Yes, it should be diesel mileage, but gas mileage is the more common term.

I was driving around with my Yakima rails and bike rack on the car and not getting very good gas mileage (about 38.5 mpg).  My car is rated 42/49 so that was pretty annoying.  I had been attributing it to the change to low-sulfur diesel.  When we drove up to New York, we decided to take the whole thing off the car and see how we did.  For that trip, we got 48.6 mpg.  Huge improvement!  I haven't put it back on and am now getting 43.9 with mixed highway/city driving including some stopping during rush hour.

I was curious how this breaks down for cost per month and cost per year.  I average about 1300 miles per month.  Keep in mind, this is an average for the whole 5+ years I've owned my car, so it include all the trips back to Wisconsin.

Mileage	Gallons in a Month	Cost per Month	Cost for the Year
38.5	33.77	160.39	1924.68
43.9	29.61	140.66	1687.92
48.6	26.75	127.06	1524.72