I always knew electric cars were cheap to run, but this is ridiculous.

After I bought the first set of replacement tires for my 2013 Tesla Model S (at 26,000 miles), I crunched the numbers and came to a startling conclusion: I've spent substantially less per mile for my electric "fuel" than I have for my tires.

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The tires weren't cheap. The Michelin Primacy MXM4 all-season grand-touring tires set me back $250 apiece, plus mounting and balancing, for a total of $1,131.

Over 26,277 miles, that works out to 4.3 cents per mile. Pretty typical for a high-performance luxury sedan.

Cost of electricity

Over those same 26,277 miles, I used a total of 8,531 kilowatt-hours of electricity.

But, thanks to Tesla's network of free high-power Superchargers, I didn't pay for all of it.

2013 Tesla Model S at Supercharger station on NY-to-FL road trip [photo: David Noland]

As best as I can figure, I drove about 5,500 Supercharged miles during that time, including a 2,500-mile round-trip to Florida from my home in New York's Hudson Valley.

That means I probably sucked up around 1,800 free kWh from the Superchargers.

ALSO SEE: Life With Tesla Model S: Three Days Of Service Nirvana (Sep 2013)

So let's say I paid for 6,700 kWh.

My local utility, Central Hudson, charges about 14 cents per kWh. (Unfortunately, it offers no special night-time or electric car rates.)

So, let's do the math: 6,700 kWh x 14 cents/kWh = $938.

Onsite service by Tesla Motor technicians on 2013 Tesla Model S, upstate NY [photo: David Noland]

Divide by 26,277, and my total "fuel" cost per mile works out to a remarkable 3.6 cents per mile.

That's 20 percent less than the per-mile cost of the tires that carried me on all those miles.

Tire costs vary

Not every Model S owner will have the same numbers, of course.

It's possible to spend a lot more--or less--on both the tires and the electricity.

MORE: Life With Tesla Model S: Battery Upgrade From 60 kWh To 85 kWh (Dec 2013)

For example, I could have bought Goodyear Eagles and saved $200, reducing my tire costs to 3.5 cents per mile.

On the other hand, Model S owners with the optional 21-inch rims can spend as much as $500 for each high-performance tire replacement--and expect to get only 15,000 miles out of a set. That pushes tire costs above 15 cents per mile.

2013 Tesla Model S in Florida, during New York to Florida road trip [photo: David Noland]

(The upside for Model S owners with 21-inch rims and high-performance tires: At least they can brag that their fuel cost is only one-fifth of their tire costs.)

...And so does electricity

Electric rates can vary even more widely than tire expenses.

At the national average rate of 11 cents per kWh, my "fuel" cost would have been a mere 2.6 cents per mile.

And many utilities offer special night-time rates of 5 or 6 cents per kWh, which would have brought my cost down to 1.3 cents/mile.

2013 Tesla Model S in Florida, during New York to Florida road trip [photo: David Noland]

But as far as I know, nobody's got a sweeter deal than my buddy Bob Meyer, a Model S owner who lives in Indianapolis, Indiana.

That city--my hometown, as it happens--turns out to be a hotbed of forward thinking when it comes to electric cars.

RELATED: Does The Tesla Model S Electric Car Pollute More Than An SUV?

The local utility, Indianapolis Power and Light, has a special electric-car-only rate for off-peak charging. It's a a mind-bogglingly low 2.3 cents per kWh.

So it costs Bob $1.95 to recharge his Model S battery completely and then drive 265 miles. His "fuel" cost amounts to three-quarters of a penny--that's three farthings--per mile.

And that doesn't even account for any free Supercharged miles.

Cheaper than tires?

Heck, in Indy in the winter time, that's probably cheaper than the per-mile cost of Bob's windshield-washer fluid.

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