How to change CO₂ to a less polluting gas?
It makes no difference where you grab the $\ce{CO2}$.  Don't burden the car.

Plant mammoth sterile forests with gene-gineered high lignin wood for building.  Or, deep bury the trunks in the Sahara Desert of the Empty Quarter.  RuBisCO is the slowest, least efficient enzyme on Earth, the first carbon fixation step in photosynthesis.  Gene-gineer a faster, more efficient (e.g, not poisoned by oxygen) RuBisCO for the forests.

http://www.cea-jaea-collaboration.net/literature/cea/ICCAP%20Nice%202007%20NaCO2finalpaper.pdf

A reactor for your tailpipe.