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On 240s with an electronic speedometer (and all 7 and 9 series cars) the rear axle ratio has nothing to do with the type of speedometer, which is driven by a signal from a tone ring on the ring gear in the differential. The tone ring is the same in ALL axles, regardless of ratio. (it may be different on cars with ABS, or for other reasons, but that's beside the point in this discussion) The speed at which it rotates is determined solely by the diameter of the tires and the rate of progress of the vehicle. Because 240 sedans and wagons came with tires of different diameters, different speedometer heads were used to make up the discrepancy. The relevant numbers are 9800 and (I think) 10002, or 0.980 and 1.0002 for the older cars with cable drive, (which used different drive gears in the transmission to make up for the difference in axle ratios) though I forget which is which at the moment, sedan or wagon.
Otherwise, you are absolutely correct, that the diameter of the wheel is irrelevant, and the only thing that matters is the OD of the tire, and your suggestion of a tire manufacturer's web site is a good one for looking up such information.
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