Yes, I misunderstood.

My mux is quite complicated. The hardware UART is off talking to the empeg. In software it has to emulate another two UARTs as well as take accelerometer measurements and some other house keeping, all running at once.

Multi-rate could be done but this code is already very complex. (Not that it's finished yet...) Not even sure I have the CPU power to pull all this off, it'll be tighter than I would like. Might end up with something other than a PIC.

There are other timing/latency issues that this ELM device should resolve in this case.