I currently have a Bluetooth FM transmitter for my car but it's microphone is really bad. Using the phone in the mount on speakerphone is far better, but on highways I can't hear it.
This is my current model: http://www.everbuying.net/product1104706.html
From my previous experiences with these devices, most of them seem to almost work only on "simplex" mode so that there's minimal echo.
It seems like when there's sound coming out of the car speakers, the microphone will be switched off and when you're talking into the microphone, the car speakers will switch off and it continuously switches between the the two. Otherwise the sound coming from your car speakers will go straight back into the microphone and cause an echo.
So when the other person is talking and you try to talk, they only get bits and pieces of what you're trying to say and vice versa.
Perhaps try testing it out by taking turns speaking and see if that is any better?
is ur car had aux in? if so then u can just get a aux cable with mic and button.
No AUX
You should try the Plantronics K100. I find that it works very well.
What sort of car?
Sometimes it's just better to upgrade the stereo to get proper bluetooth with an external mic, and have the added functionality of the upgraded stereo. Considering that on special, a new deck can be had for ~$100, it's a valuable investment rather than continually dealing with transmitters
is the microphone still bad when you have stopped moving and are parked and you are speaking down towards the microphone/Bluetooth FM transmitter?
possibly when you're using that type of bluetooth fm transmitter with microphone near the cigarette lighter then the microphone is down beneath you. whilst you're looking ahead (at the road) and speaking in a forward direction.
how handy are you with a soldering iron? possibility - remove microphone from current transmitter and reattach with longer wires and with the microphone sitting on the dash (discretely) so that you are speaking t more directly - and hence it will pick up your voice.
In my previous car I had one of these which I bought for ~$8 delivered form aliexpress. Attached to the sun visor. power cord ran discretely up around the windscreen. picked up my voice really well. (as I could talk directly towards it).
I used to have one of these