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Sorry for what may be a question with an obvious answer. I'm just wading back in RC and determined to finish some restorations started years ago, and also to run some older cars I have.

I'm trying to understand the measurement standards for wheels and tires.

I'm not following the differences between 1.9", 2" 2.2" wheels and the tires that are meant to match. Meaning; what measurement does 2.2" represent on a wheel? I assume something related to diameter, but for example I have a set of old Duratrax aluminum rims where the header card indicates 1.9" (part n# DTXC4023) but the measured diameter of the outer lip on the rim is 2.3".

There's no measurement I have found that reads 1.9..

Thanks in advance.

Should be 1.9" where the bead of the tire seats into the wheel

exactly. the flange or side of the wheel may be significantly wider than the tire bead ID. that is the guiding dimension.