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Calculating an equivalent $/Gallon for the Model S

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If it costs me 4 cents to drive a mile in my S, whereas it cost me 35 cents to drive in my Toyota, then there's no ambiguity.

Right. Fuel/electricity costs me about 2.6 cents per mile for the Model S, and about 4.5 cents per mile for the 2004 Prius (plus 6.9 cents for maintenance and tires over 146,689 miles). I don't have the tire cost yet on the Model S.

$6.652.42/146,689
$10,161.95/146,689
 
Here is the formula I came up with for my comparisons:
eMPG= =G$/(E$/(1000/Whr/M))*(Eff/100)

G$ = cost of gas per gallon in dollars
E$= cost of KWhr of electricity in dollars
Whr/M= Watt hours used per mile (from car)
Eff= Charging efficiency (percentage)

Here in Sacramento CA, gas is 4$ per gallon, my electricity is .06$ per KWhr, my Whr/m is 333 and I am guessing the charging efficiency is 85%
This gives me and eMPG of 170 miles per 4$ gallon. Much cheaper than a Prius! :smile:

If anyone has a better number for charging efficiency, let me know. I like the 85% because for an 85 KW battery, it works out the the cost for a complete charge is 100X your KWhr electricity cost, or 6$ for me. Of course that number is pretty meaningless but it is something to throw out to the Prius owners...
 
I have kept very detailed records on all my cars for the past 25 years. I do so on the tesla, but it is much easier. Based on my current cost per kw of electricity, and my usage, and compared to my last car driving the exact same route to work, comparing the mpg to mpkw, my ultimate cost for energy (electricity vs. gasoline), our tesla over the past 14 months of ownership operates for 15 - 20% of what our ice(BMW) previously cost to operate.
 
I don't have mine yet (19 days!) but I will probably answer the question this way:

My "gallon" is 10 kWh, I get about 31 miles to the "gallon", but it only costs me 1.20 USD.

(Above assumes EPA rated 265miles for 85kWh battery and 0.12 USD per kWh)
It's not precise, and might be made more realistic if the cost per "gallon" includes the during-charge loss, but it's a near-equal comparison mileage-wise.
 
One other consideration to keep in mind which I've experienced is that along with the discussion above I have found that my wife and I use our ICE vehicle a lot less which leads to considerable fuel savings as well. If I'm home and she needs to commute the 25 miles to work she takes the S. Our fuel bills have gone from $7-800/month to $150-225. In that sense it drives the per gallon cost even lower.