Equipping your classic car with an overdrive unit is a common type of vehicle customization—and for good reason. An overdrive unit allows a vehicle to drive at a sustained speed with a decreased amount of engine revolutions per minute. In doing so, there are numerous financial, comfort, and practical benefits that one can enjoy.
What Does Overdrive Unit Do?
The term “overdrive” refers to the highest gear that you can shift to in some automatic transmission vehicles. All vehicles on the road today feature at least one overdrive gear within the transmission. With today’s cars, the overdrive gear is automatically selected in many cases while you will manually need to select some sort of “Eco Mode” via a button or setting.
With older cars, a physical overdrive button or lever must be manually activated. When the overdrive button is set, the engine will run at extremely low revolutions-per-minute (RPM).
As a result, the output speed of the engine will be faster than its input speed. Anyone who loves high-speed cruise control and good fuel economy will benefit from the overdrive gear. The biggest downside is that you will not reach the maximum speed of a vehicle in overdrive. But unless you are on the Autobahn, you legally cannot do so anyway.
To learn about some of the key advantages of adding an overdrive to your vintage car, consult this guide.
Reduced Noise
One of the key benefits of an overdrive unit is that it will allow your vehicle to operate efficiently while outputting less noise. Because the engine is turning at a slower speed, it will produce less noise. As a result, you can enjoy a quieter, more comfortable ride in your classic car without sacrificing power or speed.
Less Mechanical Wear
In addition to reduced noise, adding an overdrive to your hot rod will also result in less mechanical wear. Since the engine of the vehicle will turn at a slower speed while in overdrive, both the engine and the gearbox will experience less wear and tear over time. Thus, you will be able to keep some of your vehicle’s most important components in better working condition for a longer period.
Better Fuel Consumption
Another advantage of adding an overdrive unit to your vintage car is improved fuel economy. Since the engine will turn at fewer RPMs at cruising speed, the addition of an overdrive unit will result in better fuel consumption for your vehicle. Thus, while an overdrive unit may require a bit of an initial investment, it has the potential to save you a significant amount of money on gas in the long run.
Why Do I Want an Overdrive Transmission?
It was not that long ago that overdrive transmissions were not standard equipment in every new vehicle on the road. The tried and true 4-speed manuals and 3-speed automatics were the only thing we knew, and they worked well, so why would anyone want to change that? During the late 1970s and early 1980s we saw a push to create more economical cars that produced lower emissions.
As most of us know, it had a lot, if not all, to do with the fuel shortages and government requiring the automakers to comply with EPA regulations on fuel consumption and emissions. Thus, the mainstream overdrive transmissions and pollution control systems were born into existence at the expense of horsepower. All those things combined to get the overdrive off on the wrong foot with most car enthusiasts.
Tremec Magnum 6-Speed
There can be some misunderstandings that the overdrive transmissions are not as strong or able to manage as much horsepower or torque as the older non-overdrive units. This can be true in certain situations, but most of the new transmissions will handle far more power than their older counterparts when comparing them at a stock-built level. True, the 1980’s were not a glorious decade for high powered performance vehicles and that is where most of this accepted truth comes from, and by all rights those early overdrives were not capable of the demands we place on transmissions today.
Consider how much more power a stock LS engine makes versus a stock Gen. 1 small block Chevy. Another common misconception is that the vehicle will not be as responsive due to the available gear ratios. This can be easily resolved by matching up the transmission’s gear ratios with the corresponding rear gear ratio and tire size to match the performance you are looking for. In nearly every situation we can configure a complete drivetrain package that will be much more responsive through all the gears. Even better is that you can keep that 4.11 posi for the hole shot and still be able to drive 75 MPH at 2,000 RPM. The third most common issue that comes up is the fitment aspect of the newer overdrives into older vehicles. This one is a legitimate concern, as there are many cars that were designed specifically around one or two transmission options that were available at the time. This can lead to the need for modification to the floor structure due to lack of room in the transmission tunnel. Obviously, this can be solved quickly with some cutting and forming of a new tunnel, but for some guys, the thought of chopping up an all original, rust-free car, can be pretty overwhelming.
With the advances in the technology available for the aftermarket it is possible to equip nearly any car with a modern overdrive. Regardless of if you favor 2 pedals or 3, there is a solution that will fit your needs. The newer electronically controlled automatics can be tuned to your driving style and preferences. No longer are you stuck with a “shift kit” to tune your transmission. The controllers available now will allow you to dial in the shift points exactly where you want them, and in every gear. Match that with a torque converter and transmission that has been built specifically for your car and you will wonder why you did not do it sooner. If you lean more towards the manual shifting side there are some great transmissions available to manage a wide variety of power levels, gear ratios, and shifting styles. I do not know about you, but when you look at a shift knob with a 6-speed pattern engraved into it, you cannot help but get excited. Besides, if you are going to have a manual transmission, you do so because you like to shift the gears, so why limit the fun to only 3 or 4 when you can have 6!
The bright side of the overdrive revolution in our eyes far outweighs any downside. With the ability to lower the engine’s RPMs at highway speeds you not only increase your fuel mileage, but also increase the life of your engine and the accessories that are run by the belt system. Also, with lower fuel consumption comes lower emissions. This may be the last thing on your mind while you are lowering that 632 C.I. big block in between the frame rails, but you can at least feel better about it. Over the life of a vehicle the savings can quickly add up to justify the cost of the transmission upgrade, and most cars that have been upgraded will command a higher price should you decide to sell the project. Another benefit that is often overlooked, but as important, is lower levels of noise and vibration inside the vehicle. Sure, you may be equipping your car with an exhaust system that violates most noise ordinances, and this may not be an issue, but there are many others who want to be able to enjoy conversations at a normal speaking volume or their favorite disco album through the new 6×9’s.
Add-On Overdrive Unit
If your older automatic transmission vehicle does not have an overdrive gear, it is possible to attach an add-on overdrive to your existing transmission to give it one. Truckers would call add-ons like these “splitters.”
The unit is just a smaller transmission with 2 speeds that gets bolted onto the back of your vehicle’s main transmission. With this add-on, your vehicle will be able to cruise at higher speeds with more fuel efficiency just like any other car with overdrive in it.
The term “2-speed” really means it will double the speed of your existing transmission. So, if you have a 3-speed transmission, then it will turn it into a 6-speed transmission. If you have a 4-speed transmission, then it will turn it into an 8-speed transmission.
But you do not really need to concern yourself with this because you will experience fuel efficiency regardless of what speed your current transmission is at.
Overdrive Unit Add-On Cost
If you are looking to purchase an overdrive add-on kit, you will find them priced anywhere from $800 to $3,000. There are instructions which come with the kit that can help you install it.
However, if you do not have enough mechanical skill to install it yourself, you should hire a professional. This can run you another $250 to $1,500 in labor & material costs depending upon any additional car modification costs. Additional costs can include modifications to your drive shaft assembly, exhaust system, and may even mean some adjustments to the body floor or frame rails/supports.
Finally
We have become accustomed to the cars of today that are equipped with more buttons than the Apollo 11 rocket and creature comforts to keep us spoiled, so it is common for us to want that translated into our project classic cars as well. We have realized that what we remembered from 40 years ago may be a little fuzzy and it is not so much how “nice” those cars were, but it is more about the feelings these cars inspire in us. We need those memories to come alive, but not at the expense of our sanity or safety. Adding an overdrive unit to your classic car can be a means for your old car to be a more pleasurable experience cruising the scenic roads.
It would be nice if you had some links as to where I could purchase this item.
Hello Frank Butch here from London Ont Canada–where in Canada would I find a location who sells one of these add ons that will fit a 1975 c4 transmission. –thanks
I would suggest national supply houses like Summit Racing or JEGS as a source for overdrive units.
Thanks