That's pretty surprising that the Miata is rated so low, but your first and best modification is to get some instrumentation to see what you're *actually* getting, instantaneously. You might be surprised to find the Miata is capaible of much more, with instrumentation. Since both of your cars are new enough to be OBD-2, a Scangauge or Ultragauge is your easiest option. There are also some $7 bluetooth Elm327 adapters on Ebay that you can pair with a phone or tablet, but they tie up another expensive device.
Food for thought, a '98 Civic HX is rated for 39 highway but members on here regularly exceed 50mpg (sometimes more than 60mpg) in these cars. The slightly older VX trim is capable of even better economy, while also being a roomy hatchback. Records have been set in the even older CRX's and first-gen Insights in excess of 110mpg. I can understand attachment to vehicles though - I'd be pretty well served getting rid of my Del Sol for something larger and more efficient, but it has a lot of character.
It helps to like what you drive.