Reply To: High Speed Gears or Overdrive

[Gregory Long]

I have fit several Mitchell overdrive units in Pierce arrows.

I went with the Mitchell because it is a simple synchronized gearbox. No electic solenoids, no free wheeling, no issues.. Just either in direct: 1:1 ratio or overdrive, which for our applications is 26%.

So for my ’33 836 with the original worm drive differential, a 4.28:1 ratio, the new final drive ratio in Overdrive is 3.17:1. I chose the Mitchell OD because of the ’33, ’34, ’35 stewart warner power brake system It has to have a spinning driveshaft to have brakes. An OD system with freewheeling would leave the car without brakes. The simple Mitchell OD is either in direct or OD. so unless you intentionally select a postion between gears there will always be braking available.

My ’33 836 is a dream to drive with this OD, the engine easily pulls this gear ratio at 60-70 mph on all Michigan freeways, and even at lower speeds, 40-55 mph on secondary roads are no problem. The engine runs cooler and of course quieter.

The Mitchell family has used this gearbox in a 1 ton pickup truck for 20 years, towing cars. Their pickup truck is a gasoline big block V8 truck.

I looked at an open OD at the Mitchell shop in Colusa California. The gears are very robust, the shift forks heavy, the bearings large, and the box itself very heavy duty. The OD uses standard input and output yokes and U-joints.

The only drawbacks are that there is some gear noise in overdrive, in m cars, a distant whine.. The noise level seems to be lessening as i put on more miles.

The gearbox uses a mechanical shift, so either a cable or a shift lever with a push pull rod is needed to shift the OD

On my ’33 836, I made a simple selector located under the front seat. It is not intended to be shifted on the road. I leave the car in OD 95% of the time.

I have made NO attempt to do any noise insolation with the OD.

The mounting is quite easy for anyone capable of steel fabrication work.

I have installed three Mitchell OD units in Series 80 cars. The Series 80 cars have either 4.45:1 gears for most enclosed cars and open cars. In mountainous areas, Pierce equiped the cars with 4.88:1 gears. The Pierce literature states that there also was a 4.65:1 gearset available, But I have never seen one.

With the 26% OD, the new gear ratios are : 4.45:1 x .74 = 3.29:1 . And the mountain gears 4.88 x .74 = 3.61:1.

The resulting rpm drop makes for not only a higher max speed, but much less noise and engine wear and heat.

The Series 80 installation is a bit more involved, because the Torque Arm front mount needs to be moved or offset to the right. The parking brake and rear brake pull rod needs to be moved or shifted to the left.

I made a gearshift lever that shares the parking brake handle and slot in the floor boards, It works very well, and looks pretty much like a piece of original equipment.

I prefer an overdrive over high speed gears. With an OD, the original gears are still there, and just one shift to OD will give you an additional 10-20 mph available if you wish.

Greg Long