alright...to mt people this may seem trivial but oh well. After each trip(weekends of driving in 93 one time and 66 GTO the other) my rpms were differnt by 300 rpms on the return trip. In the GTO, they went down 300, letting me go 5mph faster at the same rpms, while in the mustang they went 300rpms higher (about 12 ph slower at the same rpms). Not my clutch because, they are both solid and not slipping. Not my needle because I could hear the engine running at the indicated levels. Not becasue of inclines becasue I had the same rpms level/up/down and in town. With both cars, the rpms returned to normal the next morning when I started them up. Any ideas would be appreciated.

Wire resistance to the tach pickup will cause this, check the wire's continuity. The resistance to the connection should be under 1 ohm.

what was the wind direction on these trips, in michigan we have constant westerlies and i get getter gas millage and tach readings on my way east than i do going west.