The Energy Department said refiners are starting shifting to summer-grade gasoline ahead of Wednesday's deadline. Summer blends have lower volatility than those used during winter because emissions can increase during warm weather.
"It costs refiners several cents per gallon more to make summer-grade gasoline, compared with winter-grade fuel, which is part of the reason that retail pump prices can rise in the summer," the department's Energy Information Administration said.
Motor group AAA reports that U.S. consumers paid, on average, $3.50 for a gallon of regular unleaded gasoline Tuesday. That's about 30 cents lower than the same time last year. Regional highs were reported in Illinois, with a $3.91 average, and in California, with a $3.90 average.
Some states have stricter gasoline requirements, which may explain price variations, the EIA said.
EIA said gasoline prices during the peak summer drive season, which extends to September, should average $3.63 per gallon. Refiners switch back to winter blends in mid-September.
Notable deaths of 2014 [PHOTOS]