Welcome Charlie, I have the 3.9 model and 30 on a long run, 22 locally but I am a bit keen on the sound track so I'm sure a better mpg could be achievable if that was necessary.
I wouldn't have side pipes either, on a run you'd get sick to death of them, my dual Librands Sport system is enough on a long run of 200 +.
Haven't played around with carbs as both my cars have been EFI, others on here will have though and will show up soon.
Enjoy the car there is nothing quite like a V8 rumble.
