(May 2, 2022) BOSTON — Diesel prices have reached a new average all-time high of $5.16 per gallon, surpassing the previous record of $5.15 per gallon set on March 10, according to GasBuddy. In addition, a second record has been set — diesel prices are now $1 per gallon higher than gasoline prices, surpassing the previous record 98 cent difference set in November, 2008.