The SX was the 'sport" model & all sport oriented vehicles have tachometers, don't they?
I believe that the very last of the GX models also had the SX dash, probably to use up surplus supplies & to add another feature that might help them sell.

The SX had a lot of other sport features like 6" mags, uprated springs & shocks, along with special paint,[red, white, or blue only] SX decals, & special interior.
The engines were no diferent from all other models & i believe that all came with four speed manual trans.