O/D Off Meaning: How To Use O/D Off Effectively - Tireer

Contents

• What is Overdrive?
  • 1. Overdrive in modern vehicles
  • 2. First cars with O/D
  • 3. Implementing the O/D feature into the main gearbox
• How to Understand and Use the O/D Off Light Effectively?
  • 1. Driving on the Highway
  • 2. Driving in Urban Scenarios
  • 3. Driving on Hilly and Twisty Roads
  • 4. Towing a Trailer
• Do Modern Cars Still Have Overdrive Functions?
• What is Overdrive in a Manual Transmission Car?
• Do CVTs Have Overdrive?
• Final Words

What is Overdrive?

Although most people I know think the O/D function gives the vehicle extra power, the opposite is actually true. The word “overdrive” doesn’t come from driving over the capabilities of the car – it comes from drive ratios.

And, in mechanics, a transmission ratio that’s less than 1:1, i.e., a ratio that multiplies the engine speed rather than reducing it. In other words, any gear in your transmission that’s lower than the direct 1:1 ratio is considered overdrive.

1. Overdrive in modern vehicles

So, in a six-speed gearbox vehicle, in most cases, the 4th gear is the one with the direct ratio, while the 5th and 6th gears are considered overdrive. As a result, these gears push the engine to lower rpm, which has a positive effect on fuel economy and comfort on the highway.

Important disclaimer: when talking about the overall transmission ratio on any car, it’s important to note that every transmission has a final ratio. Thus, when you calculate the overall engine-to-wheel ratio, you also need to take that into account.

2. First cars with O/D

Okay, so why did older vehicles have an overdrive function? And how did it work in the past? Well, the first cars with an O/D function had a three-speed or a four-speed automatic transmission, where the highest gear was a direct drive (1:1).

This made the highest gears great for achieving top speed but not for fuel economy, as the engine was kept at very high rpms. Not to mention, a direct drive ratio strained the engine and shortened its lifespan if you drove your vehicle mostly on the highway, and let’s not forget about the noise.

To alleviate all those issues, automakers started putting an overdrive function in most vehicles in the 60s. The first such system actually changed the final ratio by employing another tiny gearbox connected to the main one.

Hence, after engaging the feature, it changed the ratio of all gears – not just the final third or fourth gear. As such, it was helpful on the highway, sure, but it significantly reduced the torque in all other gears and thus hampered acceleration.

In other words, you were only supposed to engage O/D when driving on the highway and disengage when driving in urban scenarios, on a steep incline, towing, or just overtaking.

3. Implementing the O/D feature into the main gearbox

To simplify things and allow front-wheel-drive vehicles to have overdrive gear, automakers started implementing the function into the gearbox itself. How did they do that? Well, by simply adding more overdrive gears; for example, a five-speed automatic or manual transmission has one O/D gear.

But since the gearbox already had overdrive built in, why was the lever/button inside the cabin still necessary? The problem was – gearboxes weren’t smart enough back then and hunted through gears when presented with higher load and/or speed.

Thus, automakers still left the driver with the decision to engage the feature depending on the circumstances. To help the driver, every car with an automatic transmission had an O/D off light on the dashboard, which told the driver when the overdrive feature was not engaged.

Put differently, with the O/D off light illuminated, your vehicle wouldn’t shift into overdrive gear and only shift to the direct drive gear (usually 4th). Why manufacturers didn’t put an O/D on light is beyond me, but that was the way things were back then.

Not that you understood what the O/D off light means, let’s see when and where you’ll need the overdrive feature engaged or disengaged.

How to Understand and Use the O/D Off Light Effectively?

1. Driving on the Highway