I ran larger tires on my 2012 Leaf for a while. The chassis of the 2018 doesn't appear to be much different, so my experience might prove useful.
I originally had 235/50R17 on both the front and back. There were no fitment problems - no rubbing of the tires at all. I found that the ABS/traction control really did not like having that large of a tire on the front though. If I'd go around a ~40 mph bend in a road, it would start kicking in and light up a warning light. Disabling traction control was a short-term work-around.
Later I dropped the front tires to 215/50R17. That solved the ABS/traction control problem. The car had no issues with the front and back being different diameters. I assume that the car just didn't like having a tire with that much larger of a diameter on the front.
According to a tire size calculator
, these tires have the following diameters:
- 235/50R17: 26.3"
- 215/50R17: 25.5"
- 235/40R18: 25.4"
- 245/45R18: 26.7"
Given that, I think you'll be fine.