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I wonder why Buick put the 15x6.00 wheel on the Century and Roadmaster and put larger wheels on the Special and Super.

Has any owner swapped out the larger wheel and tires to their Century or Roadmaster?

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According to the shop manual, the 40 and 50 series have a 16-inch rim size with a loaded radius at 0 mph of 14.01-inches.  The 60 and 70 series have a 15-inch rim and a loaded radius of 14.03-inches.  The loaded radius is nearly identical.  This leaves a larger aspect ratio, or larger sidewall depth of about 0.5-inches.  

 

The larger aspect ratio provides a softer ride as you've no doubt heard the complaint of low aspect ratio tires on modern foreign cars as being rough.  It would seem to me that the original design goal might have been ride quality, but I'm guessing and couldn't quantify what an additional ½ inch of sidewall rubber does for the ride.

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Interesting. I always assumed the smaller rim on the 60 (and later 70) series allowed for a more substantial tire due to the added weight of the 320 engine, as the diameter of the tire is almost identical. Limited had a 4.2 ratio as opposed to 3.9 of the 60 and 70, but it had 7.50x16 tires. A 15" wheel/tire on such a big car would seem out of scale?

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Exactly my thought when comparing my 71C to my model 41.  I think I will be looking for five 16 wheels. Thanks for your input.

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I bet it was purely fashion. Smaller wheels were seen as the way of the future. There was a progressive reduction of wheel diameters throughout the 30s, ultimately culminating with the 14s of the 1960s, which were the most common choice on even high performance cars. The 16s on the Limited were probably for weight considerations (15-inch tires were still new and probably limited in the number of sizes and weight classes available) and to a lesser degree, proportions. 15s were used on the Century and Roadmaster because they were the halo vehicles and needed to look cutting-edge, which was what the 15-inch tires were. There is no functional difference, gear ratios could be anything the engineers wanted without the need for tweaking it with tire sizes, and I can't imagine ride quality varies much between tall 15s and tall 16s. 

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