A pressão máxima aparece no pneu.

Because Portuguese normally uses the definite article with a subject like this.

So A pressão máxima means the maximum pressure. In English, we sometimes drop the in general statements, but in Portuguese a full sentence like this usually sounds more natural with the article.

• A pressão máxima aparece no pneu. = The maximum pressure appears on the tire.

Leaving out a here would sound unnatural in normal speech.

Because adjectives in Portuguese must agree with the noun they describe in gender and number.

• pressão is feminine singular
• so the adjective must also be feminine singular
• therefore: máxima

Compare:

• pressão máxima = maximum pressure
• valor máximo = maximum value

Since valor is masculine, the adjective becomes máximo.

In Portuguese, adjectives often come after the noun.

So:

• pressão máxima = maximum pressure

That is the normal, neutral order.

You can sometimes put adjectives before the noun in Portuguese, but that often sounds more literary, emphatic, or changes the nuance. Here, pressão máxima is the standard order.

Aparece is the present tense, 3rd person singular form of aparecer.

Here it means something like:

• appears
• is shown
• is visible

In this sentence, it does not necessarily mean that something suddenly appears. It can simply mean that the maximum pressure information is shown or printed on the tire.

So in context, aparece is often understood more naturally as is shown than as a dramatic appears.

Because the subject is singular:

• A pressão máxima = singular

So the verb must also be singular:

• A pressão máxima aparece

If the subject were plural, the verb would also be plural:

• As pressões máximas aparecem

Because no is a contraction.

• em + o = no

So:

• em o pneu → no pneu

This is very common in Portuguese and is normally required.

Other similar contractions:

• em + a = na
• em + os = nos
• em + as = nas

Examples:

• no carro = in/on the car
• na mesa = on the table

Because Portuguese often uses em (and its contractions like no/na) in places where English prefers on.

So no pneu can correspond to English on the tire, especially when talking about something written, marked, or shown on its surface.

This is normal Portuguese usage. You should not translate the preposition word for word.

Yes. Pneu is the normal Portuguese word for tire/tyre.

In European Portuguese, this is the standard everyday word.

A useful detail:

• pneu = tire/tyre
• câmara de ar = inner tube

So if you are talking about the rubber tire itself, pneu is the right word.

Yes, but the nuance is slightly different.

• A pressão máxima aparece no pneu.
  This emphasizes that the information appears / is shown on the tire.

• A pressão máxima está no pneu.
  This simply says it is on the tire.

So both can work, but aparece focuses more on visibility or display, while está focuses more on location.

Yes, they matter a lot. They are part of the correct spelling and they also help show pronunciation.

• pressão

  • ã shows a nasal sound
  • ç makes the c sound like s
  • the stress is on the last syllable: -são

• máxima

  • the accent on má shows that the stress is on the first syllable

If you remove the accents, the words are misspelled and may be harder to recognize or pronounce correctly.

Yes, Portuguese word order is somewhat flexible.

The neutral order is:

• A pressão máxima aparece no pneu.

But you could also say:

• No pneu aparece a pressão máxima.

That version puts more focus on where it appears. It is grammatical, but less neutral.

For learners, the original order is the safest and most natural one to produce.