Exactly how is the MPG figure caluclated for the Prius Plug In?

It might be a lot easier to keep EV-miles separate from gasoline-miles. I can count on about 50 mpg in most situations, and I can get about 11-13 miles on an EV charge.

I only really worry about gas-only mpg. The EV-miles cost about half as much as gasoline when I charge at home, and it's free when I charge either at work, or at a no-fee public charging station. When you start mixing the two, it starts to sound like gibberish in my opinion.

 

So if you take out the 23 miles of EV (I'm guessing this what what you are actually getting), you drive 80 miles using gas.

70MPG over 103 miles is 1.47 gallons of gas.

So if you divide the 80 miles you drove on gas using 1.47 gallons, you get 54.4 MPG while in HV, using gas.

23 miles at 54.4 MPG would have consumed an additional 0.422 gallons of gas. So that's where your calculation went wrong.. first, assuming you only got 50 MPG.. and then using that to calculate the additonal 0.5 gallons, when you really only offset 0.42 gallons using electricity.

The 70 MPG you get is simply miles driven divided by gas used. It doesnt take into account the electricity you used.

 

Miles = miles since last fill up.
Actual gallons = gallons to fill since last fill up.
Actual electric energy = kWh used since last fill up (measured out of the wall using a meter)
Actual electric cost = cost of kWh used at your rate (highest tier for the month if you are on tiered rates)

MPG-gas = Miles/Actual gallons

MPG-cost = Miles/(Actual gallons + Actual electric cost/gas cost for 1 gallon)

MPG-e = Miles/(Actual gallons + Actual electric energy in kWh/33.7) this is the EPA method

Note: the kWh shown by the car display is approximately correct however just as you pay for the gas that comes from the pump you also pay for the kWh that come from the electric company. Of course if you get some or all kWh without a meter the car display will give you a reasonable result.

 

EV miles are *not* the only miles which consume plug-supplied electricity.

HV miles take advantage of the battery's larger capacity too.

Dividing total distance traveled by the amount of gas needed to refill the tank to full is the only way to determine MPG.

 

Yes, HV miles may use the plug-supplied energy too OR they may then replace it with ICE supplied energy. That makes the "gas only" mpg rather meaningless however it is interesting to calculate as that is a major reason to purchase the car. How long was the trip, how much gas did I use, very interesting.... Most folks with a PiP make that calculation, it feels good The EPA method seems to be the most realistic.

 

Some people add 12-15% to the electric kWh because of electric loss from the wall. YMMV.

 

I'm not sure what you mean by "loss from the wall". Its certainly not that kind of loss in the EVSE or it would get very hot. If you mean loss in the "battery charger" in the car, well that's part of the cost of running the car and is indeed on the order of 12-15%. Just like gas used to warm the ICE if the car isn't moving is part of the gas used in the mileage calculation. The meter measures all the electricity sent to the car just like the gas pump measures all the gasoline.

 

Well, I agree. It's the loss between the outlet and usable EV range in the batter after charge.
I don't really no how to put it I guess.

 

I watch my electrical usage pretty closely since I bought my PIP in April. I notice when the car is charging, even though the car is getting about 1 kilowatt an hour, but electrical usage from electrical company is actually 1.2-1.5 kilowatt an hour. This of course include whatever I am using at home at the time. My home when nothing is running (except for fridge and what not) usage is about .07 to .09 kilowatt.

I guess this is the "loss" you are talking about here.

 

Yeah! We do have some charging loss, every time we charge. About 12%-15 %. I just figure 15% and add it to the cars total to compensate.

 

I got so confused and consumed with the MPG looking on the dash, looking at entune, reading a million threads so I just wanted to see my true MPG the old fashion way. I just filled up for the first time on the car. MPG is 63.5. I think I am just going to keep track of it that way to see if it gets better of worse.

 

that's the best way to do it!