I'll have to take a peak at the under carriage on mine again. It wasn't "too" bad from what I remember when I last looked at it in the summer. I think it was mainly just surface rust. But I'll have to take pics of it just for record, haha.
During the winters here, at the self service car wash, I spend a good minute just spraying the undercarriage too. I washed my Mirage bi-weekly but it really depends how 'dirty' it gets from the slush and salt.
My filters looked sorta like that after i replaced mine after 50,000 or so miles too. I bought the cabin filter and air filter for about $22 shipped on amazon. Still works as they should!
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View my fuel log 2015 Mirage DE 1.2 manual: 43.4 mpg (US) ... 18.5 km/L ... 5.4 L/100 km ... 52.2 mpg (Imp)