Grip, not speed
Why Do Tractors Have Huge Back Wheels?
A tractor is built to pull, not to travel. Everything odd about its shape follows from that one job.
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Step 1 of 5
Pulling needs grip, and grip needs weight.
A wheel can only push back as hard as the ground holds it. Most of a tractor's weight is deliberately placed over the driven wheels for exactly that reason.
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Step 2 of 5
But weight sinks into a field.
Soft ground gives way under pressure, and a sunk tractor pulls nothing. The wheels must be huge so all that weight is spread over a large footprint.
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Step 3 of 5
A big wheel rolls over things a small one falls into.
A large diameter climbs a furrow or a rut that a small wheel drops into. Height is as useful as width in a field.
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Step 4 of 5
The tread is angled to clear itself.
The deep V-shaped bars bite into soil and then throw the mud sideways as the wheel turns. A tread that clogged would be a smooth tyre within one pass.
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Step 5 of 5
Gearing trades speed for pull.
Gears let a modest engine turn slowly with enormous force. A tractor is not powerful compared with a car — it is geared so that all of what it has arrives as pull.
A tractor's rear wheels are usually run at very low tyre pressure on purpose. A soft tyre flattens against the ground and grips over a longer patch.
The short version
A tractor's huge rear wheels give grip through weight and avoid sinking by spreading it, which is what pulling on soft ground requires.
Try it yourself
Push a coin edge-on into sand, then flat. The same push gives very different results.