The Reynolds Ratio: 8.446:1 (or thereabouts)

What with everything happening in the world today, you probably don’t wake up thinking about famous ‘ratios’. Fair enough, but that doesn’t mean there aren’t some good ones nonetheless.

For instance, there’s the Golden Ratio – 1.61803.

Or the 16:9 aspect ratio of a YouTube landscape video.

And of course, our friend/foe from the 6^th grade, Pi (3.14159 etc, etc), a circle divided by its diameter.

Well, I’ve got a new ratio. And whereas that 6^th grader version of us protested that Pi can’t possibly have any practical value (if I hadn’t spent decades measuring-out circular skidpads in parking lots, I’d agree), my ratio involves that most pragmatic of subjects — money. Actually, money per mile, but first, let’s talk money.

Right now in Southern California, regular gasoline is around six bucks ($5.99 to $6.09 on GasBuddy) while local electricity costs $0.34 per kWh. Yeah, they’re preposterously over the national average.

Which is? Well, the AAA is saying regular is $4.261 while the US Energy Information Admin states that a kWh costs $0.1883. There’s a saying in human ergonomics: ‘there’s no such thing as an average person’ – which goes for energy prices too. There’s probably nowhere in the country where gas cost exactly $4.261 per gallon AND electricity costs exactly $0.1883 per kWh. But it sets my stage.

And in its spotlight taking a bow, is the ratio of those prices — the Reynolds Ratio: the cost of gas (per gallon) divided by the cost of electricity (per kWh). For my California prices, the ratio is 17.65. For the national numbers, it would be 22.63. And everywhere you go in the country there’ll be another, different ratio.

To demonstrate the ratio’s relevance, I’ll conveniently use my 2017 Chevrolet Volt. It’s sort of like two cars in one: a plug-in hybrid (still the best one in my opinion) with 53 miles of EV-only range (during which the engine never starts unless you tell it to) then, once drained to charge-sustaining level it’s a hybrid getting a claimed 43 combined mpg.

Here in Southern California, I’m facing both expensive gas and expensive electricity. So which mode is cheaper per mile? And by how much?

In EV mode — where it gets a claimed 106 mpge — the Volt costs $0.108 per mile (a little over a dime). In hybrid mode (42 mpg) the same mile would cost $0.143 – twenty-four percent cheaper. I’m incentivized to drive it as much as possible in EV mode, and charge it every night.

Now let’s extend this more generally.

I queried Moblr’s current database of specs (60 vehicles) to find the average combined mpg of traditional gas (ICE) offerings to be 29.7, hybrids at 41.1, and EVs at 118.5 (mpge). Not perfect numbers, but ballpark enough.

Now using those national average costs for regular gas and electricity, we can computes that:

• The gas vehicle costs $0.143 per mile or $2009 per 14,000-mile year of driving (yikes).
• The hybrid costs $0.104 per mile or $1451 per year ($558 less than the ICE vehicle).
• The EV would cost $0.0536 per mile or $750 per year ($701 less than the hybrid; $1259 less than the ICE vehicle). That’s more like it.

The take-away:

• The hybrid is 28% cheaper to operate than a traditional ICE vehicle.
• The EV is 48% cheaper than the hybrid and 63% cheaper than the ICE vehicle.

Importantly, these percentages are independent of the cost of either energy source as long as their ratio stays the same.

And that gets us to this: at what ratio of gas divided by electricity is the cost per mile for a typical ICE vehicle equal to a typical EV?

8.446:1

I.e., the Critical Reynolds Ratio! (Not to be confused with the Reynolds Number of fluid flow fame; when I took fluid mechanics, Dr Arthur would point to me in every lecture and ask ‘What’s the Reynolds Number!’ (Flow velocity * a characteristic length / kinematic viscosity!) The ‘thereabouts’ in the title to this story is because that silly-precise 8.446 number would certainly be refined using mpg and mpge fuel/electricity averages from a larger dataset.

So, if you divide YOUR local price of gas (per gallon) by YOUR local price of electricity (per kWh), and its number higher than 8.446, an EV is cheaper cost per mile. The higher the number, the better.

And by the way, if you want to see the cost per mile for any of the cars in our reviews, click on its ‘mileage’ topic and enter your local price for gas or electricity.