Take the number of miles you expect to drive the "gas-sipper" and divide by the mpg to get the total number of gallons used. Multiply that by the price of gas and you get the total cost of gas. Then do the same for the gas-hog and see how much less you will spend on gas with the more efficient vehicle.
Example: Replacement car gets 30 mpg (for ease of math) and you expect to drive it 12,000 miles per year for 5 years. This is 400 gallons of fuel per year. At $3.90/gallon (here in CA
) that is $1,560 per year in gas, or $7,800 in 5 years. Gas guzzler gets 11 mpg, would use 1091 gallons of fuel, costing $4,255 in gas, or $21,275 in 5 years. You would therefore save $2,695 per year in fuel, or $13,475 over 5 years.
Of course, the savings increase or decrease with gas prices and the extra car will add insurance and upkeep costs which would have to be factored into the equation. If you're getting 11 mpg from your everyday vehicle, getting a more efficient second car could very well make sense.