On flat terrain, I see my first minor dropout at the 70 dbu contour line, (nothing to panic about though) It is still listenable until you pass the 60 dbu line. That is when you should see major dropouts that will cause the station to be annoying. So, in other words, if you are within the 60 dbu you should be fine (on flat terrain). I am assuming that driving from Austin, you should loose the signal a little past Gonzalez. But again, be aware that I have experienced this on (almost) flat terrain. On a very flat terrain, I have been able to do a lot better than 60 dbu. Just something to consider because I know you are in the hill country.
A bit of advice, if you do buy a car stereo, save the receipt. ;D