Q: I would like to have information about two-speed axles - particularly the Gear Venders overdrive in five-speed pickups.

A: The company you mentioned is a leading manufacturer of auxiliary transmissions for domestic car, truck and motor home applications. These bolt up behind your existing transmission and allow overdrive and gear-splitting functions.

An overdrive gear ratio allows the engine to spin more slowly at cruising speed, reducing fuel consumption, noise, and engine wear.

Gear splitting refers to the practice of shifting the auxiliary transmission in and out of overdrive as one shifts the main box, allowing additional useful gear ranges. Some gear combinations fill gaps better than others. This sounds like a lot of shifting, but is particularly great when towing, climbing hills or racing.

In addition to the hardware, Gear Vendors supplies an electronic interface that can coordinate overdrive functions with your existing vehicle powertrain controls.

Do you remember when some automatic transmissions had only two gears? There were only two vehicle speeds where the engine could be the most fuel-efficient or develop peak power, and every other vehicle speed resulted in a moderate to dreadful compromise. Transmissions these days have up to nine speeds, and some are continuously variable - they have infinite speeds, which allows the engine to be highly efficient under every possible driving condition.

Your five-speed transmission already contains an overdrive ratio, in addition to the other four, so you wouldn't see the same benefits as the owner of an older pickup with a more sturdy three-speed transmission that suffers from wider gaps between gears and no overdrive.

From what I've heard and researched, the Gear Vendors units are very sturdy and expensive (their word). I priced one for a hypothetical pickup and the cost is around $3,000 plus tax, installation and drive-shaft shortening.

From a fuel savings standpoint, in a typical low-mileage pickup with three or four gears, and $4 gas, the unit should pay for itself in perhaps 50,000 miles, then allow in-the-black fuel savings. If you work your truck hard, either heavily loaded or towing, the added gear ratios can improve performance and reduce driveline strain.

Then there's the fun factor - truck folks seem to love shifting gears. I can't leave the up/downshift and tow/haul buttons alone on my six-speed Allison.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.