My motor has always had an odd vibration within a certain RPM range, but because I lack a tach I don't know what RPM my problem starts at. I have an A-Code GT coupe with a standard 4 Speed Toploader, stock height tires, and stock rear gears(supposedly 3.00's). The vibration starts roughly at 20 MPH in 2nd gear, between 30 and 35 in 3rd, and 45 in 4th. I'm going to use 3rd gear as an example...the vibration picks up between 30 and 35 and will stop at 40 MPH, now my question is what could be the cause of this vibration? It happens under light and heavy acceleration...I'm not a motor person so I don't know what could cause it.

 

could be bogging down..... my car hits 40 in first, 75 in second, maybe it's being shifted too soon?? It could be timing, vacuum leak, fuel problems....