I received an email with this info recently and checked it out on Snopes - here's the link:
http://www.snopes.com/inboxer/household/gastips.asp
Sounds like it's only partially true. The last paragraph mentions the following: There are much easier and better ways to make your car more efficient (and save gas at the same time). Particularly important is proper maintenance (engine tune-ups, wheel alignment, proper tire pressure, filter replacement). Also remove any little used equipment that adds weight and drag such as roof racks/carriers, sports gear or tools. Some people run with extra weight in the trunk or back of their vehicle for traction in the winter, which also reduces the gas mileage. Driving habits are especially important: jackrabbit starts and starts burn up extra fuel, as well as driving at higher speeds. All in all, the simple habit in engaging in the planning and combining multiple trips into one excursion will likely save the average motorist far more money (and time) than all four of the tips listed in teh original article.