The current difference is abnormally high. You can see the large spike for the most recent data which is way off the chart compared to normal. Usually the % difference is less than 20% but now its hit over 40%.
You can also see that it varies seasonally. I plotted monthly prices for a few years 1995 to 2000 to see how the difference varies month to month.
Generally diesel is around 5-10% higher than gas in the winter and then drops to ~5% cheaper
Overall diesel averages about 6% higher than gasoline over the long run. The mean and median are both 6%.