I have an aftermarket portable system. The nice thing is you can move it from car to car ( I have 3 right now ) - and if I don't want it in the car with me, it can stay at home. And if it gets obsolete in a few years, I can upgrade it easily!

2001 Classic Red Mazda Miata (+/- 30MPG)
1998 Toyota Sienna (+/- 23MPG)