Economy gauge reading low when cold
 

My car is a 1989 560SEL, 148K miles. Since the cold weather hit Florida, (temps in the mid 60's in the morning), my economy gauge reads about 1/4 inch off the peg when idling, upon start up. Usually sits right on the peg. No other unusual symptoms, MAYBE a rough idle, if I listen really close. When the car warms up, the needle returns to normal behaviour.

I have not even cracked the hood yet to troubleshoot, but I plan to over the weekend. Any thoughts or ideas, where to look?

As always, I appreciate the help!!

P.S. Just installed new Pilots all around. SUCH a difference in ride from the Firestones.....