My 68 GT with a 428 CJ, 4 speed and 3:90 gears got 11 mpg and it was only 335 hp, so I'll take 22 mpg at 300 hp, at least for now. I haven't had it on a trip yet, but I expect it to get 25 or 26 mpg.
You will only ever get the advertised ratings if you minimize your RPMs, skip gears, utilize 5th gear and are smooth on the gas pedal, the break and steering wheel. You are not going to drive it like you stole it and get 25 MPG.
I got near 30 on a long trip at between 65 and 70 m.p.h. Mixed driving gets betwee 19 and 23 depending on how aggressive I am. I checked the computer against the actual math for two months and it was always within 1/2 m.p.g.,so it is accurate enough. I also did one month of ethanol free gas compared to one month of gas that contained up to 10% ethanol: no difference in mileage.