Michelin X Radials and Poor Gas Mileage

I purchased the Michelin X Radials from Costco a short time ago and have taken a serious MPG hit.

I have had the tires for about 4 weeks and 4 tanks of gas for about 1200-1300 miles. I was averaging about 49 mpg and now I am averaging about 42. I'm surprised that they caused that large of a MPG change. I haven't noticed any improvement or "break in" since I've had them. I'm running them at 40/38 psi. I thought these had relatively low rolling resistance.

Should I just suck it up and be patient or try to return them to Costco for something else? Is there any way that Costco messed something else up that would decrease mileage so drastically?

What exactly does "breaking in" do to the tires?

 

<div class='quotetop'>QUOTE(Sufferin' Prius Envy @ Nov 5 2007, 04:42 PM) [snapback]535046[/snapback]</div>

I'm in Los Angeles and have a 2004 Prius, so I know what to expect. Since changing my tires I have mostly driven my usual route to and from work (almost all highway). The weather has been between 60-75 degrees. Even when it's cold here (50 degrees at worst) I don't notice much difference.

When I got my new tires, I had half a tank of gas, so I didn't notice the change until a few days later after I had filled up again. But I haven't made any other changes to the car.

 

Assuming the tires themselves aren't causing my mileage problem, is there anything else the Costco tire people could have done to cause the problem? The prius drives exactly as it did before the tires. The only difference I notice is that the ride is slightly less bumpy with the tires inflated to the same PSI as the OEM tires.

Could the problem be a combination of two problems? Maybe tire break in and a change in fuel used by the stations (winter grade)?

 

<div class='quotetop'>QUOTE(saechaka @ Nov 5 2007, 07:19 PM) [snapback]535132[/snapback]</div>

I have p185 65r15. I believe that is the same size as the OEM's (Goodyear Integrity I think).

 

<div class='quotetop'>QUOTE(H2OSkier @ Nov 6 2007, 12:39 PM) [snapback]535536[/snapback]</div>

Thanks Ken. I'll wait it out and see if things improve. I'm almost ready to refill the tank, so I'll see if things improved. What do you keep your tire pressure at?

 

<div class='quotetop'>QUOTE(evil_attorney @ Nov 6 2007, 12:46 PM) [snapback]535540[/snapback]</div>

Ok, it's been just under 3k miles or so on the new tires and the mileage has slowly improved. It is now almost to where it was with the OEMs. I guess the break in period for these was longer and more dramatic for me for some reason. Just happy it's almost back to normal with gas prices coming back up.