Figured I'd put my two cents in
MPGs and savings at the pump are very tricky. Realistically it comes down to the price of the vehicle. Bellow I've provided an example of what i'm talking about.
I assumed $4 a gallon, which is an over estimate for today's market, at least here in NJ, but you'll get the picture. The chart estimates the gas costs of a vehicle based on MPGs for 12k 24k (yearly) and 100k total miles driven.
To kind of illustrate what i'm talking about, let's take a 25MPG combined vehicle (close to Mazda 5), and let's say we are comparing to the diesel version that gets 35MPGs combined, the fuel cost difference between the two models is $549 a year assuming you drive 12k a year, or $4571 over the 100k period. Sounds good, but if the price point of the vehicle that gets 35mpgs is 5k more from the dealer. If you are not going to keep the car for 100k+ you will actually loose money i.e. it will cost you more to own a car with better MPGs, additionally one has to keep in mind that if you finance the car, you pay even more because now the interest is involved, if you buy with cash, you still loose, because that $5000 difference can be earning interest at the bank or mutual fund etc.
I've over estimated with $4, just to "future proof" but if we take the price of $3.5 difference is even less (it's only $4000 over the course of 100k).
Please keep in mind I'm keeping the "being green" factor out of equation, this is strictly "money out of pocket".
Interesting example is 2011 elantra vs 2011 prius. Due to the price point difference, you will pretty much never make your money back. Same goes with RX370 vs RX450h, I can't justify a difference of 10000+ for a 5MPG difference. The interesting things happen when you start looking at used cars though, because the upfront premium is somewhat diminished, but you can account for that as well and some situations will actually change, and it might be well worth it to go for a "better" model.
Anyhow, just my two cents, if one is interested in specific models etc, and what's better finance vs purchase I have many equations set-up to analyze the cost to own for many scenarios (can you tell i'm a math major lol), so I can potentially help with a car decision choice if anyone is interested, because some things might look and sound good, but once the numbers are on paper, it's totally different.
MPGs Pice per Gallon Cost of fuel per year at 12000miles Cost of fuel per year at 24000miles Cost of fuel for 100000 miles Difference from the chepest (45MPG)
15 $4 $3,200 $6,400 $26,667 $17,778
20 $4 $2,400 $4,800 $20,000 $11,111
25 $4 $1,920 $3,840 $16,000 $7,111
30 $4 $1,600 $3,200 $13,333 $4,444
35 $4 $1,371 $2,743 $11,429 $2,540
40 $4 $1,200 $2,400 $10,000 $1,111
45 $4 $1,067 $2,133 $8,889 $0