I tried this last night with a similar filter: Honeywell-16-in-x-20-in-x-1-in-Ultimate-Allergen-Pleated-Air-FilterIf the TYC doesn't take care of the pollen for me, I may purchase a 3M Elite Allergen furnace filter and modify it to fit the Volt's 10-inch by 8-inch filter slot.
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The 16x20x1 size works great for this. Just cut it into 4 8x10 pieces and you've got 4 filters for $20. The fit is quite snug in the filter box. I was behind a few diesel trucks on the way to work this morning and didn't smell anything. Usually I have to roll a window down at least once during my commute to get that crap out of the cabin. So far so good!
I did do quite a bit of looking for a HEPA filter that might be trimmable to fit in the Volt. With all the labeling confusion around *real* HEPA filters and the fact that most of them are plastic framed (made for air filter appliances or vacuums) and much thicker than 1", I just went with the furnace filter. $5 per filter instead of $12-15 for the TYC and a known performance rating (FPR 10 / MERV 13).
