Most diesel truck owners pride themselves on the fact that their vehicle can tow and haul anything and everything they want yet still achieve 15 to 20 mpg on the highway. While that's true, we're here to tell you there's even more fuel efficiency to be had out of the average diesel-much more. The ability to tow and haul heavy amounts of weight means that most diesel trucks are geared for all-around performance, not for fuel economy. In most cases, even in the higher overdrive gears, at 65 to 70 mph a diesel will spin about 2,000 rpm, instead of the 1,600 to 1,800 rpm that is needed for maximum fuel economy. Manual transmission and steeply geared trucks have it the worst. For example, a Ford F-450 with 4.88 gears and 31-inch-tall tires will spin almost 2,500 rpm at 65 mph-way out of the engine's peak fuel efficiency range.
| 1989 Dodge D250 Overdrive overdrive Gear
But there is a solution. It's a Gear Vendors planetary overdrive unit designed to bolt right on to the back of the vehicle's existing transmission or transfer case, which gives a 22-percent overdrive ratio in any gear. Using a 4.88-geared Ford F-450 for an example, this means that engine speed in double overdrive is reduced to a mere 1,900 rpm by the activation of the Gear Vendors overdrive-right in the meat of the engine's torque curve. Since the unit can be activated in any gear, it can be used to split gears. A Gear Vendors-equipped transmission has the ability to shift through First, First-overdrive, Second, Second-overdrive, and so on. This is very useful when towing or climbing grades, where the engine wants to be in a very narrow rpm range to keep power up and EGT down. It should be noted that you should never, ever tow or climb hills in double overdrive, as the combination of very high torque, low output shaft speed, and engine harmonics will destroy the overdrive unit.
| Before we get started, a little history lesson: Gear Vendors has been in the business of making overdrives for 30 years, so it's safe to say it knows what it's doing. It is a true manufacturer and orders its raw materials from outside sources, preferring to do almost all the machining and assembly in-house for the best quality control possible.
Theoretical articles are good, but having the cold, hard facts is even better, so we took our '89 Dodge D250 over to Gear Vendors' headquarters in El Cajon, California, to have a unit installed. Rick Johnson, the owner of Gear Vendors, told us we would see a 20-percent fuel economy improvement with our old Dodge, since it only had a three-speed transmission. As it turned out, Rick was almost spot-on. During our pre-install testing, our truck averaged 19.6 mpg at about 65 mph, and after the Gear Vendors overdrive was installed, it was able to achieve 24.2 mpg during 400 miles of testing. In addition to the mileage gain, we're now able to easily exceed 100 mph in our '89 Dodge, which means it's just about time to hit the dragstrip!
| Shown here are all the parts we needed for our Dodge's TF 727 transmission conversion. Front and center is the actual Gear Vendors unit (A), splined adapter to fit the unit to the transmission output shaft (B), replacement tailshaft housing that bolts to the transmission (C), gasket (D), and speedometer cable (E).
