my recent perfomance numbers. Also, larger fill-ups seem more accurate.


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[MaD]

I use Fuelly and the actual used and calculate from that, dash is all over the place if I don't reset it. But Fuelly is pretty close.

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[PurdueGuy]

Same experience with my wife's 2013 - often about 2mpg optimistic. Really her only 2 disappointments with the car is the inaccurate mpg estimate on the dash, and not getting closer to the 40mpg highway, but she understands that the highway rating likely didn't account for Idaho's 80mph speed limits. lol She's still pretty happy with getting about 34-35mpg.

 

[albuquerqueith]

Same experience with my wife's 2013 - often about 2mpg optimistic. Really her only 2 disappointments with the car is the inaccurate mpg estimate on the dash, and not getting closer to the 40mpg highway, but she understands that the highway rating likely didn't account for Idaho's 80mph speed limits. lol She's still pretty happy with getting about 34-35mpg.

It seems the mpg sweet spot is around 68 mph but I can set the cruise up to 70-71 and still get mid 40s mpg. It drops a little between 72-74 and significantly over 75.