At the outset of our Hot Rod Hauler project, one of our major goals was to increase fuel economy while maintaining the towing capacity of our ’73 Ford F-350. And while we will be building a new fuel-injected engine in the near future to help meet that goal, the first step in our attempt at getting better mileage was the installation of an E4OD overdrive automatic transmission from Gearstar Performance Transmissions

The truck’s original C6 had been rebuilt, equipped with a shift kit and heavy-duty components. The Ford’s shortcoming was the 3.78:1 rear gears, which meant the 460 spins three-grand plus at highway speeds. What’s desperately needed is another gear and for that, there are basically three choices--converting to a manual trans with overdrive, adding an auxiliary overdrive unit to the existing transmission, or installing a contemporary overdrive automatic.

We ruled out the first option because of the difficulty and expense, plus we like the convenience of an automatic. And even though adding an auxiliary overdrive would provide taller gearing, the issue then becomes the torque converter. The reason lockup converters were added to contemporary overdrive automatics is that the increased load from the higher effective gear ratio causes the converter to slip. That not only reduces the gains made by the higher gear ratio in Fourth but also produces transmission-cooking heat, which dramatically shortens its life expectancy. The cure for this was the introduction of the lockup torque converter. However, while slippage is eliminated, in many cases the torque capacity of the clutch pack in the converter is limited--as a result converter failures in trucks that are subjected to heavy loads and towing duty are a concern. Fortunately, Gearstar has addressed these issues by building their own torque converters. With additional heavy-duty clutch packs and premium internals, Gearstar’s triple-disc lockup converter has three times more capacity than stock.

Confident that there was a lockup converter that would do the job, the next step was to figure out which transmission to couple it to. Not surprisingly, there are a number of choices--Gearstar has adapters that allow the use of GM overdrives, the 700-R4, 4L60E and 4L80E are good candidates for an overdrive auto swap. Ford’s AOD and AODE are also viable choices. Of course, the best bet is to talk to the experts at Gearstar and let them tailor a transmission to your particular needs. In our case, we chose the E4OD.

For comparison, here are the gear ratios of the C6 and popular overdrive automatics:

Trans first second third fourth
C6 2.46 1.46 1.00
AOD/AODE 2.40 1.47 1.00 0.67
E4OD 2.71 1.54 1.00 0.71

700-R4/ 4L60E 3.06 1.63 1.00 0.70
4L80E/ 4L85E 2.4 1.48 1.00 0.75

Ford’s replacement for the C6 was the E4OD, which featured a fourth forward speed, electronic shift controls, and a lockup converter. Introduced in 1989, it was Ford’s first fully electronic transmission. The E4OD is a large transmission so it’s not a great candidate for all swaps; it’s 4-inches longer than a C6 and considerably larger in diameter with a big pan.

An unusual feature of the E4OD is the shifter pattern, which is P-R-N-OD-2-1. In the (D) range, the transmission shifts through all four gears. A lockout button disables Overdrive operation allowing the transmission to shift through the first three gears only. Selecting 2 allows the transmission to shift from First to Second and holds it there, while 1 holds the trans in First.