This book outlines dozens of methods of saving fuel that have been around for decades, but have not been made common knowledge.
For example, water injection was used in World War II to increase the performance of fighter aircraft. Water injection can be applied to modern carbureted and fuel injected vehicles. A simple water injection system can be put together for less than $5.
Note: If you apply any combustion enhancement technology, remember to apply the Carburetor Enhancer or EFIE too