Why overall tire diameter shrank over the decades?

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Tire diameters have decreased significantly over the decades, with a notable reduction of about 2 inches every ten years from 1915 to 1985, despite increasing vehicle size and engine power. This trend is attributed to several factors, including improved road conditions that reduced the need for larger tires, changes in vehicle design that favored smaller diameters, and advancements in tire and wheel technology. The shift from open wheels to integrated body designs necessitated smaller tires, while stamped steel wheels improved balance and strength at higher speeds. Additionally, modern suspensions and wider tires have enhanced handling and ride comfort, allowing for lower vehicle profiles. Overall, the evolution of tire size reflects a complex interplay of design, performance, and road infrastructure improvements.
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Automobile tires has been gotten smaller, diameter wise over the years. Below were based on Cadillac of different era.
  • 1915 - 51-Eight with 36x4.5 tires with overall dia 36.8" and 4.75" wide
  • 1925 - V63-Eight with 33x5 tires with overall dia 34.88", and 5.88" wide
  • 1936 - V16 with 750x16 tires with overall dia 31.45" and 8.2" wide
  • 1945 - v8-75 same as 1936
  • 1955 - 8-7500 with 820-15 tires with overall dia 29.59" and 8.5" wide
  • 1965 - De Ville - 820-15 same as 1955
  • 1976 - De Ville - P235/75R15 with overall dia 28.9" and 9.4" wide
  • 1985 - De Ville - P205/75R14 with overall dia 26.1" and 8" wide
  • 1995 - De Ville - 215/70R15 with overall dia 26.9" and 8.7" wide
  • 2005 - De Ville - 225/60R16 with overall dia 26.6" and 9" wide
  • 2014 - XTS 3.6L - 245/45ZR19 with overall dia 27.6" and 9.6" wide
From 1915 to 1985, tire diameter shrank 2" or so every decade, and yet the engine output increased with more torque. Other than '85 De Ville being much smaller than '75 De ville , I would think vehicles increased in size and weight every decade over that period.

Bigger heavier with more power but smaller tires, why?
 
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Look at improved/paved road mileage over the same time period.
 
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Bystander said:
Look at improved/paved road mileage over the same time period.

Can you detail the relationship between condition of road vs tire diameter?
 
Cars have also gotten smaller over the same period.
 
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Rose Gardener said:
relationship between condition of road vs tire diameter?
Try pushing a grocery cart through potholes in a supermarket parking lot with those little bitty wheels. Same thing going on a century ago driving along cow paths, wagon ruts, and other unimproved roads --- the large diameter wheels/tires were necessary for everyday driving.
 
There's no one reason tire sizes have changed over the period of a century. Some of the reasons are:

1. Early cars were designed with open wheels / flimsy fenders. Tire size wasn't affected much by this type of arrangement. In the 1930's, open wheeled car styles became passe, and pontoon-type fenders meant tires had to get smaller in diameter to fit. After WWII, even pontoon fenders gave way to body styles where fenders were no longer "pasted"onto the cars, but faired into the body. For example, see the 1949 Ford sedan, versus just about any pre-war car.

2. Using large diameter wheels and high speeds means that it is critical to keep the wheel in balance, otherwise the tire will start to vibrate and wear unevenly. Switching from wire wheel construction to stamped steel wheels of smaller diameter means that it is easier to keep the wheel/tire assembly in balance at speed. Stamped steel wheels are also stronger than wire wheels.

3. Better roads meant that cars could go faster, which meant they needed better brakes to stop. Stamped steel wheels can resist the larger braking forces better than wire wheels.

4. Better car suspensions to increase ride comfort also work better with smaller wheel/tire assemblies. This is especially true as cars switch from solid axles to independent suspensions at the front and rear. The change in suspension geometry as the wheels travel up and down are not a severe with small stamped steel wheels as with large wire wheels.

5. As wheel diameters shrank, tires got wider as a result to keep the same area of contact with the pavement or more contact area. More contact area means the car can handle turns better at high speeds.

6. Car styles also gradually got lower over time, which required a different design of the frame and suspension to accommodate this change.

7. Different tire construction methods (i.e., radial v. bias ply construction) work better with small wide tires rather than large skinny tires.

There may be other reasons, but the above are what I can think of off the top of my head.
 
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I can think of some reason for larger tires off the top of my head:
  1. Largest dia tires should last longer? less rev to cover the same distance?
  2. Can accommodate bigger brake rotor.
  3. In case of off roading, can climb over larger objects.
  4. In case of electric wheel motor, more space.
 

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