Velocity vs. Engine RPM

Historically my 1990 Mustang LX 5.0 5-speed would do 80 mph in fifth gear at an engine RPM of 2000. Last night driving home from work, it would do only 70 mph at 2000 rpm in fifth gear. This morning, it was back to normal. THis was done on the same road, same grade. The only difference was that it was about 95 degrees last night and about 70 this morning. Any thoughts?

With the direct link of a manual tranny then a set rpm in a specific gear should give you the same speed. Either you where in 4th last night or you got one hell of a slipping clutch.

I was in fifth gear for sure. I do suspect a slipping clutch.
Thanks.

I noticed a trend: in the morning I can do 80 mph with a tach reading of 2,000 RPM in fifth gear on mostly level and slightly uphill interstate. In the afternoon (5:00 PM - 6:00 PM) on my way home from work the tach reads 2,000 RPM, but the speedometer registers only 70 mph. Even going downhill the tach doesn't back off at 70 mph. I duplicated this three or four times over the last week. The only major difference is the ambient temperature; in the morning it might be 65-69 degrees, in the afternoon it's in the 90's and the car has been sitting in the sun all day. So, is this an instrument problem (tach, speedometer) or a drivetrain issue? It's got me baffled.