[Mike C]

Not sure where the 28 came from. 275 40 20 tire: Section width of your tire is 275mm which is the width between the sidewalls or approximated tread width. Aspect ratio is 40% of the section width. That is the sidewall height. Rim diameter is 20”.

275mm/25.4mm per inch= 10.83”
40% of 10.83”=4.33” and since you have sidewall at top & bottom multiply by 2.
4.33”x2=8.66” add to rim diameter to get close to actual height.
20”+8.66”=28.66” so figure a 28 and 1/2 inch tall tire.