To get the cost per mile is used these formulas:

Electric Cost per mile = kWh x Electric cost per kWh x 1.2 / Miles Electric

The electric cost per kWh was 13.6 ¢. 1.2 is the charging efficiency factor and the kWh is what is displayed after a drive on the Volt energy display in green. I measured the charging efficiency by using a Watt meter while charging my car on 110V at 12 amps. I divided the kWh used by the kWh value displayed in the car since the previous charge.

An alternative formula is:

Electric cost per mile = 33.7 x Electric cost per kWh / MPGe

MPGe is the value displayed in green on the Volt energy display. 33.7 is the MPGe conversion factor from the EPA’s formula. I prefer to use this 2nd formula.

For gas cost per mile the formula is simply:

Gas cost per mile = Gas cost per Gallon * 100 / MPG

I used $2.25 for the Gas cost per gallon. MPG is what is displayed after a drive on the Volt energy display in blue.

Since I bought the car my average cost for electric has been 4.7 ¢/mi. For gas it has been 6.8 ¢/mi with a combined average of 4.9 ¢/mi. I have minimal data for gas miles since I almost always use just electric. Most of my gas miles so far have been using max heat on the highway.

3.4 ¢/mi - my best day so far on a warm day with no highway miles

8.1 ¢/mi – worst day on a very cold day and a short trip with max heat

4 ¢/mi – typical on a cold day with only heated steering wheel and seats

6 ¢/mi – typical on a cold day with max heat

I set my Engine Assist Heat to Deferred and found that this was cheaper to run on electric only with the same heating comfort level. Just to compare, the Volt replaced my very efficient Passat diesel which got 44 MPG over its 2 year lifetime at an average cost of 5.5 ¢/mi for fuel. So the Volt is 10-20% cheaper than the Passat in winter. I am planning to run these numbers again in the summer and I expect it to be significantly better.