Anyone know what would cause the speedometer needle to jump erratically, other than the speedometer itself? At highway speeds the needle jumps from 40 to 75 mph.

 

check ur electricals......had this prob b4 an it turned out to be a loose battery terminal/clamp

 

Your VSS may be going out, common on our cars. Do a quick search on the forums for Vehicle Speed Sensor. Its either the electrical part, the harness, or the small mechanical gear that turns, before it is converted to an electrical signal and sent to your ecu and dash. In my case the harness was corroded and wasn't getting a proper signal from the VSS resulting in a CEL and error P0500. The harness I picked up at a wrecking yard for 5 bucks. VSS is common on ebay for less than 20 bucks. The mechanical part I found on craigslist for 10 bucks locally, so it should be a cheap fix no matter what you find the problem is.

http://www.hondacivicrepairs.com/views/honda-civic-speed-sensor.JPG

Its on top of the tranny, left of the air intake to the engine.

 

vss