The hide selector was incorrect. I hid the blocks at page load and showed the selected value. I also changed the car div id's to make it easier to append the radio button value and create the proper id selector.
<div id="myRadioGroup">
2 Cars<input type="radio" name="cars" checked="checked" value="2" />
3 Cars<input type="radio" name="cars" value="3" />
<div id="car-2">
2 Cars
</div>
<div id="car-3">
3 Cars
</div>
</div>
<script type="text/javascript">
$(document).ready(function(){
$("div div").hide();
$("#car-2").show();
$("input[name$='cars']").click(function() {
var test = $(this).val();
$("div div").hide();
$("#car-"+test).show();
});
});
</script>