Transmission Swaps and Overdrive for Classic and Pro-Touring Cars and Trucks
Published on 12/30/2023
An introduction to transmission swaps and upgrades available for classic cars and trucks. Read on if you are looking to add overdrive or switch from auto to manual (or vice versa) in your classic car or truck. This post will get you geared-up and pointed in the right direction!
Have you ever gotten onto the highway with your classic muscle car and wished you had an extra gear to shift into? There are many reasons to swap or upgrade the transmission in your classic car or truck. The transmission is one of the most determining factors in how the vehicle drives, accelerates and shifts (obviously). Many people will upgrade their rear differential gear ratio to a lower ratio for better off-the-line acceleration only to find that engine RPMs at highway speeds are now way too high. This is common and the best way to get the perfect balance of acceleration and highway cruise-ability is adding overdrive. What is overdrive? Simply put, overdrive a gear in a vehicle transmission that provides a gear ratio higher than that of direct drive (the usual top gear), so that the engine speed can be reduced at high road speeds. The result of overdrive can also lessen fuel consumption and allow higher overall speeds.
Options - So what are overdrive options for your transmission? There are many and it all depends what you have now or what you want to end up with - auto versus manual that is. Many folks prefer to swap out an automatic transmission for a manual - examples of this are a 3 speed GM Turbo-Hydramatic (TH350) being replaced with a Muncie M21 4-speed manual. Both of those transmissions are non-overdrive so a swap like that changes the shifting for the driver but does nothing for overdrive. Now take that same swap and use a Tremec 5 Speed and you will have overdrive. This is a common swap from either auto or 4-speed manual trans cars like Chevelles, Camaros, Firebirds, Impalas, Mustangs, Challengers, Cudas, etc. F100 and C10 pickup trucks as well.
Example of a Tremec 5 Speed Manual Transmission (Courtesy of Modern Driveline):
Automatic OD - There are many automatic swaps that include overdrive (OD) as well - using the same TH350 example as above, the most common way to swap and achieve overdrive is swapping in a 700R4 OD automatic transmission. This is basically a TH350 with the added 4th gear for OD. Be careful choosing a 700R4 for your swap if you have a lot of horsepower though - the cars the original 700R4 transmissions cam in barely made more than 250 HP - so a 500 HP engine could easily damage a stock 700. Many beefed up 700R4 models are available and parts to beef them up if you are so includes are also readily available.
Example of a 700R4 Overdrive Automatic Transmission:
Gear Vendors - Another interesting option for adding OD is an add-on product from a company called Gear Vendors. They offer a replacement tail shaft housing for many transmissions (manual or automatic) that adds an overdrive gear. The kit comes with the new tail-housing and a switch with wiring to you can engage the OD (or set it for auto OD mode) - the new housing is bigger and longer than your original SO make sure you have enough tunnel and exhaust space. You will also need to shorten your driveshaft to accommodate the length. These units have been around a long time, can handle a LOT of power and derived from the truck industry so they are tried and true. Check out
https://www.gearvendors.com/index.html for more info.
Example of a Gear Vendor Unit attached to the back of a TH400 Automatic Transmission:
Things to consider with any transmission swap - see below.
Gearing - Choosing the right gearing combo is critical to making decisions on what OD to go with. You must know your rear tire diameter, diff gear ratio and OD transmission choice and either do some serious math (or use a handy online calculator) to come up with the right combo. Check out this nice calculator which will help you by telling you what your engine RPM will be at certain speeds based on the tire size and gearing you enter. https://www.tremec.com/calculadora.php
Below is a screenshot example of what a 5 speed transmission can do for you at 70 MPH
Other Required Items for Swaps
There are a handful of items to consider when doing a transmission swap - see this brief list below to steer you in the right direction:
- Tunnel Clearance - is there enough physical room to fit the transmission you are considering?
- Exhaust Clearance - Is there enough room or will you need to modify your headers or exhaust pipe location to fit the new trans?
- Compatibility - If there a transmission model or bellhousing (if manual) that will fit your engine?
- Driveshaft - You will likely need to take precise measurements and have your driveshaft modified to fit (or replace it) with the new trans.
- Speedometer Drive - You will need to get the correct drive and driven gear for your speedometer cable to correct the speedo accuracy (unless it's electronic, those can usually be calibrated digitally)
- Shifter and Linkage - Make sure retrofit parts are available for your application or be prepared to fabricate
- Crossmember/Mount - Same goes for the trans mount - you are a decent fabricator, aren't you?
- Clutch Type (if manual) - Clutch options vary from cable, hydraulic and linkage/z-bar style - make sure you know what your options are.
Pro Tip: Contact a specialist for a complete kit, or parts from a swap kit. Companies that sell transmission swap parts and kits are out there and they are very knowledgeable about what you will need to get the job done. Research what you can but we highly recommend you contact one of these companies below to get assistance - you'll be glad you did!
List of Resources to Consider for Trans Swap Parts and Kits - Pro-Speed has worked closely with each of these vendors. We trust and recommend them.
Hopefully this article has helped you with your journey to a transmission swap or upgrading to overdrive (OD). There are obviously many other options and things to consider - feel free to contact us for advice (we do these swaps ALL THE TIME). This article was merely designed to help get you started and understand what your options are and what's available to help you swap from automatic to manual (or vice versa) or achieve overdrive.
PRO-SPEED builds, services, upgrades and restores the hot rods and classic cars of your dreams.
Please visit the PRO-SPEED Autoworks YouTube, Instagram & Facebook Page to learn more & check out some of the Restomod projects currently underway.
Contact us today about your restomod project - info@prospeedautoworks.com - 571.250.5908
