Timely question Richard - especially after last week's announcement that the top-selling vehicle in the US was no longer the Ford F-150 pickup, but the lowly Toyota Corolla.
IMHO, there's two things to think about when considering this question.
First, while it's tempting to consider buying a new vehicle that delivers better gas mileage, it's important to think about how much you'll be paying for that new ride that gets better mileage. As an example, if you own a pickup that delivers 15mpg and are considering selling it to buy a new ride that delivers 30mpg but which costs $10K more than what you'll get from selling the pickup, then you'll need to factor in that additional $10K cost as an offset to what you'll be saving on gas. In other words, you may be saving money on gas but at the cost of an additional monthly payment or hit to your savings account.
I won't bore you with the math or the notion of depreciation, but no matter what kind of vehicle you own, the bottom line is given how popular new high-mileage vehicles now are, there's no free lunch in saving money on a new vehicle.
Second, getting better mileage is just one way to reduce transportation costs. A news article the other day stated that the average two income family now spends over $6K per year just in gas - a major chunk of their after-tax disposable income. Getting better mileage is part of the solution to controlling fuel costs, but so too is reducing the amount you drive. Which is why so many employers are now switching to 4-day workweeks and telecommuting.
Changing your driving habits is just as big a factor as what you drive. That's one of the reasons more and more people are fishing closer to home instead of regularly driving across the mountains to fish the Yak, the basin lakes, Idaho or Montana. Believe it or not, the fishing isn't always better the further away from home you drive. Our sport has always been about a quality experience and for more and more people cost has become a component of the flyfishing experience.
K