My overdrive went out on 2001 Kia Sephia

The car still drives ok but I guess it uses more gas. doese it hurt anything to drive it without overdrive?

Asked by for the 2001 Kia Sephia
Kia have changed there recomendations for transmission oil for this transmission from Dexron 3 to Mitsubishi SP 3 oil.
I don't see a separate fuse for the Over drive, I wonder if the switch could be bad? you assumptions are correct the engine will be working a little harder and the gas mileage will be a little worse.