With Americans driving less, the federal flat tax on gasoline of 18.4 cents per gallon is bringing in fewer dollars. Some states, however, are having less trouble getting money from drivers.

Since January, the average tax per gallon has risen in 16 states, according to a new report from the American Petroleum Institute, Forbes.com reported. That has pushed the July 1 nationwide average tax on a gallon of gasoline, including the federal levy, up to 49.4 cents per gallon from 47 cents in January.

The six states with the highest current per-gallon charges -- California, Connecticut, Illinois, New York, Michigan and Indiana -- all levy taxes based on a percentage of the price of gasoline.

California motorists pay an average, including the federal tax, of 74.9 cents a gallon, up 11 cents a gallon from January. Indiana jumped from 12th place to sixth on the high-tax list, thanks to an April increase in its general sales tax rate from 6 percent to 7 percent.

Another legislated increase came in Minnesota, where the gas-tax increase, which is being phased in, will total 8.5 cents a gallon by 2012. Still, the state's tax as of July 1 is just 24 cents a gallon, 7 cents below the average state tax levy.

The lowest tax on regular gasoline is in Alaska, where the state collects 8 cents a gallon. By region, the lowest taxes are in the South, where combined federal, state and local taxes averaged 38.5 cents a gallon as of July 1. The West is highest, with an average of 64.6 cents per gallon.

Diesel taxes run even higher, with a combined nationwide tax averaging 56.4 cents per gallon in July, up almost 3 cents from January.

The state's with the highest gas tax rates are:

1. California: 74.9 cents per gallon
2. Connecticut: 70.8 cents per gallon
3. Illinois: 66.6 cents per gallon
4. New York: 62.3 cents per gallon
5. Michigan: 60.7 cents per gallon
6. Indiana: 56.9 cents per gallon
7. Washington state: 55.9 cents per gallon
8. Florida: 51.6 cents per gallon
9. Wisconsin: 51.3 cents per gallon
10. Nevada/Hawaii: tied at 51 cents per gallon