If you're looking only at the cost of fuel, you have to really compare apples to apples.
Consider your diesel vehicle get's 20 miles a gallon. Consider the cost of diesel per gallon is $4.00 per gallon. 15 / 4 = $0.20 (twenty cents) a mile.
Consider your gas vehicle gets 15 miles a gallon. But the cost of gas is $3.60. The price per mile is $0.24
For example:
Diesel (realistic in Indiana right now) is $3.70, my truck gets 14 mph that equals $0.264 (twenty six cents) a mile to run.
My car (gas) gets 34 mpg. Gas, right now is abut $3.20. That costs me $.064 (six cents) a mile to run my car!
That's how you should REALLY determine fuel costs!
Do you math and then see what works best for you.