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I currently have a T5 in my 57 w/ 350 engine. First gear ratio is 4:1, and is too much like a granny gear. I have been keeping my eye out for T5 with 3:1 first, but for my type of driving a 3 sp OD would be just as good. I recently missed an opportunity to bid on an MY6 on ebay, and believe that would be the absolutely ideal trans for me. HOWEVER, I have long been aware of the 3 sp ODs used in Chevys of the 1950s, and they are often offered for sale. My concern is about how they function, and how reliable they are. For example, is the OD useable in all three gears? I have seen reference to a governor in these units. What is that for? I have also heard that with the OD engaged, the vehicle free-wheels when decelerating. Is that so?

Would appreciate any insight that should influence my decision to buy or not.

The governor is there to prevent engaging the OD below about 30 MPH, depending on rear end ratio. It's possible to run fast enough in 2nd. gear to use the OD, but it's mainly useful as a 4th. speed after you shift the normal 3-speed pattern. If you use it as intended instead of bypassing all the automatic functions, all that's necessary to engage the OD gear is to let off the gas in 3rd. gear, listen for a click, and reapply power. When road speed drops below governor speed and you let off the gas, it downshifts back to direct drive automatically. Ditto on a full stop. Going to full throttle and tripping a kickdown switch in the throttle linkage downshifts out of OD for passing power or hill climbing. The car will "free-wheel" at times, so get into the habit of using Reverse as a parking gear. It can roll away even when parked in one of the forward gears. OD is locked our in Reverse. There's also a cable-operated lockout for preventing OD engagement while compression braking on a downgrade, trailer towing, etc.

Most people either don't understand how to make one work right, or are too lazy to set it up properly, so they install a toggle switch and bypass the governor and the automatic downshift. Done right, it's almost idiot-proof.
Jerry

I have had BW overdrives for 45 years. The T86 will handle a 350. The T85 and T89 will handle big engines. Olds used them behind their big blocks. Overdrive is usable in all 3 gears, but it is not practical. Overdrives do become free wheeling at under 28 mph which is awkward. However, the overdrive can be locked out with a cable operated lever. The governor is what senses the 28 miles per hour. I drive mine around with the overdrive locked out around town and use the overdrive on the highway. I like 3.89 and 4.11 rears. It gives you plenty of acceleration in town. A .70 overdrive gives me an effective rear ratio of 2.73 and 2.88 which gives excellent fuel economy. Back when I ran on quarters marked off on country roads, I used 2nd over which gave me just the right gear to finish the quarter. Overdrives were never very popular with GM big they were big with Ford, and there is much parts interchangeability. Few people use them because they don't know how to drive them to get the most out of them. My first overdrive was in a 56 Ford I bought over 45 years ago. I have always had overdrives, and I am strongly thinking of putting one behind a 302 GMC and use the factory 56 GMC 3.90 rear. An overdrive is not free wheeling at speeds over 28 mph.

What they said, but parts are very hard to find as are the trans units themselves. They are very good units tho and not complicated to work with.
dg