A number of letters and e-mails have reached my desk regarding our most recent test of compact economy cars in the July 2008 test group.

One of the more frequent comments was why we didn't test Volkswagen's diesel models. We would have loved to include a TDI Jetta or Rabbit, but there's one, slight problem: neither are currently for sale in the United States. We tested a Jetta TDI in 2006 (available to subscribers) and got 34 mpg overall. VW plans to reintroduce the Jetta TDI this August; we'll buy and test one as soon as we can. (Read "How we test fuel economy.")

Another comment burning up my in-box was from readers saying how their 3-cylinder Geo Metro got 50 mpg gallon and "Why the heck aren't modern cars as fuel efficient?" One of the reasons cars today haven't made greater strides in fuel economy is that while engines and transmissions have gotten more powerful and efficient, the cars themselves have gotten heavier. You see, over the years, we've all become accustomed to air conditioning, power windows and door locks, quiet (heavily insulated) interiors, and high-end, multi-speaker stereo systems. Plus we take comfort in having front and side air bags, side-impact door beams, and solid overall structures. Plus, larger vehicles are often fitted with all-wheel drive and third-row seats. All of these creature comforts and safety gear add weight. And the extra poundage detracts from fuel economy.

Also, the engines and transmissions in cars today have helped improve performance to levels that was unheard of 20 years ago. For example, the last Toyota RAV4 V6 we tested went from 0-60 mph in 6.7 seconds and still returned 22 mpg overall. Our model had a third-row seat, standard stability control, and could tow 3,500 pounds. Put this in historical perspective and it's impressive: A two-seat Nissan 300ZX we tested in 1992 made it to 60 mph in 6.1 seconds, but certainly didn’t have the utility, safety equipment, fuel economy, or light-duty off-road ability of a modern RAV4.

However, in some respects, while cars haven't made the fuel economy strides they could have, all the vehicles are a LOT safer. While most of this safety equipment adds weight, we think it's a fair tradeoff. And when you think about it, the 1980s micro cars were all very thrifty on fuel, but none had antilock brakes, stability control, or front-, side-, or head-protection air bags.

Ideally, we think fuel economy should not come at the expense of safety. Today's cars are safer than anything built 20-30 years ago; they’re also cleaner and more comfortable. I don't miss the "good old days" in terms of automotive safety. And the future looks exciting with all the emerging technologies (electric cars; plug-in hybrids; cleaner diesels). And yes, we'll test all that we can, once they become available.

—Mike Quincy

For information on the most fuel-efficient cars and alternative fuels, see our guide to driving green. Also, see our video above on how to improve your gas mileage.