These tires kill mileage

that is very odd. premiers shouldn't cost more than a couple mpg. have you checked the pressures?
as far as determining if they are new, i would get a second opinion. where did you buy them?
we put premiers on my wife's hycam, they were black when ups dropped them off. but even an old tire doesn't turn brown, unless it's the arizona sun?

 

costco has a pretty good tire rep.

 

Look at the date code on the sidewall, it will tell you what week and year they were made.

Tire Tech Information - Determining the Age of a Tire

And a lot of Prius owners like to overinflate their tires to get better mileage. I run mine at 42 in front and 40 in back. Most modern tires have a max pressure of 44 psi.

 

Have you checked the tire pressure?

 

I agree, though my Costco Bridgestone's Ecopia are the first set of tires that have out performed the origional Goodyear integrity series. I made many mistakes recommend by Americas Tire, Pirelli, Les Schwabe, Toyo. Take them back, get what CostCo wants, your satisfaction and loyalty. After 400,000 miles they have earned my business by beating the others.


iPhone ?

 

If COSTCO values your membership, and they should, get them to replace the tires with a set of Ecopias.

 

I wonder if these are "directional" tires that have been mounted wrong......so that they are turning the wrong way ??

Anyhow.......I seriously doubt that ANY set of tires alone would make that much difference.
Over how long a period, and how many gas fillups have you made this current calculation ?

If that difference in mileage continues, you probably should be looking for a different cause.

Coincidences can be brutal sometimes.

 

maybe the 120 degree temperature, and battery protection mode.

 

New tires have more thread depth and are not perfectly rounded until broken-in by driving enough, so there should be some loss of mpg for new tires. Some new tires have pointy treads that eventually wear off.

I bought a new set of Pirelli P7's (215/45r17 91V) 2 months ago. My mpg instantly dropped from 55mpg to 40mpg for the first month until I managed to adequately wear them in. After two months, my mileage has risen back up to the 50's.

You tried the new tires for a couple of months? How much driving did you do?

I also noticed that disconnecting the battery terminal kills mileage too. The ECU has to re-learn the proper fuel ratio.

 

Those are listed as "Ultra High Performance" tires.........and as such you should expect a permanent drop in gas mileage, maybe on the order of 5 MPG .......and you should also expect a shorter life span.

I personally don't think that kind of tire is a wise choice for a Prius. But it is your money.

 

Princess, your tires are blooming.

PRO TIP: Tire 'Blooming' and Why Your Tires Turn Brown - The Adam's Detailing Library - Adams Forums

http://www.incrediblydetailed.com/how-to-get-rid-brown-tires-permanently/

 

I bought Pirelli P7 Cintario All Season Plus tires sized 215/45r17 91V. These are Grand Touring All Season tires with a 70,000 mile warranty. Refer to Tirerack:
Pirelli Cinturato P7 All Season Plus | 215/45R17

It was rated very high by consumer reports and tirerack that uses a rolling resistance technology, so I bought it. You are probably referring to the W and Y speed rated version which is high performance.
Pirelli Cinturato P7 (W- or Y-Speed Rated)

 

I looked at a place like Tire Rack and the P7 was listed in the category of: "Ultra High Performance".
Sorry if that was incomplete information.

BUT.....I'm not sure I would trust information like that from Tire Rack.
I'd want to see what Pirelli themselves had to say.

Sounds like you are satisfied so that's all that really counts.

 

You sure you weren't in the grocery department and they aren't cookies?

Seriously, I'd just go back, calmly and politely tell em you aren't happy. See what can be done.
Your tires shouldn't be brown, and that mileage hit is unacceptable.