One good thing: as you approach the speed of light your vehicle will contract in the direction of motion, making it more difficult for speed cameras to detect (and, of course, for police cars to catch). Unfortunately your mass will also approach infinity, which will be counter-productive in terms of your weight-loss program.

However, if you’re a bit lazy and you’ve got a car that supports dual trip-odometers, reset one every time you fill up your fuel. You can use this as a easy way to gauge how well you’re doing on the fuel conservation front. I do this and used to always try to beat 400miles per tank. Now I can routinely beat 420 miles a tank, though going much further than that leaves me a bit uncomfortable that I may run out.

This has the added benefit of giving you a heads up when something may be wrong with your car, since a lot of times a loss in MPG is the first indicator that there is a problem. If I was to get 350 miles per tank one week, I’d be suspicious. Two weeks in a row, and I’d have a mechanic look at it.

Issue #2 with turning the engine off–what happens if you can’t get it started again? This, alone, is why I never, EVER, turn an engine off in traffic.

With gas, one concern may be quality of fuel. Some mechanics say that not all gas is the same, and some cheap brands may hasten problems with fuel injection systems. Which could easily negate any savings.

Finally, the best savings is not driving. Parked, a car uses no fuel. Thus, the less one drives, the better. Plus, this can result in lower insurance rates, and will lower maintenance costs.

I’m not sure I would agree with this. Turning over the engine to start it back up requires a significant amount of electricity to power the starter motor. Naturally, this energy is pulled from the car’s battery. Even though the alternator is recharging the battery as you drive, this puts extra wear and tear on your battery, and extra wear on your transmission. It might save gas but it could cause you to need additional maintenance more quickly than you would have otherwise.

Don’t forget that speeding is simply not safe. My car is as efficient at 55 mph as it is at 80 (I’ve tested it several times – always 32 mpg) But I’m much less safe if I’m going 80 than 55 depending on the traffic situation. Safety will always trump everything else I do in life, but generally I go a few miles above the posted speed limit if conditions and traffic allow on the interstate. I get both maximum mileage and time efficiency that way.

I’ve also heard others discuss the merits of different grades of gasoline on their mileage. The guy who buy my 1989 Nissan Stanza swore that he got his best mileage out of mid-grade (specific brand) gasoline than he did any other grade or brand (I can’t remember the brand he used). He said it’s car specific and he’d test the Nissan to see what worked best for it. I’m not sure I have the patience to test all those brands, but I have done some grade testing. My car, which advises you use premium fuel, does burn more efficiently by using premium gasoline. I’ve done the math and I am better off paying for premium as my car burns it at a more efficient rate than 87 grade. No more than I drive the savings aren’t that great, but it might be worth it to run some tests of your own if you drive a lot.

I appreciate the point you were trying to make here, but I think it’s a little oversimplified.

Most cars achieve maximum fuel efficiency at around 50-55 mph. So while you’re correct in pointing out that going 75 mph in a 55 zone is not only dangerous, but also very inefficient, it’s also true that going 55 in a 45 zone is INCREASING your fuel efficiency.

Think about it. Crawling along at 5 mph in rush-hour traffic is terribly inefficient. Likewise, obeying the speed limit in a 15 mph zone is operating your car at well below maximum fuel efficiency. In this case, speeding (up to a maximum of 55 mph) actually INCREASES fuel efficiency (though clearly is unsafe).

Ignoring the safety aspect of the argument, it’s inaccurate to make the blanket statement that “When you’re speeding, your car uses fuel much less efficiently.” A more accurate statement would be, “Traveling at any speed other than 55 mph, your car uses fuel less efficiently.”