I have read this discussion many times, saying one size is an inch larger than the other, but when I research the tires, at least with the tires made by Goodyear, they are the same diameter, 30.7 inches.
The Marathon:
http://www.tirerack.com/tires/tires....omCompare1=yes
The G614:
http://www.trailertiresandwheels.com...l-trailer-tire