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I own a 2005 Toyota Prius. Up until about 3 weeks ago, I was averaging 52-54 MPG/tank consistantly. I badly needed new tires so I got a set of 4 Goodyear Viva Fuel Max tires. Immediately after I had these installed, I noticed my MPG dramatically went down. I now cannot break 46 MPG/Tank.

I thought I would be getting better gas mileage with the fuel max tires. Obviously something with the tires is affecting my gas mileage. Nonetheless, I am PO'd. I did not get an alignment, but the car seems to move pretty straight when I let go of the wheel. What could be affecting my gas mileage? Tire pressure is good. I have not changed my routes or driving habits.

Any help? Please? Thanks!  

thats strange.. but not totally unheard of...   audi quattros were found to get better fuel economy than the 2 wheel drive versions..

i am going to say.. you need to search out a prius web form..  discuss it with them...

are the tires exactly the same size???  as they originals were new???   in revolutions per mile...   some tires are slightly taller or shorter even though they are the same specified size.. this throws off the gear ratio sometimes.. effects fuel mileage..

shorter tires let the engine run at a slightly higher speed than taller tires..  this can make a difference..  i would imagine the goodyears are slightly taller...  as that could get better fuel economy on most models..  or at least show up as better fuel economy as the tires spin less times per mile changing the odometer reading..

it could also just be the difference of the new tread thickness..  the car needing more engine input  to move with the taller new  tires.. or the battery pack is getting tired..    

these are all just thoughts..  but they are worth thinking about..  

When new tires are applied to a vehicle you should expect the fuel economy to drop.  

1)  Worn tires have better rolling resistance (RR) than new tires - all other things being equal.

2)  There is a technology triangle involving fuel economy, tread wear, and traction.  Improvements in one area require sacrifices in one of the other areas (or both!)  So tires with great wear properties are not going to have great fuel ecomony (or traction).

3)  OE tires typically are designed with fuel economy of paramount importance, where replacement market tires are designed with wear as most important.  Unfortunately, OE tires typically do not wear well and sometimes do not have great traction.   Goodyear's Fuel Max - as a replacement market design - moves the bar closer towards better RR, however, OE tires are typically better still for RR.

I cover this in more detail here:

http://www.barrystiretech.com/rrandfe.html

So what you are experiencing is to be expected.  The fuel economy will get better as the tire wear, but may not ever reach the levels you experienced before.  

here is the link enabled...

http://www.barrystiretech.com/rrandfe.html