In order to replace an old wheel barrow tire, you need to measure the tire. Proper measurement of the single-axle wheel requires three different measurements to be taken. Use a tape measure to get the proper sizing. You will take the inside diameter of the hub and the outside diameter of the tire itself. In addition to these obvious dimensions, you need to get the inside hub length to ensure a proper fit onto the wheel barrow's axle.
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Place one end of the tape measure at the edge of the tire. Open the tape measure directly across to the opposite side edge. Take the measurement of the tire, in inches.
Set one end of the tape measure at the outer edge of the inside hub -- where the axle goes into. Extend the tape to the direct opposite side of the hub circle and take the measurement, in inches.
Place the end of the tape measure at the inside edge of the axle hole and push it through the other side. Take the measurement where the tape hits the opposite side of the hole, in inches. This is the "bore" measurement.
Write down the measurement with the diameter first, hub size second and then a notation of the bore size. For example, 15.5/4.8--6" bore.