have a 1977 z28 Camaro have owned the car for the past 37 years has 21000 original miles it is a survivor/driver not a show car. the tach will only read 500 rpm no matter what speed you are going. no corrosion on connections, good ground on fire wall, circuit board for dash is new. Could this be a filter problem or has the tach gone bad ?
When you rev it tap the glass infront of it I have to occasionally other than that you could likely find a replacement for $10-30 filter is unlikely I'd say the needle gets stuck