gas mileage

Actually I just got this car from my 85 year old Aunt who has had it since it was new, and its my first automatic transmission ever, and I have noticed the tach is always just under 3000 which seems high, but I just assumed that's where it wants to be. There is an Overdrive button on the shifter - maybe I hit it so its off? Thanks for the lead. I assume that if its not that button then there's something serious(ie - expensive!) with the transmission? Really appreciate your help!

No - a dash light tells you when the OD is off and it hasn't been off. So I guess my new question is what's causing the high revs and what's going to need fixing. Would be grateful for any insight - always better to go to the shop with some knowledge!

There is no engine or other warning light on. It was fully inspected before I drove the car home from San Diego to New Orleans. It got a clean bill. Just needed the transmission fluid changed and they recommended new spark plugs when I got home. The mileage on the 2000 mile trip was great. The mile and revs seemed normal until about a week ago. Going to do the spark plugs and ask them to check my fuses and hoses. Will let you know the results. Thank you for the guidance. I let it warm up this morning before pulling out. The idle revs went from 2000 down to 900 and seemed lower once I started driving but didn't get past 40 yet today.