Always remember that the profile of the tire not the inches is where the difference lies. If you use a 20" wheel and an 18" wheel, the tire profile is where the differenc lies. The heigth of the tire. 45 profile on an 18" wheel and a 35 profile on a 20" wheel. All things being the same 45 to 45 then the 20" wheel would be taller. 315 R 35 to 315 R 45.