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Regular wheels/tires on a typical passenger car sized like so:

Tire size: 235/55R19 235mm -width (235*0.55)mm sidewall height 19" - Radius

But the spare wheels for the same model and certain trailer wheels have dimensions in form of 19x4 for example. I guess 19 is the diameter of the rim. but what 4 means? is this the width or the sidewall height?

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On a trailer you may find something like 4.8-12. These are cross ply tires, and the two numbers represent the width of the tire, and the diameter of the rim it fits on. They're used on trailers mainly because they're cheap to make. You may also find cross-ply tires on off-road vehicles, where they're used because the continuous ply running from bead to bead makes the sidewalls stronger than radials.

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When you see something like 34x12 on a tire(bigger number first), and you'll mostly see this kind of designation on off-road oriented tires, this is really a radial tire, and is fully designated as maybe 34x12R17 (R denoting radial). The 34 is the overall diameter of the tire, 12 its width, and 17 the diameter of the rim it fits. This same tire could also be described as something like 305/70R17, where 305 is the width (now its millimeters rather than inches) and 70 is the profile - the ratio of the height if the tire above the rim to its width. The rim size is still in inches, metric rims really didn't catch on.

Image from here

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