ok well this is the deal. i just got done swapping a US spec gsr into my 96 civic hb so i decide to take the car out for a drive. well i was just driving normal shifted to second then when i shifted into 3rd it acted as if im in neutral and now basically it wont move whatsoever. so basically now i can start my car, everything sounds fine but i can shift gears without pressing the clutch but yet it doesnt stall out my car or anything. also as i switch from gear to gear my mph are still going up even when im not moving..so a couple of my friends claim it might be the clutch then some argue it might be the differential..i just really have no idea

 

Isn't the clutch, because if it was your speedometer would stay at zero. The speed sensor reads off the outside of the differential/ring gear, so that far is OK.

Check for stripped splines on either end of each axle. Rule that out before you blame the differential itself.

 

Yep, if your mph is moving but your car is sitting still then your diff is moving(speedo gear is connected to diff).. Maybe you stripped out the axle splines or maybe an axle broke since all the power would be going to the broken/stripped axle..

Get a light under the car(DONT GET UNDERNEATH OF IT) and check the hub where the axles go into the trannie.. Get someone to put it into first and let out on the clutch and see if the inner hubs are turning(or at least one of them).. If neither is turning you probably stripped out the splines on the axle(s) or inside the diff.. If one of them is turning then you broke an inner cv joint or something else axles related..

Good luck..