20s use a 50 aspect ratio while the 18s use 60 aspect ratio. Is a 50 considered low profile? Depends if you think 50s are enough or would you prefer 35s? OR are you coming in from 65s?
The overall diameter is still the same. 30.5 inches diameter. No difference in mileage. HOWEVER, finding replacement tires for 18s are much easier and plentiful than 20s, unless you go up one size to 275/55/20. The original sixe 265/50/20 limits to a few all seasons and only ONE all terrain, the Nitto TGs, in factory size. IF you go up one size to 275/55/20, then you will need to consider changing out the spare to 255/70/18 to retain the same diameter as the 275/55/20.
Lots of threads on this dilemma and lots of suggestions too. I'd rather have had the 18s to save on having to swap sizes---and you can get 18s if you twist the salesman's arm and demand the 18 or else a deal breaker. 18s do come nicely polished too.
My wife and I both drive 2012 hemis V8. I have actual hand calculated and EVIC 20.0-20.1 on her GC on the hwy at 65-70, flat, no headwinds, sea level, with some stop and go. City is about 15.5-16 mpg. Going to the larger size tires on mine and in A/T tread lowers mileage about 1 mpg.