Im looking to get new rims for my 2012 + model because it seems everyone here in San Diego Has the same rims. I dont know if i want to go to a larger rim. If i do, how much mpg do you think i would expect to lose on average?
You would only lose MPG if the total diameter of the tire increases, not if you increase the rim size and keep the outside tire diameter size the same as factory. So, if you get a larger rim, you need a tire with a smaller sidewall.
Factory wheels are usually heavy. If you pick the right aftermarket wheels, you can reduce the weight of the rolling mass. This should actually help fuel economy.
But like was said, larger rim, smaller side wall... keep the same tire circumference.
My 2012 Soul+ came with 205/55R16 on a 16" wheel
The Soul! comes with 235/45R18. You could buy 18" aftermarket wheels and run those tires.
Using a tire calculator (Tire size calculator) you can find similar size tires to your stock tire size. Issue with some tire calculators is that they allow you to type in a size that doesn't exist, even though it's a perfect match.
I replaced the OEM 16's with a 17" wheel and a 215/50 17 and so far I've lost about 1.5 miles per gallon. I need to check with the dealer to see if I need the car re-calibrated for the different tire/wheel size.