If you don't need a self-inflating pad, you might consider the Exped MegaLite 12, Medium (73" x 20"), air mattress that is available from REI-Outlet right now. It is nearly five inches thick, has an R-value above 5, and can be inflated to whatever softness or hardness works best for your back. I have the Long Wide version (77" x 30") for car camping and it is excellent.

If you include the 'Schnozzle' bag that makes inflating it much easier, the whole setup weighs about 2 lb. 6 oz. That's hardly lightweight according to the usual practices of most of our members, but for car camping the thickness/comfort it offers makes it a very reasonable choice.