Re: 5518 tires
I made a mistake on the Honda OEM turf tires, the ratio should have been 1.2 and not 2.0.
I cut grass with it some times but I use it more for show. As far as the ratio goes the tires I have on the tractor now gives me a ratio of 1.1940 which is good. There are to many variables to acutely calculate the ratio. Not everybody inflates the tires to the proper inflation, air gauges will vary between each other, consider the tire wear but the one that gets me is Honda itself. With turf tires the ratio is 1.2:1 under perfect conditions and the for the ag tire it is 1.26:1 (Honda did not change any gear in the transmission to achieve this, just changed tires).
Your calculations are based on the tire being 20" tall for the front and 24" tall for the back when in fact they are 19.8" for the front and 24.1" for the back which gives a ratio of 1.212. It is easy to find out how tall the tires are by going to the manufacturer web site such as Carlisle which says that the tires on my tractor are 20.1" for the front and 24" for the rear which gives a ratio of 1.1940:1.