Average diesel fuel prices by state are available at AAA.com. The site provides a daily fuel gauge report that shows the average price of regular, mid-range, premium and diesel gasoline. The site sorts information alphabetically by state, but users can also sort by the average price in a category.
The state with the highest average diesel price, as of April 2015, is Hawaii with a price of $4.50 per gallon, while the state with the lowest average diesel price is Oklahoma with a price of $2.44 per gallon. AAA.com's daily fuel gauge also provides data on the national averages for each category. This data includes the current average, as well as averages for the previous day, week, month and year.