replaced bearing gear and wheel.
you measure the inside of the tube, not the outside
Cut you off apiece and take it with you and match it up at the store.
Gears (all kinds) tend to 'strip' when they become mis-aligned, and one of them is harder (material wise) than the other. Check the alignment of the drive axle, or perhaps there's wear on the shaft (diameter). This could apply to gear or wheel. Wobble in either is likely, and needs the 'fix'. Re-shim either that or the wheel to reduce sloppy tolerances to give a tighter mesh between them. There are shims available of varying thickness, but perhaps a coarse flat washer would move the gear (or wheel) sufficiently to re-engage them tightly. Worn shaft (wheel or gear) would take more finesse and perhaps need to be replaced. A correct sleeve over the shaft might bring it within tolerances, but that would get expensive if machining is needed. If you have an extensive drill bit assortment, a stub of copper tubing might be jury-rigged to provide the needed dimensions. Good luck.
