here as well diesel is slightly more expensive that premiium fuel -about 5-10 cents/litre( so about 18-37 c/ US gallon. The fuel consumption difference should be about 40-50 % at least in something like the frontier.. I expect based on road tests of local cars to get an easy 22 mpg in the city, probably more and up to 28 + on the open road. My current 3.8 litre petrol workhorse (which is approx 1200 lbs lighter than the Frontier) gets around 18 mpg around town, and 26 on the open road.
When towing a heavy load (currently looking at towing approx 2200 kg (4800 lbs) plus passengers plus 400 lbs in the back) my current workhorse get around 10-11 mpg, and really struggles with it!. I expect the fronter to be something like 15 mpg and be a far better things up the hills. I expect it'll save me approx. 52 AUD per trip away with the trailer, and about 10 AUD a week otherwise, so perhaps 1000 AUD a year over what I have now ( about $2.50 US!

!!!) or about $1500 AUD (3.75 USD!) a year over a petrol Frontier
Cheers
Rowds