Accelerometers and/or gyros would be my choice. But I guess that would need substantial software for the empeg, wouldn't it?

Another option would be speed input - I miss the speed dependent volume adjust I had in my miata before.

That reminds me: My all time favorite would be an obdII input for the empeg! Of course the software would need to be configureable to display any of the variables read out via the interface. I have the right connectors, since I use them to read out obdII data onto my laptop. But it would be the epitomy of luxury (in my eyes), to use the empeg to read out obdII data. BTW, that could also be used for speed dependent volume adjust