The 1990 Honda Accord does NOT have a speedometer cable. The speedometer gets its signal from the VSS (Vehicle Speed Sensor). The drop in mileage may be related to the VSS problem. The fault could lie in the wiring between the VSS and the speedometer. It could be the VSS. Check fuse #1. It supplies power for the VSS. Checking the VSS circuit, with a digital multimeter, is the way to go.
Click on this link for the wiring diagram of the VSS circuit, then click on Fig #37: http://www.autozone.com/shopping/repairGuide.htm?pageId=0900c1528005f9fc