What is the overdrive for on a 1993 Lincoln town car?

In a 1993 Lincoln Town Car, overdrive is a higher gear ratio in the automatic transmission. When engaged, it allows the engine to run at a lower RPM at highway speeds, improving fuel economy and reducing engine noise. Essentially, it's a way to make the engine work less hard at higher speeds. Disengaging overdrive (usually via a button or switch) forces the transmission to use a lower gear, providing more power for passing or climbing hills.