FM transmitters are not that good, they are a compromise as almost no cars had a cassette anymore, and jonly the last years cars have had an AUX / iPod input.
I have to say that the results are depending on a lot of factors, so if you are keen on having better sound try to install an AUX input or use the simple cassette I KNOW that it is sooooo Yesterday....but is bad sound the present Future ?.
maybe FM transmitters will surprise me now, but have no great hopes for that.
But as I say I am very picky, I use at least 192kbs when ripping, but still sound can be of dubious quality... but very handy