Adding to the "effective overall gear ratio" part of this discussion, it should also be noted that the bias ply tires (5.90, 6.00, and 6.40's) grew a couple of percent at highway speeds, while the radials stayed the same diameter (or revs per mile) almost regardless of speed.

It looks like the sedans went to a (roughly 2% taller) 6.00-15 by about 1965, and IIRC these were tubeless, as opposed to tube-type 5.90's on the earlier cars. At some point late in the production run, there were also 120 series cars shipped with bias-ply 165S15 tubeless tires. I cannot be certain, but I believe both my '68 122's came on that rubber. Unfortunately, I can't prove this, as the window stickers from both cars (I bought them both new) simply indicate "Low profile white sidewall tires". I remember the tires on those cars definitely being bias ply, but I can't recall if they were 6.00's or 165's.
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Gary L - 142E ITB race car, 73 1800ES
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