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Is the electricity that goes into a Tesla actually a cost savings over the cost of gasoline that goes into a gas powered car?

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Yes, definitely, even with high-electricity-cost utilities.

Even the big, heavy older Teslas get the equivalent of at least 60 mpg (say, 300 watt hours per mile at 15 cents per kWh vs $3 for a gallon of gas).

Electricity is still cheaper than gas, but electricity in many European countries these days is at least 30 cents per kWh (and rising every year), instead of the 15 cents you mention.

The eMPG is an attempt to convert WH/m into MPG based on some fixed numbers for $/kWh and $/gallon.

a WH/m x b $/kWh / 1000 = x $/mi

c $/ga gasoline / x $/mi = mpg equivalent.

For a=300, b=0.15, c=$3.00 you get 66mpg.

You can plug in your own numbers as you see fit.

The cost savings is then estimated by plugging in a number of miles driven, and comparing gallons of gas versus a hypothetically less efficient car (not sure what MPG rating Tesla uses for its comparison)

Note that the Tesla Model 3 is closer to 150 WH/mi. And what does gasoline cost in those European countries?

€1.3 per litre, so that's $5.53 per gallon.

The amount of savings obviously depends on electricity and gas prices where you live, but yes, the electricity will be less expensive for the same mileage.

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