I bought a 90 Miata about 6 months ago with 172,000 miles. Now I have about 187,000. In the past month the performance has gone down noticably. Climbing a hill requires being in 4th or 3rd and my speed dropping to 40 or less. My milage has dropped from 35 mpg to about 29. When just started, it pops when I come off the gas. Any ideas?

sounds like normal wear for a miata. when was the last oil change? changed the plugs/wires recently? the pop is pretty common, miata ecu's tend to run rich. try replacing that cat w/ a test pipe

That kind of stuff started happening to me recently. I'm doing an engine swap now. The rear seal was leaking oil onto my clutch. Ruined my clutch and flywheel. Someone first told me that it was the slave cylinder, but the problem wasn't fixed when I replaced it. I had to look further into it, and the problem was a lot worse than I thought it would be. Be careful. (The slave cylinders to wear out fast on those cars.)

It does sound about normal though. Oil and plugs and wires are good to check.

I have a test pipe instead of a cat. It doesn't really do much for the gas mileage...

Did you check the releaf valve connecting valve cover to intake manifold.