My mileage penalty is ~ 9% with new tires

What were the stock tires? Some of it might be just due to them being brand new, hard to say. How many miles have you got on the new tires now; a tank or two?

 

Pretty good article over on TireRack, but even changing over to the same tire the new tire will always be worse due to various factors but revs/mile is different due to wear. In your case there could be other differences. So it should get better. The Defenders are thought to be cost-effective due to longer life with small MPG loss. I have Defenders on my minivan and they are noisier than I might have expected, but I would not read that over to a Prius.

 

could improve with age, and the diameter of the new tire with full rubber might affect the odometer.

i just put on michelin energy savers. whisper quiet and no hit to mileage.

on the plus side, the defenders might last long enough to make up for the gaso cost.

 

Do they have the Green X label? I've got a stack of four in the garage for our son's Civic, going to put them on in a week or two. IIRC they do have the Green X label, but my set in under wraps and a few things piled on top.

That label seems to come and go: our Prius has Michelin Pilot hx MXM4, made in USA Feb 2009, and they have the Green X label. But now, TireRack says our size (215/45R17, and virtually all the sizes) are NOT Green X. Frustrating and confusing when they keep shifting the product spec's.

Being Green X doesn't necessarily prove anything, but it at least intimates it's among Michelin's LRR set.

 

Costco tried to sell me on the Defenders the other day when I inquired about the Bridgestone EP422 plus. I told him that the consensus on Prius Chat was that the Defenders deliver less mpg. He was surprised. I forgot to tell him that the Energy Saver were Michelins LRR tire.


iPhone ? Pro

 

I initially had the Goodyear Assurant fuel max and didn't like them, had to fight the wander on grooves in the road and lot of road noise. I then went with the Defender because of the 90,000 mile warranty, they only made it to 60,000 miles and I had them replaced with Continental Control Contact at Discount Tire. They adjusted the price to reflect the difference in the actual miles vs the warranty on the Defenders. I am real pleased with the wear on the Continentals, quieter than the Defenders and a solid 3 mpg better. I now have 32,000 miles on the Continentals and they are between 7 and 8/32 tread left. If I see I'm keeping the car when these are worn out, I will definitely go with the Continentals again and I've had excellent service at Discount Tire in Lou, Ky and the one in Fla. where I bought the Defenders.

 

I had the same drop in my 05 with the Defenders. Thankfully Michelin had a 30 day return policy at the time and I didn't hesitate to return them.