18's are a bad idea if you actually want to drive the car, 17's ride like crap already and not much room for any sidewall at all if you go 18's.
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I just shipped a set of RPF1 in 16x8, 4x100, back to the company I bought them from and have some 15x8 6UL on the way instead, only bought the RPF1 due to not finding a 15x8 to fit over my DIY BBK with 11.75" rotors. Later I was able to get a file to print out the 6ul template and with a bit of tweaking on the BBK can make it work.
The 16x8 RPF1 would not of cleared the standard Fiesta WW 12.19" rotor BBK, it would of been very close but not quite fit unless a bit of modding done which most should not attempt.
The red car is a custom setup as is mine with front and rear BBK, 6/4 piston on my car.
I love the RPF1 but have had many sets and wanted something different and love the 15x9's I run on the car for track wheels, summer tire wheels and want 15's for some AS tires.