what are the differences over the years? Gear ratio?
Diff between TR and TRS?

thanks

All TRS drives are 1.5:1.

All TRS from 1977-1993 are the same with the exception of the input yoke. (MercTrans used a female spline)

TR drives are 2:1, much deeper lower unit, can swing a 20" prop. Mainly found on cruisers and houseboats.

TRS stands for transmission sport I heard. Like Biggus said, the TR's were deeper cruiser type drives.

Also the TR and TRS share the same upper unit.