As Tim Winters points out, most auto accidents are due to driving errors, so you would be comparing perfect piloting with imperfect driving.If we were all perfect pilots and never made an error, I would guess that flying GA would be far safer than driving? By how much?
The following is an older Australian paper that examines cross-modal accident rates:
http://www.atsb.gov.au/media/36229/cross_modal_safety_comparisons.pdf
For GA to have the same rate as ground vehicles, the GA rate to ground rate would need to be less than 1/(1-.71) = 3.45. As you can see in that study, the estimated rates vary, depending on the normalization and methodology used. The lowest rate relative to autos appears to be 4.15.
So even with perfect piloting the accidents caused by mechanical problems would appear to keep GA more dangerous than imperfect road drivers.