I've noticed that every time my MPG drops to the 40-42 range, I find that I'm also driving on tires that are almost flat.

I refilled the tires and the MPG jumped back up to 46.5 (normal for my terrain, driving style, and available gasoline formula).

So... Is it *normal* for the tires on an HCH to go flat every 3-4 weeks, or do I have a bum set of tires?

What is the temp where you are? lower temps will deflate tires faster. And what do you consider flat? 10-15 psi or 20. I try to keep my tires around 37-40psi and in the colder months i am adjusting them every 2 days or so, but ive never had them go , "almost flat". if they are doing that you might want to get them remounted to seat the bead better. hope this helps. Dean