The only way you'd get 25 MPG out of a 66 mustang (even the V6) is if you pushed it the whole way w/ the engine off. A 2011+ if you drove very conservatively or just did freeway the whole time you could may be do it, but for a classic one it's never going to be that high.
Though as Doorknob said he got 20 doing while road trips probably on the freeway and driving conservatively, but of course if you're looking at combined mileage there's just no way 25's gonna happen. Especially considering that up to 2010 25 MPG was the highest ESTIMATED freeway mileage of most V6 mustangs to that point
honestly I think you'd be lucky to get more than 12-15 MPG combined mileage out of it,Classic stangs DO NOT get anything close to good gas mileage by modern standards.
I have a 351C in my 72 Mach and I maybe get 10-12 max out of it.
Ours with the auto trans and stock size tires did about 15 around town combo driving. The couple road trips I took it on it did around 20. The thing that made it seem better is that when the gas gauge gets below 1/4 tank it only takes around 12 gallons to fill. At least mentally that is easier to process than filling up the truck with a 30 gallon tank.