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C-Max computation of MPG

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#1 OFFLINE   mocnarf


    New Member

  • C-MAX Hybrid Member
  • 3 posts
  • Region:U.S. Pacific Coast
  • Locationkitsap county
  • My C-MAX's Year:2015
  • Current Vehicle:Ford C-Max PlugIn Hybrid

Posted 14 March 2019 - 04:17 PM

I have a 2015 C-Max Hybrid PlugIn and I only drive only short distances and plugin whenever I stop. so I seldom ever use any Gas.  My MPG is averaging over 300 MPG which  I believe is miss-leading. I don't believe the formula the computer is using takes into consideration the cost of a full charge.  I figure the cost of a full charge is about the same as the cost of a 1/3 gallon of gas.  I am able to go about 24 miles in EV mode before it will switch to "Hybrid" mode.  So I figure my true MPG should be in the 60 to 70 MPG range.  Does this sound correct to you?

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#2 OFFLINE   Plus 3 Golfer

Plus 3 Golfer

    C-Max Hybrid Member

  • C-MAX Hybrid Member
  • 2,087 posts
  • Region:Decline
  • Location"Valley of the Sun" - AZ
  • My C-MAX's Year:2013
  • Current Vehicle:2013

Posted 14 March 2019 - 08:23 PM

Cost has nothing to do with MPG (see below). Your statement "and plugin whenever I stop" may be problematic. Have you gone to the C-Max Energi forum? They will be able to better answer your questions and you can read how some recommend to treat your battery (rbort has very good posts on how to operate an Energi to minimize high voltage battery capacity loss) . Do you know how many kWh of usable capacity you have? Normally, it's around 5.5 kWh when new and can be significantly less depending on how the High Voltage Battery was used. You do not want to charge to full capacity multiple times a day nor when temperatures are extremely high as you will accelerate capacity loss.

"So how does the EPA get from 32 kWh/100m to 105 MPGe? To create the mpg equivalent, the EPA uses an established energy standard of 115,000 BTUs (British thermal units) per gallon of gasoline. Grossly oversimplified, this means that if you ignited 1 U.S. gallon of unleaded gasoline, it would generate that much heat. To create the same amount of heat, you would need 33.7 kilowatt-hours of electricity."

MPGe = (33.7 kWh / gallon) / (32 kWh/100 miles) = 105

How many kWh did you use to go 24 miles? So, if you went 24 miles on x.x kWh, MPGe = (33.7 kWh / gallon) * (x.x/24 miles)
If x.x = 5.5 kWh, then your MPGe = 144 and your equivalent gallons are 24/144 = 0.167 gallons

Now, if you drove another 8 miles on 0.2 gallon of gas, then your MPGe = 32/0.367 = 87

Your cost of the 5.5 kWh, is your electricity all in variable cost rate ($/kWh at either the average rate or marginal rate) multiplied by the amount of electricity at the electrical outlet where you plug in. There are electrical losses to convert the wall power to DC power to store in the high voltage battery. If it took 7 kWh to put 5.5 kWh in the HVB at $0.10 / kWh, then the cost to go 24 miles would be $0.70 or $0.029 per mile. If gas cost $3.00 per gallon, it cost $0.60 for 8 miles or $0.075 per mile.

Edited by Plus 3 Golfer, 15 March 2019 - 07:11 AM.

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