Back before about 1980 not many cars had an overdrive ratio available. Early overdrives were not an incorporated part of the primary gearbox. They were a secondary gearing that was engaged either after the primary gearbox, or in fewer cases, prior to the primary gearbox. In fact, with cars up to around 1980, the driver did not even use the gearbox to engage overdrive. It was often done using a completely separate lever or control placed elsewhere in the car—typically on or under the dashboard. Usually something you turned or pulled to engage the overdrive ratio.
The idea of overdrive, and the reason why it is called overdrive, is that with overdrive engaged the driving shaft coming from the engine rotates less times than the output shaft going to the differential. Thus the output shaft is being ‘overdriven’ or de-leveraged.
From the first gearbox designed and put into a motor vehicle the final or ‘top’ gear was set up so that the ratio of drive from the input shaft to the output shaft is 1:1; or very close to 1:1 (like 1:0.98 or 1:1.01). For one rotation of the input shaft there was pretty close to one rotation of the output shaft. This 1:1 gear ratio is also known as the “top gear”.
For the average ‘family’ car built up to about 1990:
- they weighed close to, or more than, 2 tons,
- aerodynamics was not a big consideration in the design, so as the vehicle went faster the wind resistance increased almost exponentially, and
- engines had significantly lower horsepower and torque compared to now per cubic inch of cylinder capacity.
Because of the three points above no overdrive gearing was possible or even considered back then. Cars were battling to push themselves forward at anything over 80 miles per hour with 1:1 gearing—in fact often the differential had to be geared to compensate for this so that a 1:1 ratio could even be used as the top gear in the gearbox.
- car weights decreasing by as much as 50 percent across model ranges due mainly to the use of alloys and plastics,
- amazing benefits being achieved with aerodynamic designs (the wind resistance of a modern passenger car at 60 miles per hour is only 60 percent that of the average car built in 1970), and
- internal combustion engine technology advancing significantly (including the development of reliable and long life turbo-charging and super-charging technology),
the use of overdrive gearing ratios started to be become common by the late 1990s.
It would be hard to find a car built since 2005 that does not have overdrive available either as an add-on to the gearbox or incorporated as additional gearing ratios directly into the gearbox. Some more expensive cars with very powerful engines have two overdrive ratios, being two gears that have ratios beyond top gear.
In gearboxes that have overdrive the 1:1 ratio gear is still called “top gear” even though it is not the last gear available in the gearbox. So, technically, in a 6 speed gearbox with overdrive the top gear is 5th, and 6th is overdrive. Or in an 8 speed Ferrari with two overdrives, top gear is 6th—and 7th and 8th are overdrive 1 and overdrive 2.
The Problem(s) with Overdrive
There are two main problems with overdrive gearbox ratios.
- Due to the drive shaft rotation being less than 1:1—typically something like 1:0.8 (it takes 80 percent of a rotation of the drive shaft to cause one full rotation of the driven shaft)—there is significantly less power being delivered to the wheels of the car.
The engine has no ‘leverage’. In fact it is under-levered. Because of this it is necessary to de-select overdrive when going uphill, or when needing pick-up to overtake another vehicle, or when pulling a caravan or other load.
- Again, due to the severe lack of leverage the engine is able to exert on the car when an overdrive ratio is selected, there is very little breaking effect applied by the engine when decelerating.
Because of this overdrive needs to be deselected in those cases where the breaking affect of the engine is required when decelerating. This would be when slowing down (coming into a town on the open road) or when travelling down a decent and you don’t want the vehicle to pick up speed (as the engine has no breaking or slowing affect on the car).
The Purpose of Overdrive
So, considering the above, why have an overdrive gear, or two? Basically there are two reasons for overdrive, and really, both are the same reason but just stated differently.
Firstly using overdrive saves petrol when the engine does not need to provide much power or is required to act as a break when decelerating. This is because the car moves further, due to the overdrive ratio, using less engine revolutions—hence uses less petrol (generally).
As soon as the car requires power the overdrive should be deselected. In most automatic transmission cars this happens automatically (funny that, in an automatic transmission car) as soon as the transmission system detects that the driver wants power. In modern manual cars, where the overdrive ratio is incorporated into the gearbox, the highest numbered gear is the overdrive gear. So as soon as the top gear is deselected, so is overdrive.
Not so ‘automatic’ is the de-selection of overdrive when engine breaking is desired. This is when going downhill or decelerating when changing speed zones to a slower zone (such as coming into a town on the open road). In both of these cases the driver needs to manually de-select overdrive and put the car back in ‘top gear’ to engage the breaking effect of the engine.
Secondly overdrive allows the car to travel faster without needing to rev so much, providing you are not towing something or going up a lot of hills. This works up to a point until the wind resistance gets too high due to the speed of the car. Fortunately for most drivers the speed at which overdrive can no longer push the car through the air due to wind resistance is not a problem—this starts to occur at about 140 kph or 85 mph, depending on the aerodynamics of the car and the engine it has.
Good ‘Overdrive’ Habits
Taken from the British Automobile Association’s handbook, following are the top two habits for drivers to adopt in relation to using overdrive in a modern car:
- Deselect overdrive when driving in 60 kph or lower speed zones. In the case of manual gearboxes this will stop your engine from labouring. In the case of automatic gearboxes this will stop the gearbox from hunting between overdrive and top gear. In both cases deselecting overdrive provides a more responsive driving experience which is generally desirable when navigating city driving conditions.
- Deselect overdrive when ascending/descending inclines/declines. This will prevent your gearbox from ‘hunting’ between top gear and overdrive as you climb the incline, and will provide engine breaking to prevent your car from picking up speed as you descend a decline.
Drivers who do not deselect overdrive when descending an incline are likely to over-use their brakes as they try to keep their speed within the required zone. Over-use of brakes will result in brake fade (the brakes become less effective the longer they are kept on and heat up), warped disk rotors (due to the intense heat generated of continual braking), and premature wear on brake pads.
Points to Note
Not all cars have an overdrive ratio. It is possible that the last gear on a manual transmission is simply top gear (1:1) and is not an overdrive ratio. Ditto for automatics. Not all automatic transmissions have an overdrive ratio.
With automatic transmissions the overdrive ratio is sometimes selected by a separate button on the shifter or a button or switch elsewhere in the vehicle. Typically three speed automatic transmissions (marked 1, 2, D) either do not have an overdrive ratio (“D” will be the 1:1 top gear) or overdrive is selected/deselected by some other button.
For those automatics that have overdrive and where no separate button is provided then the overdrive ratio is generally the final ratio selected by the transmission. Such transmissions usually have at least four selections (e.g., 1, 2, 3, D). In this case the “D” is the overdrive and to deselect overdrive and select top gear (1:1) simply move the selector to 3 (or in the case of 5 speed autos marked 1, 2, 3, 4, D then 4 will be top gear, and in the case of 6 speed autos marked 1, 2, 3, 4, 5, D then 5 will be top gear).
Importantly, when in doubt check the manual provided for your car.