Version 02.00, Dec. 11, 2007 - Updated after 26 months.
Version 01.01, Apr. 23, 2006 - Corrected MPG payoff interval. After 6 months, getting 49.9 MPG after 9,000 miles.
Version 01.00, Oct. 21, 2005

Why not a gas only? Scion xB vs. Prius

Our requirements narrowed the field down to a Scion xB (a micro-SUV Echo, 30/34 mpg) or a used Prius (sedan, 52/45 mpg). J.D. worked up a Scion xB quote with the same options as this Prius: 
$16,424 - Scion xB (30/34 MPG)
$17,300 - Prius (52/45 MPG)
Will we save enough in gas to equal the $876, purchase price difference? If so, how long?

The answer will be found in the data but a reasonable model can be constructed upon the past history of 13,500 miles per year from our 1991 Camry and the city 30 MPG Scion and 52 MPG Prius. We can divide the fuel cost, dollars per gallon, by the miles per gallon and get the dollars per mile:

From the above table, divide the $876 price difference by the price per mile to calculate how many miles to reach breakeven:

It looks like the breakeven point driving 13,500 miles per year is within three years. But this is just a model and the actual breakeven point won't be known until we get the metrics for a whole year. Still, the model is encouraging. (NOTE The annual mileage did not account for four years of working in Washington DC where public transportation was used for commuting.)

26 Months Later

It has been a little over two years and the odometer reads: 95,500 miles. My GreenHybrid database mileage is 52.6 MPG for over 36,000 recorded miles out of 46,500 miles driven. My last tank of gas came in at $2.999/gallon. Bottom line, the ~20,000 Prius miles have saved nearly twice the purchase price difference over the Scion xB.

The car corners great due to the low center of gravity. The low noise levels makes it a joy to drive on long distances. Adding the 1kW inverter has provided emergency power during two outages. It has provided bedroom AC in the summer and furnace heat in the winter. I'm still using the original brakes and at this rate, they look to be good for at least three more years. Best of all, it looks like the car mileage and utility continues to improve.

One unexpected result is freed from ordinary gas costs, I'm driving more than before. In the past, the cost of a trip might lead to deferal or not even running an errand. Freed from the cost of gas, I'm enjoying a higher quality of life as I don't have to make decisions between the trip and say eating when I get there.