Most states, and the fed, tax on a per-gallon basis, so the tax on 10 gallons is the same, regardless of price. These states actually make out better when prices are low, as folks tend to drive more when it costs less. As an example, Idaho's fuel tax has been 25 cents/gallon for years for both gasoline and diesel.
Way back in 1998, fuel price for regular was around $1.10 in some areas (price wars). At that point, 22.7% of the price of that gasoline was Idaho tax. Today, around my hometown of Nampa, ID, the price is around $2.69/gallon. The same tax of $0.25/gallon is now only 9.3% of the purchase price. Where I live near Albuquerque, NM is seeing prices in the low $2 range (lowest near me is $2.129/gallon).
Only a few states (CA is one) include sales tax on top of the price.
MI puts a 6% sales tax on top of their per gallon cut.
And then there are states like Illinois where Gas is taxed at the State, County and Municipal level IN ADDITION TO having a state sales tax on every gallon of gasoline, which in effect "taxes the taxes" on every gallon of gasoline sold here.
Bastards here double-dip!
CT is another, that's where it hurts.