[jQuery] How to create a dynamic parameter in getJSON url?

[K Bouton]

How do I add a dynamic parameter in getJSON url?

[code]
function onSelectChange(){
  var whatChecked = $("input:checkbox:checked").val();
  $.getJSON("select.php", {whatChecked:"somevalue"}, function(json)
{
     ...stuff to do
     }
[/code]

I want the url that getJSON uses to be dynamic and look like:
select.php?whatChecked=somevalue
for example select.php?carType=compact


I can get the somevalue, no problem, but I can't figure out how to
make whatChecked be the value eg carType and not the word whatChecked!