Cody,
The X-91 is an updated version of the X-85, with a few new features. The X-91 has a 200 kHZ transducer that comes complete with temp sensor. That differs from the 192 kHZ transducer used on the X-85. This new unit also features Fast-Trak, which is like a real-time flasher which appears on the right side of the screen. This Fast-Trak allows you to see bait, fish, or structure before it is drawn on the screen like a traditional LCD unit.

The speed feature you can add to this unit is not another transducer, it is a seperate paddle wheel that must be mounted on the transom of the boat. Lowrance does not offer a transducer that includes both speed and temp. The temperature addition to the transducer of the X-91 is a new feature.

One last thing, the 200 kHZ transducer on the X-91 will allow you to run two units at the same time. Anyone with an older unit with a 192 kHZ transducer, can run that unit simultaneously with a new unit without any interference between the two. Since they work at different frequencies you can run the console unit at the same time as the bow unit, no matter what the depth and both will read independently. I know this for a fact, because my X-85, and my new LCX-15 MT (200 kHZ unit) never have interferred with one another. The LCX-15 also has the Fast-Trak feature, and I can tell you it is a useful tool for staying on bait, or trying to vertically catch fish you can see on the graph.