built up 318 oddly low MPG's? hp rating?

Ya I am pretty sure I have at least 8:1 compression. It runs really well and has a lot of get up and go, they guy I work for is a car nut and has 9 classic muscle cars including a 79 corvette with a 355hp 350, 67 gtx with a 375hp 440, and a 67 el camino with a 425hp 427. I've driven them all a lot and can tell you that my car can hold its own quite handily.

I have a 87 VW Jetta that gets 30mpg. Im well on my way to the Naval academy, haha believe me this car comes second to most things, but yes I am getting a little tired of working on it and want to drive it.

When I installed the cam I checked the timing and timing chain and calculated that it is in correctly as it should be. Went through the carb. last night and changed a bit, it runs a bit better now. the 64 jets it used to have before toms performance DID run a bit lean and had hesitation so I dont they were too far off from putting in 68's. I have new plugs, cap, rotor, wires, mag pick up, ballast resistor. I may invest in a new coil shortly. I just installed my 3.55 posi rear end and put new-used 225/60/15's and I Know my speedo is off 6% and my odom. by 11% when I checked it with my GPS a week ago. I will run down a complete leak down test soon

but I mean I read on Holley's site that 16-18hg of vacuum is what you are looking for and as off last night I went from 14 to 16. so that's a check mark. the timing is only about 14 degrees initial so it's not that bad, sounds great and is a good runner I'm really curious to dyno it but I'm guessing it has 280-290hp. So do all my car/mopar guys who helped me build it, so nature of the beast lol.

Thanks guys