Since you are on the freeway it will probably be worth it.
stock is 165/70/13 weight 14lbs 22.2 inches in dia. 949 revolutions
...........175/70/13 weight 15lbs 22.7 inches in dia. 927 "
...........175/70/14 weight 15lbs 23.7 inches in dia. 886 "
the 14" tire is 7% larger in diameter. above 5% will show mpg improvement
Make sure the larger rim is not too much heavier
Check to see if there is a shock or strut tower that would limit how tall you can go.
For example:
I have the tallest tire that fits up to the bottom of the front stut tower
