Originally Posted by maverickfhs
Originally Posted by Inspecktor
I would keep it out of overdrive. In addition to the weight, you will also have aerodynamic drag to deal with.
Don't think my car has an overdrive, does it mean I stick to 'D' ? Thanks
"Overdrive" is any gear where the output shaft of the transmission is turning faster than the input shaft. Older transmissions sometimes had the overdrive unit separate from the main transmission(the factory-optional overdrive on my MG, which I don't have, is that way-the top standard gear is 1:1, and the O/D has an electrically operated cone clutch that can be engaged in top gear), but on modern transmissions-both auto and manual-the top gear is almost always an overdrive gear. I don't think it's unheard of-especially with some autos now having 8+ gears-to even have multiple overdrive gears.
In any case, every car auto box I've had has specified towing with O/D disabled, although the way it's labeled has changed over the years. On older Fords(and Lincolns) for example, the standard "D" position had a circle around it to designate overdrive, while there was a separate D position that disabled overdrive, or sometimes a instead a button on the end of the stalk(column shifter) or floor shifter to turn O/D off. I had a Nissan that was that way also. My dearly departed Lincoln LS had "D5" for normal driving and "D4" for O/D off.
In any case, though, I default to "read the manual and do what it says." Chances are it will also specify a maximum "safe" speed in D3/OD-off, and it's probably a good idea to follow that.