I'm very sorry to have to post this.
I've been a satisfied user of Mastercraft Glacier Grip snowtires on my Prius for three winters.
But I just got burned by Cooper, and their "new" Glacier Grips are not usable for Prius.
I always get 51 to 53mpg** on my summer tires (Goodyear Integrity).
When I switch to my "winter" tires (Mastercraft Glacier Grip) I used to get around 47-48mpg.
(not bad for aggressive snow tires).
But a month ago, one of my Glacier Grips got damaged by a pothole.
So I got (what I thought) were identical, new Mastercraft Glacier Grip tires.

39mpg !!!!!!!!!! WTF????????

So, I put my summer tires back on early, and instantly got my 51mpg back.
I immediately contacted the dealer, then the distributor, and finally Cooper Customer Service.
Cooper Tire Company did their best to convince me that it was all my fault:
-I was just imagining it, and my new tires got the same mileage as the old tires
-It was my fault for overinflating the tires by 1psi.
-It was my fault for underinflating the tires by 1psi.
-New tires cannot be expected to get the same mileage as worn out, bald tires.
-I was not making a fair comparison of mileages (** see notes below).
-I did not know what I was talking about.
-My car must obviously be defective.

Anyway, after weeks and repeated conversations the truth finally came out:
Cooper is putting ABRASIVE COMPOUNDS in their snow tires to make the tire surface rougher,
and improve traction.

Apparently, at the expense of 25% of my fuel mileage.
Bottom line: Cooper offers NO apology, refund, exchange, or credit for tires that produce poor mileage. If your Cooper tires cause your Prius to get below normal mileage, TOUGH!
So now, I have a brand new set of 185-65-15 snow tires. And if I put them on my Prius, it will cost
me over $500 in extra fuel cost over the life of the tires.
Anybody need a set of tire-swings for the tree in their back yard?

Caveat Emptor

** NOTE: All of the above mileages are quoted for the round trip Buffalo, NY to Albany and return.
on the I-90, clean, dry pavement, in cruise control at 65 to 68MPH.

I make this trip at least 15 times a year, and my mileage is very consistent.