As the owner of a TDI vehicle, I have definitely noticed this discrepancy. Whenever the media trumpeted a picture of a gas drop, if there was a diesel price, it was far closer to the national average of diesel. Still the prices I am paying here in mid-Florida have now dropped below the price I paid 4 years ago, a very pleasant gift to my wallet. Another fact noticed in those 4 years is in times of sudden gas cost spikes like Southern California about a year ago (?) when even regular gas was above diesel.
Just some personal observations.
I bought diesel yesterday for $3.25 at Willco on the Interstate. Across the street at Pilot it was $3.26
I expect the same was $3.15 or $3.20 at Murphy(walmart) or 3.10 at Murphy (Sams). I filled up not long ago at Sams for $3.05
Those prices are down from just shy of $4.00 in early ‘14 as I recall