taiyahaba ga hiroi.

It is a compound noun made from:

• タイヤ = tire
• 幅 = width

So タイヤ幅 means tire width as a single technical-style noun. Japanese often combines nouns this way.

A less compact version is タイヤの幅, which means the same thing but is phrased more explicitly as the width of the tire.

が marks タイヤ幅 as the thing that has the property 広い.

The basic pattern is:

X が 広い = X is wide

So in this sentence, タイヤ幅 is what is being described as wide.

Yes, but the nuance changes.

• タイヤ幅が広い: neutral, straightforward identification of what is wide
• タイヤ幅は広い: makes タイヤ幅 the topic, and can sound more contrastive, like as for the tire width, it is wide

So が is often the more neutral choice when you are simply stating a property.

Because Japanese commonly describes width with 広い and 狭い.

Very common patterns are:

• 幅が広い = the width is wide
• 幅が狭い = the width is narrow

This may sound a little unusual if you think too literally in English, but in Japanese it is a normal way to talk about width.

Yes. タイヤ幅が広い and タイヤの幅が広い are both natural.

The difference is mostly stylistic:

• タイヤ幅 sounds more compact, technical, or label-like
• タイヤの幅 sounds a little more explanatory and conversational

You may see タイヤ幅 in specs, catalogs, charts, or product descriptions.

Probably, yes. Japanese often leaves out the larger topic when it is clear from context.

For example, the full idea in context might be something like:

• この車はタイヤ幅が広い。
• このモデルはタイヤ幅が広い。

So the sentence by itself focuses only on the point being made: the tire width is wide.

Because 広い is an い-adjective, and in plain style it can end the sentence by itself.

So both are possible:

• タイヤ幅が広い。 = plain style
• タイヤ幅が広いです。 = polite style

The sentence you were given is just in plain form.

No. They are probably only there to help learners see the parts more clearly.

Normally, this would be written as:

タイヤ幅が広い。

Japanese usually does not put spaces between words in ordinary writing.

Not exactly.

• 幅が広い is a normal sentence pattern: the width is wide
• 幅広い is a set expression/adjectival form meaning wide or sometimes broad-ranging

For literal physical description like tire width, タイヤ幅が広い is clear and natural.
Also, 幅広い is often used in figurative meanings too, such as:

• 幅広い知識 = broad knowledge
• 幅広い年齢層 = a wide range of age groups

So they are related, but not interchangeable in every situation.

Grammatically, it is describing タイヤ幅, not タイヤ directly.

That is important because Japanese often describes a feature of something rather than the thing as a whole.

Compare:

• タイヤが広い = the tire is wide
• タイヤ幅が広い = the tire width is wide

The second one sounds more precise and technical.