My absolute maximum was 442 miles on a single tank during a Boston-NYC 2-day visit where I happen to fill-up just before leaving. It was 95% highway at ~75 mph and the trip computer calculated 26 mpg. I had the fuel light on for the last 35 miles, which was not too clever. It was also the largest fill-up so far, some 17.5 gallons.
I guess driving at 60-65 mph exclusively on the highway could get you to 500 miles with one tank, if you stay awake...

I regularly get some 330 miles/tank at 19-22 mpg from a 50/50% highway/city 20 mile daily commute.

 

i would say the g35 would be a nice upgrade compared to the mustang especially if your making long trips to la to sf, since the g35 is more of a luxury gt compared to the mustang. I average about 18-19mpg city and about 24-28mpg highway. i average around 21-23mpg combined.

Basically i get the epa numbers, with my manual shifting at on average around 3k rpm and basically rowing into 6th as quickly as possible on the highway. On the on ramps, i gun it like everyone else to redline.

Best mpg on the highway seems to be 75mph and below, you'll average around 24-28mpg depending on if your going 65-75mph. At 80mph, the timing changes significantly and your mpg goes to crap. My mpg at 80mph goes down to 20-22mpg.

Its a 20 gallon tank and the light comes on after you use around 17gallons. Most people don't drive there car to empty and is not something that i would recommend, so when post people say tank that's still with around 3 gallons left. So yes in theory its possible if you do all highway at the highway mpg and use all 20 gallons to get close to 500

 

Biggest pros over stang IMHO:
Exterior Looks
Interior Quality, feel
Handling
Gas Mileage (I get 26 in my 07 6MT on the highway, 16 when I play)

Biggest Cons:
V6 (not the worst, but man V8s sound nice)
Expensive to Maintain (compared to a ford)
You'll be more afraid of door dings in parking lots

 

They did a study to prove that, on newer cars, a clean air filter doesn't really make a difference regarding gas mileage.

http://www.fueleconomy.gov/feg/pdfs/...02_26_2009.pdf

I wish it helped..