How dirty does your rear window get after being lowered, or with different tires?

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03 MP5 / 92 TSI AWD
When ever it rains on the stock wheels / tires, spray gets all over the back window. Out of boredom I tossed on a set of 17 x 7.5's with Yokohama A 520 225/40R17's, and now the back window doesn't get dirty at all while driving. This surprised me. I don't know if it's because the car is sitting slightly higher (than stock ride height), or if it's the tires not throwing the water up in the air.
I know this is sounds stupid, but dirty windows get annoying to me.

So, how is it with the car lower (with stock tires/wheels), and how is it with different tires/wheels?
 
my back end stays filthy. its a real pain in the ass too. that back end creates a vaccum and pulls all the s*** off the road on to the back bumper. i think this is natural for any wagon/van. notice the next mini van your behind. im lowered on eibachs and have aftermarket rims and tires but it still got just as dirty before. the biggest problem with being lowered is the front end, i pick up every bug/rock on the road
 
my cars ass is always dirty, especially after raining.
i wonder if its possible to make some sort of diffuser or something to go under the back to help keep that down.
 
I've seen the pics of the mid wings on the cars. I don't know if that helps or hurts it. Now I need to reconsider a few things.
 
yea,its a good excuse to wash it all the time but it would be nice if the back stayed cleaner between washes.
 
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