The 74t 155/60R15 tires are load rated at 827 lbs.
The 77t 175/55R15 they replaced were load rated at 908 lbs.
A 74t is 2 lbs lighter than a 77t. 2 x 2 = 4
Narrow wheels are 4 lbs lighter than wider wheels. 2 x 4 = 8
(2 x 908) - (2 x 827) = 1816 - 1654 =
162 lbs lower load rating - (4 + 8) = 150 lbs lower load rating
Rated load capacity for the 2008 Pure with a full tank = 507 lbs
New load capacity = 507 - 150 = 357 lbs
Still okay for me, a passenger and light cargo - or me alone and a lot of stuff.
I usually hypermile alone with a half tank and about 20 lbs of stuff.
(4.35 gal x 7 lb/gal) + 357 = 387.45 lbs
I can pull an additional 850 lbs on the 150 lb trailer with a 30 lb tongue weight calculated at 2x for dynamics of 60 lbs
That gives me about 100 pounds of gear inside on long trips with the trailer, which is still within range. (I've traveled this way from California to Houston, Chicago and Laramie on separate trips.)
http://www.cleanmpg.com/forums/showthread.php?t=24446
So I expect the primary result will be significantly improved mileage with few drawbacks.