Hi, i have a 91 accord with 155,000 miles. The first problem is the speedometer does not show the right speed, it changes between 5mph -15 mph over or under the actual speed. The second is it idles rough when it is put into drive, but not into neutral. Somebody please help. Thanks

 

NotaTuner, I'll separate this into it's own thread because it's not the same as the other guys in this thread.

Transmission??
IF it's automatic, then your rear engine mount is vacuum-operated. The mount is supposed to switch into "soft" mode when idling in D or R.

There's a vacuum hose to that rear mount, follow it through a solenoid & look for cracked, leaking, disconnected hose or something like that.

Then the test...
Get the car warmed up. Engine idling with a helper in the driver seat. Get prepared to unplug the wiring harness to that solenoid valve on the vacuum hose to the rear engine mount.
- Hold the brake and shift into "D" or "R".
- Unplug the wire. This should make a noticable difference in smoothness.
- If no difference, then somethings wrong with the engine mount vacuum system.