Is it better to leave o/d on or off? It has been on for 3 years

O/D or "overdrive" is the gear/ gears in your transmission above a 1:1 ratio. Usually, 4th gear is 1:1, meaning that the amount of cogs on the gear that your engine is driving is equal to the amount of cogs on the gear driving your wheels. So when "overdrive" is on, it allows the vehicle to shift into 5th gear (5th and 6th for 6 speed transmissions) the main reason to turn it off is for climbing steep hills or driving with a heavy load. This will keep the transmission from the cycle of trying to shift up into a higher gear, then downshifting when it starts losing power, then shifting back up when it thinks its the right time again even though its not. This will reduce unnecessary wear and tear and strain on the transmission. However, if you don't live in incredibly hilly terrain, or haul heavy equipment everywhere, then theres no point to ever turn off the transmissions ability to use 5th gear as it sees fit.

OFF,when not towing.

The reason it's called an automatic transmission is .. well it's AUTOMATIC! .. It knows when it needs to shift to a higher or lower gear, for some funny reason! .. ▪️Owners manual is a good source for the details!!▪️