MPG Comparison: 2012 Volt vs. 2010 Prius

I suppose I’ve always been a bit of a car guy. At the age of 16, my brother and I already had three cars and performed a nearly successful engine rebuild on our 1957 Chevy convertible. During my adult life as a parent, engineer, runner, amateur cook, and car aficionado, I’ve had several GTOs, BMWs, and other non-performance cars. My two most recent BMWs are shown below.

2008 BMW 135i 2000 BMW Z3

However, my 2012 Volt is unique among all of them. Although not as peppy as the 135i that I traded-in on my Volt, it is nearly as much fun to drive.

My recent fascination with electrics started a few years ago when I started a consulting gig for a battery supplier. Although not responsible for the design of vehicles, my job brought me close enough to get bitten by the electric car bug. I knew I had to have one. At this point I must give some credit to my dear wife of nearly 40 years. As an environmentalist of long-standing she convinced me back in 2004 that the new model Prius would be a wise investment. So, now on our second Prius (a 2010 model IV), we have some street credibility when it comes to environmentally friendly vehicles.

2012 Volt and 2010 Prius IV (blue in background)

So How Do They Compare?

As an engineer you just knew there would be some tech talk. Yes, but it will be short and sweet. In comparing my 2012 Volt and 2010 Prius I want to know which car is better to drive on a trip. For example, if we are planning a 100 mile journey which car is most economical? How about a 200 mile trip or longer?

Before I can get down to comparing the two vehicles, I needed to install one piece of monitoring equipment for the Volt, i.e. an energy meter that would tell me how much energy my Volt consumes during a recharge.

This is necessary because the Volt’s internal readout only tells how much electrical energy (KWh) is used from the battery to travel a given number of miles, i.e. net energy used. I call this the battery specific energy (KWh_b/mile). However, for a valid comparison what is needed is the total amount of energy to charge the battery, i.e. gross energy used. I call this the true specific energy (KWh_T/mile). The battery and true specific energies are different because of the pumping and fluid heating/cooling energy required to maintain the battery temperature during charging. The true specific energy should always be greater than or equal to the battery specific energy.

Figure 1 shows the gross and net specific energy history over the past three months or so. Again, the difference between these is that the net specific energy is determined using the Volt instrument readout while the gross specific energy is determined using the KWh meter readout. There appears to be a 0.032 KWh/mile overhead associated with the pumps and heaters when charging the Volt’s batteries.

The graph below makes the decision easy. Figure 2 compares the Volt and Prius based on the energy used (mpg_e) as well as on the cost of the energy (mpg_$). It shows the breakeven trip at around 90 miles or 60 miles depending on whether energy or current cost is the basis of the comparison, respectively.

The assumptions used to generate figure 2 are given below:

1) average EPA gasoline mileage

2) see figure 1 which is based on data from my volt between 11/18/11 and 2/16/12

3) recent price of 89 octane gasoline in my locality

4) based on recent utility electric bills

5) average Prius mileage between 9/18/10 and 2/17/12 ~ 50.1 mpg

6) data averaged from several websites

Figure 3 is presented to show the effect of $4 per gallon gasoline on the comparison. This is a very real and I feel reasonable projection given recent trends.

Figure 3 now shows the breakeven trip at around 90 miles or 67 miles depending on whether energy or cost is the basis of the comparison, respectively. The two curves overlap at $6 per gallon gasoline. This suggests that the true cost of gasoline based on its energy content should be around $6 per gallon.

Conclusions:

1) The criterion upon which a comparison can be made between the performance of the Volt and Prius is technically straightforward. If we use as a criterion the amount of energy consumed, the Volt shows a miles driven breakeven of about 90 miles. Whereas, if we use the cost of the energy consumed, then the miles driven breakeven falls to about 60 miles.

2) A clear implication here is that as gas prices rise relative to the electric rate, the $-breakeven will approach the energy-breakeven at around $6 per gallon.

3) Of course the electric rate and gas price are both relative to the local market in which the comparison is made.

4) Also, the time-of-year and temperature are additional variables that will affect the comparisons.

5) During the time of my comparison when I had the KWh meter installed there was not much variation in the amount of energy required for pumping and heating the battery. The average remained fairly constant at 1 KWh.

Source: GM-Volt.com