Could someone pls explain when I should use Overdrive? Have never bothered using it. Thanks.

overdrive should be used for cruising. Turn the o/d off to go a bit quicker, you should turn it on again at high speed (bout 120-130 i believe) to maintain good acceleration up to insane speeds.

So anytime I'm on the motorway I should be in OD then. What does it actually do (in layman's terms)?

Its just like an extra gear for cruising - simple!

Use it anytime you havnt got your racing head on and just want to cruise

and up the fuel economy a bit.

Do All Zeds Have O/d As My Auto Import Has A Button In The Top Of My Gear Knob But Ive Never Notice It Working.

P.s. Ive Got A After Market Gear Knob

All the autos have overdrive. When you press the knob in (the one on the top of the gearlever) it activates overdrive and should extinguish the light on the dash. The light tells you that overdrive is off.

Overdrive on an auto is the 4th gear ratio available (ie 4 speed auto) and to make it more complicated there is also a lock up clutch which by-passes the torque converter making it feel like yet another gear ratio. So overdrive in set off position leaves you in the same top gear ratio as a manual left in 4th gear. If you want to be pedantic about it a 5th gear on the manual is also overdriven in that the gearbox output speed is higher than the shaft input speed - so overdriven.

 

When crusing at speed in overdrive with the lock up clutch engaged you should be showing about 2750rpm @ 80mph

 

Cruising at speed on light throttle in overdrive should mean the least fuel consumption but for instant response on a country road leaving it off and letting the revs rise can be more enjoyable