Ignore the code 58. It'* a known bug in the 92 PCM.
Inspect the wiring harness for your cam and crank sensors. Both share a common connector underneath your alternator that migrates towards the front of the engine.
In this pic, you can see it. The connector is in the top right corner of the pic, and the wiring harness is the shinier black loomed harness going from right to left in the pic. The first 'tail' downward you see behind the water pump is for the cam sensor. The next 'tail' goign down is for the crank sensor, and the rest of the harness travels on to the ICM.
Ultimately, your crank and/or cam sensors are bad, and yes, that'll kill your mileage.
Your car should be able to achieve 28-30mpg well-tuned, with a trans that doesn't slip.