[Healeys] Tach heat sensitive?

[Tom]

I've noticed that when I drive to work in the morning when it's cool
(50-60F), my tach shows about 2500 RPM at 65 MPH.  When I return home at
night when it's warm (80+ F), my tach shows about 3000 RPM at 65 MPH.

Any ideas about what's going on?

thanks,

Tom