you'll get a small gearing increase, your speedo will be off a little, gas mileage may go down a little.
Depends on what size you're looking at but I went from my stock 225/60/17's to 275/40/18's and love the change (27.6" to 26.7"). I still have 2.73's out back though so every little bit helps. Its a 3.5% difference with those numbers so probably not worth it just based on that. Go a lot wider and it is
Actually, depending on the weight of the new setup, you could be getting better gas mileage. Dropping rotational mass increases fuel efficiency, which is usually a bit offset by change in gears, so it normally evens out unless you have an unusually high % reduction in weight.
However, it doesn't change the final gear by much. I reduced the diameter of my OE wheels by 2.5" and is the equivalent of going from 3.31 final drive to 3.49. Figured out by cross multiplication table of circumference.
A/B = X/Y
A = OE Tire Circumference
B = OE Final Drive Ratio
X = New Tire Circumference
Y = New Final Drive Ratio (unknown value)
Tire Circumference = pi x tire diameter
Takes a big reduction in diameter to get a little change in final drive ratio.