2% is the usual "golden rule" allowed difference but always use caution when figuring and how it's going to be used. When changing tires it's the "rolling circumference" specification that is used for comparing sizes.how close to the spec. ratio of 1.691 must it be?
No two tire companies make tires the exact same size as another in these smaller type bias ply tires, so even if the size is listed as the same it might be off substantially. Carlisle Tires most generally are the closest match to what
is listed as size, the rest of the older major companies Firestone Goodyear Titan really close too. Off brand imports are the worst and I've seen ATV tires that were actually 2 inches shorter than the size molded on the tire.
Example: 22x11-8 which should be 22 inches tall and when you measure it mounted and aired up, 20 inches tall. WTH! is right!
Another factor that clouds things up some is the fact that all bias ply tires grow with age, so when looking for replacements it's best to use the factory specs on new tires when shopping.
ALWAYS COMPARE ROLLING CIRCUMFERENCE ON FOUR WHEEL DRIVE/MECHANICAL FRONT ASSIST TRACTOR TIRES
Al