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Actual Tire Sizes


NAXJA Forum User
I know the listed size on tires and their actual sizes differ, but I measured mine today, and they're only 28.5" tall mounted up, which is more of a difference than I expected. They're labeled as 31x10.5R15's. Is this normal? I'm planning on 265/75R16's next, most likely BFG AT's. Does anybody else have information on their actual tire sizes compared to the labeled size for different brands?
How did you measure them? Was the weight of the vehilcle on them?
Here is how I would do it if you don't want to take the wheel off the truck. You may need an assistant. Get a bubble level and lay the end of it on top of the tread in about the middle and get the bubble centered. Now measure from the bottom edge of the level to the center of the wheel hub. Now double that and that should be good enough for government work.-------------Dave
xj92 --

I assume you measured from the ground to the top of the tire while it was on the vehicle, right?

Most 31x10.50s are listed by the manufacturers as being approximately 30.5" in diameter -- half an inch smaller than the nominal size. The same is true for 30x9.50s and for most of the "floatation" type tire sizes.

However, that diameter is for an unloaded tire. If you look at the charts on most of the tire company web sites, you'll usually also see a column called "static radius" -- that's the height from the ground to the axle centerline for a properly inflated tire loaded to some reference loading. This static radius is always less than half of the nominal diameter because under load the sidewalls squish down.
The weight of the vehicle was on them, and I measured them vertically in the center. The tires are pretty well worn also. I just tried it like you said Dave and they measured at 29" this time instead of 28.5. At least I didn't notice until now, because that's annoying.