MPGe

There’s all this talk about MPGe (Miles per Gallon equivalent), what it really means, and how it’s calculated. The truth is, there’s no good way to compare EVs to internal combustion through MPGe. If you really want to compare in any meaningful manor check out the miles per $ metric.

That said, there’s really three ways to calculate MPGe. I call it the three E’s, Efficiency, Emissions, and Economy. It’s really about comparing some measurable characteristic of an EV and cross multiply to get rid of the unwanted units.

The following calculations are based on June 15th data from an earlier blog post.

MPGe Efficiency

This is how the EPA, auto companies, and Big Oil want you to calculate MPGe. Basically, a gallon of gasoline can be burned and rendered useful as kWhs of electricity, thus comparable to an EV that uses kWh of electricity. The conversion is: 33.7 kWh electricity per gallon of gasoline.

So, my Leaf which gets:

(3.36 mi/kWh) X (33.7 kWh/gallon) = 113 MPGe

MPGe Emissions

Using the calculator at AfterOil EV, one may compare MPGe by total CO2 emissions from generation to wheels by emissions ratio. I used 50.3 miles @ 35MPG, comparable vehicle to my Leaf, and a very liberal estimation at that.

(31.8 lbs CO2 Ave Car) / (14.09 lbs CO2 Leaf) = 2.25 ratio (times fewer emissions)

(2.25) X (35 MPG) = 79 MPGe

MPGe Economy

This is how I’d like to measure MPGe. After all, why compare vehicles using a metric of economy (MPG) unless one does so using units of economy.

(3.36 mi/kWh) / ($0.12/kWh) = 28 mi/$

(28 mi/$) X ($3.999/gallon) = 112 MPGe

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112 MPGe Total System Economy

My average daily commute taken on June 15th was 50.3 miles. That night a full charge was 14.97 kWh.

(50.3 mi) / (14.97 kWh) = 3.36 mi/kWh

So my car is getting approximately 3.36 miles per kWh used including charging losses. On Tier 1 with PG&E I’m paying $0.12/kWh.

(3.36 mi/kWh) / ($0.12/kWh) = 28 mi/$

I’m getting 28 miles per dollar total system economy with my Leaf.

(28 mi/$) * ($3.999/gal) = 111.97 MPGe

The Leaf gets 112 MPGe

This is based on ‘wall’ to wheel economy.

Leaf EnVy

It seemed the Hummer’s always found me, now it’s personal.

136 MPGe ‘Tank’ to Wheel Economy

My car is getting approximately 4.1 miles per kWh as reported by my Leaf. On Tier 1 with PG&E I’m paying $0.12/kWh.

(4.1 mi/kWh) / ($0.12/kWh) = 34.167 mi/$

I’m getting 34 miles per dollar with my Leaf. Compare that to 7.5 mi/$ with my Civic. Now,

(34.167 mi/$) * ($3.999/gal) = 136.63 MPGe

The Leaf gets 136 MPGe, that’s miles per gallon equivalent.

This is based on ‘tank’ to wheel economy.

Breaking it in

First Day

Made it to work and back with 30 miles left.

My New Leaf

First charge

From 83% SOC (17% DOD) it took 3.5 hours on trickle charge. Given a near linear charge curve, that’s about 20.5 hour total charge time. About 12 minutes trickle charge per mile. My commute of 52 miles round trip would take approximately 10.5 hours charge. Can’t wait for my fast (8 hour) charger.

My Leaf Texted Me

Reminded me it was charging 😀

Ready

Picking up my leaf.