A plunge in the price of gasoline over the past six months has drastically changed the result of the calculation new car buyers and drivers perform every week: How much is it going to cost me to fill this thing up?

The national average price of Monday was $2.13 a gallon, according to AAA, after falling for a record 109 days in a row. Six months ago it was $3.63 a gallon.

That Chevy Tahoe you were eyeing will cost $39 less to fill up than it did in July. The Honda Fit now costs you just $23 to fill the tank. Maybe you can afford to throw in the booming premium stereo.

For hybrid cars or plug-in electric vehicles, though, cheaper gas means it will take much longer for fuel to make up for the extra up-front cost of a car with a big battery.

Here are some examples:

Chevy Tahoe

Fuel tank: 26 gallons

Fill-up cost 6 months ago: $94.38

Fill-up cost now: $55.38

Savings: $39

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Ford F-150

Fuel tank: 23 gallons

Fill-up cost 6 months ago: $83.49

Fill-up cost now: $48.99

Savings: $34.50

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Dodge Grand Caravan

Fuel tank: 20 gallons

Fill-up cost 6 months ago: $72.60

Fill-up cost now: $42.60

Savings: $30

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Toyota Camry

Fuel tank: 17 gallons

Fill-up cost 6 months ago: $61.71

Fill-up cost now: $36.21

Savings: $25.50

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Honda Fit

Fuel tank: 10.6 gallons

Fill-up cost 6 months ago: $38.48

Fill-up cost now: $22.58

Savings: $15.90

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Volkswagen Jetta TDI (diesel)

Fuel tank: 14.5 gallons

Fill-up cost 6 months ago: $56.41

Fill-up cost now: $43.51

Savings: $12.90

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Toyota Prius

Approximate price premium: $4,300

Annual fuel savings based on July gas price: $534

Payback years: 8.1

Annual based on current gas price: $313

Payback years: 13.7

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Nissan Leaf

Approximate price premium, including electric vehicle tax credit: $7,330

Annual gasoline savings based on July gas price: $796

Payback years: 9.2

Annual gasoline savings based on current gas price: $281

Payback years: 25.8

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Sources: Auto company websites, AAA, Department of Energy.