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The generally accepted rule is that when switch sizes that the overall maximum diameter variance should not exceed 3% compared to OEM.
Switching from a 245/60R18 to a 255/55R20 would have a variance of 5% and is therefore not recommended.
The difference in sizes and parameters between the 245/60 R18 tire and 255/55 R20 tire. Detailed calculations and graphics. Calculate the difference in inches.
tire-calc.com
It would have adverse affects on the braking and suspension due to the extra weight as well as affecting the electrically programmed Traction Control and ABS.
As to whether or not the larger size would fit, I'm guessing perhaps yes. I've gone the other way with my previous 2017 Platinum which had the 20" wheels and I used a comparable size in 18" tires on the 18" steel wheels for winter driving.
You could get by with a 255/50R20 at a difference of 1.6%
Peter