Wear a helmet. It's better than the alternative...

[Ms. Toad]

258. Because the helmet is the sole protection on bicycles

And a secondary element in cars which, by and large, have two features to minimize the risk of a head injury (seat belts and air bags). It is far more important to provide some protection where there is none than it is to add marginal protection where there is already significant protection.

I also find it a bit ridiculous that you are even apparently seriously asking this question - because a smidgen of thought would tell you that a helmet also has a much more significant negative impact on safety in a car than on a bicycle. Likely far more than any added safety you would gain from wearing it. In a car you are surrounded by a box much larger than your self with various bars impairing your vision in all directions - the helmet adds moderately to that impairment. But even more significantly, you do not have the same freedom of movement in a car you have on a bike. Most cars have headrests which the helmet would bang into even when driving routinely on a smooth road. In addition, when faced with a need to quickly check to change lanes (or to avoid an accident), you are highly likely to bang the bill of the helmet into the window potentially causing you to lose control of the vehicle depending on how startled you are and how significantly the impact effected your ability to see what you were looking for and recover.