Good question. Let’s say the Jeep gets 18 mpg, and Car X gets 36 mpg – twice as much. Let’s say that gas is $2.50/gallon, and you drive 1,500 miles/month. The Jeep would burn about 83 gallons of gas in a month, and Car X would burn about 42 gallons. Savings would be about $52/month.
Let’s say Car X costs $16,000. Assuming a constant $2.50/gallon for gas, and a saving of $52/month, it would take about 25 years to ‘break even’. (16000/52 = 306 months, or about 25.6 years.) Clearly, it’s not worth buying a new car just to save on gas. Better to find a $1,000 beater and ‘break even’ in a year and a half. (FWIW, I recently saw a Chevy Sprint for $650. It would pay for in a year.)
But there are other reasons to get a new car. A Jeep Wrangler isn’t very big, so this might not be a factor; but my Cherokee – even though it’s about as long as a Honda Accord – is, I feel, too big. I prefer very small cars. They’re more fun to drive, and they’re easier to park. For me, the ‘fun factor’ is as important as fuel economy.
I also like the idea of Diesel. The cool thing is that they’ll run on vegetable oil or other renewable fuels. (I asked the question a while ago, and the concensus was that if everybody wanted to use biodiesel we mightn’t have the agricultural capacity to do it.) The bad thing about Diesel fuel is that it seems it’s more expensive than petrol. A gallon of regular in my neighbourhood costs $2.50. A gallon of Diesel costs about $2.67. But the cost difference is counterbalanced by the greater mileage (given the same vehicle with different powerplants).
As for multiple vehicles, I’ve always believed the vehicle should fit the mission. My Cherokee fit the mission I had for it when I bought it – hauling gear, off-roading, and towing. I still need its cargo capacity for my film gear, but I no longer go off-road because it’s a poor use of fuel. I still need it for towing my boat. But I don’t need it for everyday driving – which gets back to fuel economy.
As I calculated, better fuel economy in and of itself doesn’t make financial sense. On the other hand, sometimes I have to make long trips. Sometimes I get the urge to drive to California. What stops me is thiking of a $500 fuel bill that month. Psychologically, it’s better to have a ‘spare’ fuel-efficient car so that my fuel bill will only be 250 for the trip even though I've spent XX,000 on the car itself.
So at the end of the day, it’s an emotional thing. It’s unlikely that I’ll drive the same car every day for 25 years. But every day I do drive it, it’s paying for itself just a little bit more. And, as I said, it’s just more fun to drive a small car. 