This might be relevant:
From howstuffworks:
"So, for most cars, the "sweet spot" on the speedometer is in the range of 40-60 mph. Cars with a higher road load will reach the sweet spot at a lower speed.

"In general, smaller, lighter, more aerodynamic cars will get their best mileage at higher speeds. Bigger, heavier, less aerodynamic vehicles will get their best mileage at lower speeds."
http://www.howstuffworks.com/question477.htm

Other statistics from How Gas Prices Work on howstuffworks.com:
"The United States consumed an average of 19.5 million barrels of oil per day (bbl/d) in 2000, according to the Department of Energy. Of that, 43 percent was used for motor gasoline.

"1950 - $1.91 per gallon
1955 - $1.85
1960 - $1.79
1965 - $1.68
1970 - $1.59
1975 - $1.80
1980 - $2.59
1985 - $1.90
1990 - $1.51
1995 - $1.28
2001 - $1.66

"Where your costs go:
Distributing and Marketing Costs and Profits: 5%
Taxes: 27%
Refining Costs and Profits: 32%
Crude Oil: 37%

"In California, the state government has set its own reformulated gasoline rules that are stricter than the federally mandated clean-gas laws. This is why Californians pay a higher price for cleaner fuels -- this, plus a local sales-and-use tax of 7.25 percent, an 18.4-cent-per-gallon federal excise tax and an 18-cent-per-gallon state excise tax. California's distance from the refineries located near the Gulf of Mexico can also add to the cost of gasoline if it chooses to obtain gas supplies from those refineries.

"The other area where prices can far exceed the U.S. national average is the Midwest. In 1999, the Midwest region became subject to new reformulated gasoline rules. The Midwest uses a special gasoline that is produced using ethanol instead of methyl tertiary butyl ether (MTBE). Ethanol is used in the Midwest because of the region's abundance of corn, which is the main raw material used to make ethanol. Few refineries outside the region produce this type of reformulated gasoline, which means there may often be a limited supply of the product. "

I'd recommend reading the whole thing at:
http://www.howstuffworks.com/gas-price.htm

Note: MTBE reduces air pollution while poisoning the water.