The lowest cost ways to save on fuel costs is to increase telecommuting, bicycle and public transportation convenience and usage. This can be done individually and city, state and national levels.
Transportation statistics from bicycle universe More bicycle statistics are in this pdf
A Canadian study by Auto-Free Ottawa found that 86 percent of the American workforce commutes to work by car, and more than 90 percent of those commuters park for free. The average national value for a parking space is approximately $1,000, so that means $85 billion in annual subsidies. Ending these free subsidies would reduce the number of solo commuters by as much as 81 percent.
Studies should be done on increasing bicycle and public transportation usage.
I think subsidizing electrical bikes and folding bikes would be useful.
One could replace an automative commute with an electric folding bike and use public subways and lightrail and buses to help on longer commutes.
One could also park farther from a downtown area (where it is probably more expensive to park) and use the electric folding bike to go the rest of the way.
The electric folding bike would allow the commuter to still have a larger range to go to multiple destinations (restaurants, errands, work, etc...)
Conversion to electrical assisted bikes.
The fuel to power the electricity of an electric bike converts to 500+mpg. If the USA could get bicycle usage as high as the Netherlands (28%) from its current 1% and public transportation usage up from 2% to 7% then driving would be reduced to 62% of the current level.