[crxgames]

These "badly tuned" cars could also have worn out valve seals/piston rings which can cause excess amounts of oil to burn during combustion, adding to the chemicals spewing out the exhaust.

[sliverstorm]

(Worn seals & rings leads to oil burning, not excess fuel consumption)

[crxgames]

Indeed. That is what I meant, but alas that is not what my brain wanted to type.