Picking an Overdrive Automatic...
...And The Gearing To Go With It

Picking an Overdrive Automatic... ...And The Gearing To Go With It

Over the years, the trucks have certainly become more sophisticated. Take transmissions for example. At one time the only choices were “three-on-the-tree” or “four-on-the-floor” manual boxes. When automatics became more common, trucks could be equipped with transmissions like Ford’s Ford-O-Matic, GM’s Hydramatic, and Chrysler’s PowerFlite, but that was then and this is now. Today most of us want to put more modern transmission in our trucks, and that usually means an automatic transmission.


While automotive transmissions may have two, three, four, or more gears, the basic internal parts remain the same; there are just more of them in some than others. We’re not going to get into transmission rebuilding here but if you’re making a decision concerning the transmission in your truck, here are some parts you should be familiar with.


This is what connects the engine to the transmission. Not only does it allow the engine to keep running while the car is stationary, but it can also provide torque multiplication when accelerating from a standstill. For a simplistic explanation of how torque converter works, picture what would happen if two electric fans were facing each other and one was turned on. As the one under power began to turn and move air, the other on would also begin to turn. If you can visualize that, you’ve got the basic idea of how a torque converter works. In a torque converter, both fans are in a container: one is connected to the engine and the other to the transmission, and oil is used rather than air.

Even the most efficient low-stall torque converter will never completely stop slipping; this is why so many overdrive automatics now have lock-up converters. A hydraulically applied clutch inside the converter hooks the transmission directly to the engine for increased efficiency. While better fuel mileage is often considered to be the advantage of a lockup converter there is another, often overlooked, purpose. The higher ratio provided by an overdrive Fourth gear can put an additional load on a conventional converter, cause excessive slippage even in light-throttle, cruise conditions. That slippage creates heat, and heat’s the enemy of an automatic transmission. When used with an overdrive automatic, a lockup style torque converter will lower engine speed torque in cruise conditions and lengthen the transmission’s life.


Made up of three elements – a sun, ring, and planet pinion gears – they can provide forward or reverse rotation, a speed increase, constant speed, or a speed reduction.

Three things are necessary to make planetary gears operate: an input (power from the engine), an output (power going out), and a reactor (one of the elements is held stationary). The gear ratio and the direction of travel depend on which element is performing each function. Most transmissions have more than one planetary gear set to provide a variety of gear ratios.


Bands and clutches hold the reactors stationary. An automatic transmission goes into gear by holding one part of a planetary gear set stationary with a band or a clutch that is applied by hydraulic pressure. One reactor is released and another applied when the transmission shifts gears; the transmission is in Neutral if no reactor is applied. If you’ve ever been in a car and the transmission felt like it was slipping, that’s exactly what was happening – the band or clutch pack wasn’t holding the reactor stationary and it was slipping and not transferring full power. In an extreme case, the clutches, or bands, don’t hold at all, and one or more gears (and in some cases, every gear) stops working as a result.


The valve body is the hydraulic “brain” of the transmission; it controls the shifting of gears by controlling which reactor is applied and when. Some transmissions use hydraulic pressure to determine shift points, white many contemporary versions use computer-controlled electromechanical servos.


One of the most common alterations is increasing the stall speed of the converter. Simply put, stall speed is the rpm that the engine will reach with the transmission in gear, the brakes applied, and the throttle held wide open. The higher rpm simply allows the engine to produce more power, which will launch the truck harder from a standstill. The downside is that high stall speed converters will slip more in “normal” use. That can create excessive heat, which is what damages transmissions. Of course, the perfect solution is a lockup converter with increased stall speed – that’s the best of both worlds.


Most stock automatics are designed to be seamless; that s the shifts are smooth to the point of being hard to detect; this is done by timing the release and application of the various reactors. One may begin to apply as the other begins to release so there is a split second of overlap. While this results in a smooth shift, there is a certain amount of slippage that takes place in the process, which wears the friction surfaces.

Shift kits generally do two things: they change the timing of the release and application of the reactors, which results in a firmer, more noticeable shift and may also lengthen the life of the transmission. In addition, the hydraulic pressure to apply the reactors is often increased too, which means the clutches and bands have more holding power, thus increasing the torque capacity of the transmission.


All automatic transmissions use hydraulic pressure to apply friction components, either bands or clutches, to shift into each gear. Some like Chrysler’s A-500 and A-518; GM’s 200-4R, 700-R4, and 4L60; and Fords AOD use a mechanical device called a throttle pressure valve connected to the throttle linkage to establish that pressure.

As Phoenix Transmission explains it, the TV system provides the transmission with pressure increase proportionate to throttle opening. This is done so the transmission will shift smoothly when cruising slowly or gently accelerating and keep the engine at a practical and efficient rpm. When accelerating quickly, the engine is in a higher rpm range where it makes more power so the transmission needs increased pressure to apply the bands and clutches and keep them from slipping.


Later transmissions, such as Chryslers 40RE, 46RE; GMs 4L60E, 4l80E; and Ford’s AODE and E4OD use a computer and electrically operated valve body. In stock configuration these transmissions are controlled but the OEM computer, however stand-alone computers like the Compushift II from HGM Automotive Electronics are readily available. Mike Hoy of HGM describes these transmissions without a controller as being “brain dead”. However, with a properly designed management system they are unable to provide high performance for a variety of applications. Computers offer total control of shift points, shift quality, and converter lockup and eliminate throttle cable adjustments. In simple terms, these computers control the transmission based on the input they’re provided from a variety of sensors, such as:


As the name implies, it measures the speed of the driveshaft. This sensor can also supply a signal for an electronic speedometer.


Again, as the name implies this sensor detects the position, or opening of the throttle.


Ford’s use this sensor, or a variation of it, to tell the computer what gear the transmission is in. On GM transmissions this function is done internally.


Here is an overview of some of the more popular overdrive automatic transmissions:



            Introduced in 1989, these transmissions are essentially an updated 904 with an overdrive added to the rear. They were found behind 3.9L/239 ci V-6s and 5.2L/318 ci V-8s.


            The A-518 is an A-727 derivative with overdrive. Starting in the early 1990s, it was used in trucks, vans, and Jeep Grand Cherokees behind 5.9 Liter (360 cid) V-8s.

The cases of both these transmissions are very close in size to their predecessors, the 904 and 727 except for the length and bulk at the rear to house the overdrive assembly. Later versions of these transmissions are electronically controlled. The later versions of the A-500, the 40RE/42RE/44RE are electronically controlled as is the later version of the A-518, the 46RE.

Chrysler Transmission Ratios

First     Second        Third               Fourth


2.74     1.54                 1.0                   0.69


2.45     1.45                 1.0                   0.69



            Ford introduced the AOD in 1908; it combined elements of C-4 and the FMX and was used behind 5.0L/302 ci engines. In 1992 electronic controls were added and the AOD became the AODE


            This transmission is based on the C6 and is unique in that it has the overdrive on the input side of the transmission. E4OD’s have always been an electronically controlled transmission and require a “standalone” computer.

It should be pointed out that Ford overdrives have some unique features. Like most transmissions, Park, Reverse, and Neutral are standard shift positions, but unlike many other four-speed overdrive automatics, there are only three other shifter positions: D, which allows the transmission to shift through all four gears; 2, which locks the transmission in second gear only (the vehicle starts in Second and stays there); and 1, which locks the transmission in First and keeps it there. There is no Third gear position that is normally handled by a button that locks out Fourth. The Compushift II comes with what is called a “cancel switch” which prevents the transmission from shifting into Fourth (however the converter will still lock).

Ford Transmission Ratios

First     Second          Third               Fourth


2.40     1.47                 1.00                 0.67


2.71     1.54                 1.00                 0.71




This compact overdrive was used from 1981 to 1987 in everything from full size Cadillac’s to the Buick GNX. It is the same overall length and output spline as the short-tail TH350 and comes in the Chevrolet, Buick, Olds, and Pontiac bolt patterns.


This trans has a low first gear ratio of 3.06:1 and an overdrive ratio of 0.70:1. This trans was built with a Chevy-only bolt pattern, however Phoenix has adapters to put them behind Buick, Olds, Pontiac, and Cadillac engines.


Introduced in 1993, the 4L60E is basically a 700-R4 with electronic controls. Gone are the governor, TV cable and conventional valve body. This trans must use a computer to function.


The 4L80E was introduced in1991. This was the first fully computer-managed transmission from GM. These transmissions require a stand-alone computer and are capable of handling 1,000 ho when properly prepared.


GM Transmission Ratios

First     Second          Third               Fourth


2.74     1.57                 1.00                 0.67


3.06     1.62                 1.00                 0.70


2.48     1.48                 1.00                 0.75


When selecting a final gear ratio for a race car, the general idea is to take advantage of the engine’s power curve and make it go as fast as possible in the confines of the course it’s competing on. With a street driven classic truck, taking advantage of the power curve is a little harder to define because it depends on how it’s being used; but we will say the most common mistake is over-gearing and it’s usually the result of several factors. One is tire size.

As an example of how tire size impact gearing, the engine in a truck with a 1 to 1 high gear in the transmission, 3.70:1 rear gears and tires with an effective diameter of 25 inches (effective diameter accounts for some “squish” under a load and is usually tire diameter x 0.094, or in this case 23.537 inches) will turn 3,081 rpm at 60 mph; with 29” tires (27.26” effective diameter) engine speed drops to 2,656 rpm (these numbers may not be exact, but they’re close).

Now let’s take that same combination and add a 700-R4 overdrive automatic with a 0.7 high gear. With the 25” tires the engine will turn 1,859 rpm at 60 mph.

At a glance many truckers would assume that high gears and cruising at such low rpm would be advantageous. However, the fact is running an engine too slowly will reduce its efficiency. Throttle response will be poor, engine vacuum will often drop to the point that mileage suffers, and the slightest attempt to accelerate usually results in the transmission downshifting one or more gears, which is not only unnecessary, but also annoying.

After discussing this issue with Joe Abbin, of Roadrunner Engineering, he took some time to do some computer simulations and his research suggest that peak part throttle operating efficiency for many gas engines falls in the area of 2,00 o 2,500 rpm; which confirms why most factory overdrive transmissions keep the engine in this approximate rpm range at highway speeds. Keep in mind with higher performance engines there are variations. With a more aggressive cam and related parts the horsepower and torque peaks move up the rpm scare, and the same will hold true for part throttle numbers. Simply put, mild engines are happier at lower rpm than their high-performance counterparts and gearing should reflect that; as one engine builder put it, “you don’t plow with a race horse or race a plow horse.”

Although the ideal rearend gear ratio will vary with a number of factors, including with how fast you like to drive, here are some ballpark numbers (using 29” tires). With a mild engine and a 700-R4 transmission, a good choice would be rear gears in the area of 3.90 to 4.10:1; 60 mph would result in 1,960/2,286 rpm and 70 mph would be 2,060/2,403 rpm respectively. For a hotter engine with peaks slightly up the rpm scale 4.30:1 gears might be a better choice and would result in 2,161 rpm at 60mph and 2,521 rpm at 70 mph.



There are a variety of decisions to be made when choosing components for your truck, and the most challenging part of making many of them is ensuring that all the mechanical components are compatible. There are varieties of automatic overdrive transmissions available as well as a wide range of rearend gear ratios. Fortunately, there are a host of computer programs and free calculators on the Internet that will help you pick a transmission and a final gear ratio. However, if you want to do your own pencil pushing, here are some handy formulas:

Mph = (rpm x tire diameter) / (gear ration x 336)

Rpm= (mph x gear ratio x 336) / tire diameter

Gear ratio = (rpm x tire diameter) / (mph x 336)

Tire diameter = (mph x gear ratio x 336) / rpm

Do some number crunching before you start building; it’s much easier and less painful to use an eraser to change a simulation than the time and money it takes to change your truck.

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