Did John Parker tell you how to jet the Mikunis & the ignition timing?

Craig Coburn is getting 31mpg at speed with the same head & cam combo as you. He has HIF SUs instead of Mikuni's, but Mikuni's should be able to equal SU's at part throttle, so you should be getting close to 30mpg cruising the highway at 70mph.

The exhaust really needs to be bigger. It would be OK cruising on the highway, but anytime you stick your foot down harder than average, it will be costing you some horsepower & economy.

Actually i put about 200lbs. in the trunk in the winter, (4 snows of course) and It goes through anything.

Change the springs & swaybar, then you only need 50lbs & will save some fuel.

Do you plan on driving your Miata to work in the snow?