I own a shop, and a Nova. You may be surprised at how good it will do out on the highway. It should crank out 18-20 mpg, and 14-16 around town, but the drawback is; It doesn't have the gearing for truly good mpg. You can do what I did; I replaced the transmission with a 700R-4, and mine now has 4 forward gears with a lock-up converter that really makes for 5 forward gears. I can get 30 mpg with no problem. I have a pretty high geared rear end (2.72:1), and the engine isn't above 2,500 rpm's at 55 mph. The way it is stock, won't do that good on mpg, so don't expect to see any better than what I posted. I base this on a well tuned engine too.
Glad to add my 2 cents worth!!!