Overdrive: on or off??

[freenote]

This may be a stupid question.

During city driving, what is best for MPG? Having overdrive on or off?

I can't decide as with overdrive on it drops the revs as soon as possible but then I guess the engine works harder to build up again when you want a little speed. O/d off keeps the engine at more consistent but slightly higher revs.

[jbi]

I leave overdrive off unless i'm on the motorway or exceeding 60mph for any length of time.

Leaving overdrive on can confuse and even damage older automatic gearboxes around town and it also makes them less responsive as they are slower to drop down when you give it the beans

[Tunku]

When I had a car with overdrive I left it off until I needed it as an extra cruising gear. This was on a Triumph Dolomite 1850, a Volvo 740 2.3 manual, and a Volvo 740 automatic. The best thing I found with having an O/D, was that when in top gear with O/D on, if you needed a bit of oomph to overtake, you just floored the throttle and hit the O/D off and when up to speed, put O/D back on. I loved the old Triumph because it had O/D in third and fourth, giving you six gears way back in time.

[mattmoxon]

Yep same here I only use it for cruising at same'ish speeds for long periods, generally above 40mph. When around town around town it is O/D off always, the 'box is far more responsive, also cruise control doesn't seem to like O/D at lower speeds either

I also have it off if I am on a bit of a hoon as it shifts down nicer when nailing it out of a corner.

Edited by mattmoxon on Thursday 9th September 00:39

[Pigeon]

The purpose of overdrive is to give you a high-ratio cruising gear. Making a 4-speed box into a 5-speed without actually having to manufacture a 5-speed box, back when 5-speed boxes didn't exist, sort of thing. No point using it for anything else, just select an appropriate "normal" gear.

Never really understood why a lot of overdrive-fitted cars had it available on third as well as top. The overdrive ratio was pretty well always the inverse of the third gear ratio, so overdrive third was pretty much 1:1 overall - the same as top, but with two extra sets of gear losses (one lot in the box and the other lot in the overdrive) so slightly less efficient.

[Timberwolf]

Pigeon said:

Never really understood why a lot of overdrive-fitted cars had it available on third as well as top.

The only thing I can think of is as a labour-saving device for those situations where your speed is varying between "top of third" and "bottom of fourth" - I'm not sure what stresses and potential wear are involved cutting an overdrive unit in and out though so not sure whether this is a reasonable idea or a staggeringly bad one that will result in the need for a new O/D unit within 10,000 miles.

(I've only driven one car with an overdrive - a 1979 MGB GT - and that had a pleasant enough gearshift that flicking between gears wasn't much of a chore anyway. The equation might change for a car with more of a "magic wand"/"sack of porridge" gearchange, though.)

[davepoth]

Pigeon said:

Never really understood why a lot of overdrive-fitted cars had it available on third as well as top. The overdrive ratio was pretty well always the inverse of the third gear ratio, so overdrive third was pretty much 1:1 overall - the same as top, but with two extra sets of gear losses (one lot in the box and the other lot in the overdrive) so slightly less efficient.

One reason - full throttle instant shifts. There are a couple of times when this is handy. One is on winding NSL roads where 3rd and 3rd OD will allow you to keep things bubbling along without troubling the clutch.

The other one is accelerating onto the motorway off a roundabout. 1st>2nd>3rd>3rdOD>4thOD moves the final clutched gearshift up past 80mph on my Sprint (grumbles a bit at less than that in 4th OD), and the 50-70 is really rather rapid indeed.

[Pigeon]

Umm... I always used to use the clutch anyway so it didn't thump the transmission so much

I remember once having my dad as a passenger and he decided I was driving too fast so he switched the overdrive off when I wasn't expecting it. Made a heck of a thump and he didn't do it again

[chard]

I usually use the clutch engaging/disengaging OD, however going down through the box at lowish speeds I can match the revs with the throttle. I never engage/disengage under load.

A feature of my box is that OD is disengaged when the lever is moved to the left of the "H" so a down change from 4th O/D to 3rd non O/D can be done without touching the switch.

I don't use it much in 3rd, but in 4th treat it as an extra gear where appropriate, pretty much as you would with a 5 speed.

Edited by chard on Thursday 9th September 07:48

[richw_82]

I always use the clutch when putting overdrive in or out. Unless the engine is barely turning there's a huge thump if you decide to try it without.

I don't know how its set up on other cars, but on my XJ, third plus overdrive is about the same ratio as fourth - so there's not much to be gained by using it in lower gears. Overdrive is more for cruising long miles.

I have found out that it gets annoyed if you try to pull away in first plus overdrive, and as we all know, reverse is a big no-no!

I must fix the lock out switch...