I used to work for a major tire manufacturer.

Changing the diameter of the wheel effectively changes the gear ratio of the drive train. This applies to cars and bikes.

A larger diameter wheel gives more leverage, effectively “gearing down” the vehicle. A smaller diameter wheel gives more RPMs over the same distance, effectively “gearing up” the vehicle.

Bicycles have larger wheels to make them easier to pedal. If you’ve ever ridden a folding compact bike, you may have noticed the smaller tires make them harder to pedal or have larger gears to compensate.

Car tires have a smaller diameter to reduce material cost. Truck and SUV tires tend to be a little bigger to pad the final torque figure. Tractor tires are huge to maximize torque. Big offroad tires are mostly to give more ground clearance, the extra torque is just a bonus. The reason offroad tires aren’t as big as tractor tires is because they still need to fit under a wheel well and because there are height restrictions on street legal vehicles and people occasionally drive a few miles on public roads in their 4x4.

Tread width improves cornering and acceleration performance, to a point, by giving a larger contact patch, which gives more friction, but reduces fuel efficiency and eventually reduces performance. Economy car tires aim for a good balance between braking performance and fuel efficiency. Sports cars aim for a tread width commensurate with the desired performance capabilities; wide enough to put power to the road and stay stable through the corners, but not any wider, so as to keep weight to a minimum.

Bike and motorcycle tires have a curved bottom to give traction while leaning, so this is factored into tread width also.

Tires are part (sometimes the only part) of the suspension of any vehicle. The air in the tire gives cushion. More air means more cushion. Higher tread wall means more room for air. The height of the tread wall is determined by giving enough to soften the ride, but not too much because tires are heavy and additional mass means lower fuel efficiency and reduced performance. Tires are considered “unsprung weight” because they’re not supported by the springs of the suspension. When calculating performance, one pound of unsprung weight is equivalent to approximately three pounds of sprung weight, so optimization is important in that area.

Weight capacity, wear rate, and shear strength are determined mostly by the profile design, material composition, and manufacturing process. Diameter and width aren’t (usually) adjusted to suit those parameters.

The TL;DR is that at one point in history, automobile wheels did in-fact use the same construction as bicycles. But the needs of automobiles diverged somewhere in the first half of the 20th Century. And since tires are mounted onto wheels, we need to discuss those first.

I’ve written prior comments here about bicycle wheel/tire fitment and wooden carriage wheel design.

Basically, early horseless carriages used the same wooden wheels that horse-drawn carriages had used for centuries, which have a squared off profile that contacts the ground, sometimes with a steel band – a tyre – to both hold the wheel together and reduce wear on the wheel itself. The only requirements for carriage wheels were to: 1) roll, and 2) bear weight. And using thick wooden spokes, a wagon wheel could achieve those objectives just fine but were really heavy.

When the bicycle was invented in the 1820s, the first iterations used slender variants of wagon wheels, but since 100% of the moving power came from the human rider, this is still unnecessary dead weight to haul around. So bicycle wheels evolved to use very thin spokes, which by the late 19th Century were made of steel in tension, rather than the compressive loads through wood that wagon wheels used. Although steel is heavier than wood, a thin steel spoke has more tensile strength than the same weight of wood has in compression. So overall, it’s a weight savings. Specifically, we say that a bicycle wheel must: 1) roll, 2) bear some weight, and 3) allow for leaning.

The last requirement is crucial for bicycles: they cannot use squared-off carriage wheels, or else leaning the bike will start riding on the edge of the wheel/tire. The solution is simple: round off the contact point so that leaning doesn’t change the profile.

As it turns out, by the 1910s or so, automobiles also realized that wooden wheels were too heavy, and so they also adopted the steel spoked wheel. But they kept the squared off rubber tire, precisely because an automobile does not (normally!) lean during a curve, and instead should be firmly planted on all four wheels. So at this point in history, both automobiles and bicycles are using spoked wheels but just have different shapes for their rubber tires. Great!

But this wouldn’t last: the spoked wheel – which already is a phenomenal structure, essentially being a suspension bridge wrapping upon itself – has one small quirk which bicycles tolerated but automobiles do not. When a spoked wheel is subjected to a straight downward force, the structure distributes the force essentially evenly. But if the force is sideways from the left (ie pushing leftward at the axle), the spokes on the right are heavily stretched but the spokes on the left aren’t. This is uneven loading, that then reverberates from side to side.

This is no issue for bicycles, because they usually lean and so the sideways force is often zero. Sure, a BMX rider can intentionally ride the bike askew, but it’s workable. For an automobile, sideways forces are a regular occurrence, such as during a sharp turn. But also during motorsports where the car is sliding. Spoked wheels can disintegrate when subjected to enough sideways force, which is why cars switched to wheels using sheets of steel rather than spokes. This added weight but was necessary.

Also around this time, cars got very heavy – some would say “land yachts” – and this required making the tire wider to deal with the weight. Since the tire and wheel are the same width in cars, this means wider rims as well. Bicycles have no such issue, because most bicycle tires are “balloon shaped”, and so already are wider than the rim, sometimes almost comically. From a purely materials perspective, making the rim match the tire width does not add strength but does add weight, so cars have to accept that penalty but bikes do not.

In the end, the closest that bicycles and automobiles got was in the early 1900s, and have diverged ever since. Fatbike bicycles and now ebikes pushed the width of tires to some 4+ inches (100+ mm) while touring cars are luxury vehicles meant for long distance, high speed cruising on the Autobahn, and so need wide, high aspect ratio tires.

As for wheel diameter, that’s much simpler to answer: as Jeremy Clarkson noted in the Vietnam Top Gear Special, smaller wheels fall into potholes easier. Bigger wheels roll over them. Automobiles for paved roads use modest diameters, capable of slowly rolling over a 4-6 inch curb to access a driveway. The same diameter on a bicycle would be the 27-inch (aka 700c) or 29-inch class used for road cycling or mountain biking. Whereas smaller folding bikes used exclusively for last-mile commuting can tolerate smaller wheels, because the benefit doesn’t outweigh the diameter penalty when folding it down. All the meanwhile, a motor scooter (eg Vespa) also has small diameter wheels, because they don’t go as fast and urban streets are paved.

For overlanding or bouldering, 4x4 automobiles have some enormous tire diameters and even then, they sometimes have to intentionally reduce the air pressure, so the tire can conform to rock surfaces and thus get more traction. But such tires are wholly inappropriate on a roadway at freeway speeds.

I can speak to motorcycle tires.

Moto tires are rounded, not flat like car tires. They’re designed to provide an optimal contact patch no matter the lean angle. Often, Moto tires won’t be perfectly rounded, but come to a bit of a point in the middle. This makes for a smaller contact patch going straight, for better fuel economy, and a bigger one while turning for more traction. Finally, moto tires are often made of multiple kinds of rubber: harder in the center for lower rolling resistance, and longer tread life; softer on the sides for more traction while turning.

I’d bet that small bicycle tires share some of these characteristics, being rounded for sure, possibly others.

The pressure the tire exerts on the road is always equal to the pressure it’s inflated to. When the vehicle weight increases while tire PSI stays the same, the contact patch (area squished flat against the pavement) increases in size.

Bike tires are narrower than car tires because bikes are much lighter (so the contact patch doesn’t need to be as wide), and also because they lean into turns (so the contact patch can’t be wide). Bike tires are often larger diameter than car tires because they have more gyroscopic effect and thus make the bike easier to balance. They also make it easier to ride over bumps, but on a road bike (as opposed to a mountain bike) that’s probably a relatively minor reason.

I think motorcycle and ebike tires are a little wider (but still round in cross-section, so not like a car tire) for durability reasons because all the forces they’re subjected to are larger.

More rubber on the road means more grip. As the weight and corner speeds increase, you need more of it.

In adverse conditions, wet, mud, you need profile on your tires to channel the water while keeping rubber on the road and maintaining grip. Wider tires make up for the rubber lost in grooves.

More grip also means more resistance. So there is always a balancing act between the grip you need and the resistance you can put up with.

The answer to width is rolling resistance.

Tires stick to road which is great for accelerating and breaking, but it makes maintaining your top speed harder. This doesn’t matter if you have a big engine, but it makes a difference if you’re cycling.

For a road bike, you want to minimize the contact area of the tire with the road so you have very narrow tires and inflated a lot so that they don’t deform much under your weight.

The type and width of tires changes depending on what you’re riding on. For off road you have wide knobbly tires that will catch in mud and push you forward. For riding on a beach you have very wide smooth tires for traveling over sand.

The diameter is about stability. If you have small wheels turning fast it effectively lowers your center of gravity due to gyroscopic effects (this is a massive over simplification of the physics) big wheels turning more slowly results in a higher center of gravity, which is more stable.

Cars have four wheels and don’t need to care about this.

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How could they not have the width and diameter they do?

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