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I'm going to clarify a couple of items. Not meant to be argumentative, just a little different look at it. :-)
First - if you're talking 120 series in the USA, the ONLY car that had 165R15 radial tires as OE was the 123GT, in 1967. All other US spec 120 series sedans came on bias ply 5.90-15 or 6.00-15, the wagons on 6.40-15. Late (67-68, I believe) 120 sedans were on the newer 6.85-15, which was basically a 6.00-15 with different numbers on it, but it was still a bias ply tire. (The 1800 series meanwhile, was on 165R15 radials from the get-go, switching to 185/70R15 radials in 1972.)
All other aspects being equal, I would agree that skinnier tires get better fuel mileage, but not just from less wind resistance... they also typically have less rolling resistance.
And that brings us to disagreement, because the lower rolling resistance is at least in part due to having less rubber in contact with the road. There are a whole lot of variables here (inflation, rim width, wheel camber or lack thereof, etc.) but IMO and generally speaking, a wider tire of the same overall diameter and of the same brand and basic design will have not only a different footprint shape, it will in fact have a larger footprint. This is particularly true when the "skinny" and "wide" tires are mounted on their respective recommended rim widths.
I do agree wholeheartedly with the unsprung weight aspect. If you're going to put significantly wider wheels and tires on your car, you'd better be prepared to spend some money to get significantly lighter wheels.
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Gary L - 142E ITB race car, 73 1800ES
BlueBrick Racing Website
YouTube Racing Videos
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