I thought a larger diameter would be better for mpg by raising the
effective gear ratio.  Engine turns fewer revolutions/mile.  'course
if you are figuring your mpg using the stock odo, smaller diameter
tires would make it look like you had better mpg because the actual
distance would be fewer miles than the odo says.   You would also be
travelling slower than indicated on the speedo and saving some fuel
there too.  I think that makes sense.
Edward

At 01:41 PM 9/9/2009, Kim Brennan wrote:

>Larger DIAMETER in your tires will have a more significant impact on
>mileage (adversely) as it will take more energy to spin your tires
>(even though you are traveling further on each revolution, your net
>will be an overall drop in fuel economy.)