I keep the underside clean for the most part. Living in Pa its seasonal event to clean the bottom of the car...especially after winter for road salts and spring for road salts and mud.
Also...I would like to suggest info to anyone taping their wheels...Tape both the outer and the inner side of the wheel. I was accumulating quite a bit of dirt in the pockets...which I'm now contributing to the out of balance shakes I was getting near 70 and contributing partially to the excessive tire wear on the taped wheels.
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