Pip lies to Mrs. Joe and Uncle Pumblechook because he does not want to tell them what happens at Miss Havisham’s house.  He does not think they will understand.

When Pip returns home from Miss Havisham’s house, he is a little dazed.  The events, and Miss Havisham, are “incomprehensible” to him (p. 46).  He is quite sure that if he tells them what actually happened, it would not go well.

I felt convinced that if I described Miss Havisham's as my eyes had seen it, I should not be understood. Not only that, but I felt convinced that Miss Havisham too would not be understood. (enotes etext, Ch 9, pdf, p. 46)

Even though he is quite young and inexperienced, Pip realizes that Mrs. Joe and Pumblechook are vain and superficial.  Mrs. Joe is harsh, and Pumblechook "is little more than a stereotype of a snob" (enotes characters).  They would never believe him if he told them the truth, and he would probably sustain quite a beating. 

So he decides to make up vain and superficial likes that fit what they expect to here, and "makes up a story about Miss Havisham sitting in a black velvet coach, huge dogs eating veal-cutlets out of a silver basket, and playing with flags" (enotes summary, chapters 8-9).  He is trying to protect Miss Havisham, and himself.

When Joe confronts Pip later about his lies, he is hurt.  Pip feels bad for having lied, but he realizes that Joe would never understand why he had to lie.  This is the first division between Pip and Joe as part of Pip's new life.

For the full text the page numbers are from, see here: http://www.enotes.com/great-expectations-text

For the full chapter summary, see here:

http://www.enotes.com/great-expectations/part-1-chapter-8-9-summary-analysis

For more on characters, see here:

http://www.enotes.com/great-expectations/characters

Why doesn't Pip tell the truth about Miss Havisham in Great Expectations?

In my opinion, Pip has realized an advantage he has over the adults in his life and exercises his right to explore that advantage. See, none of the adults know the real truth about who Miss Havisham is, but they have vague fancies about what must go on in her home. So, when Pip talks about the great dogs, and the flags, and the coach in the house, and eating on the gold plates he has them all going.

Another purpose this served for Pip is that he discovered that although Pumblechook claimed to know much about Havisham, Pumblechook really doesn't have any clue.

In Great Expectations, why does Pip lie to Mrs. Joe when he is asked about Mrs. Havisham?

After Pip's first meeting with Miss Havisham, he is ashamed of his circumstances, of his family, and of his own rough, ungentlemanly ways.  Before he went there, he narrates that he was constantly frustrated with the way that his family and their friends teased him, analyzed him, insulted him, and took every aspect of his life and hashed it out in detail, making fun of it and making it seem like it was worthless.  The greatest instigator of these conversations were the insulting comments of Uncle Pumblechook, whom Pip's sister deferred to in all things because of his slightly higher social rank.  He would come over and humiliate Pip constantly, and Pip's sister would let it happen out of respect for his rank.

So, when his sister asks about Miss Havisham, he doesn't want his special experience that had so personally altered him to be out there in the open, to be openly mocked.  Also, they had heard such fantastical rumors about Miss Havisham, that he felt that the truth, the very sad truth of her state, would be a disappointment to them.  So, lying and exaggerating was easier, and got them off of his back for a while.  If he could get them going on some tangent, it meant that at least they weren't questioning him, and he could be left alone with his thoughts of inferiority, and to process the experiences that really occurred.  He wanted to keep his world at home separate from the strange and dazzling world at Miss Havisham's.  I hope that those thoughts helped; good luck!

Why does Pip lie to Mrs. Joe and Pumblechook about Miss Havisham?

Pip grows weary of Mrs. Joe and Pumblechook's interrogation of the time he has spent with Miss Havisham. He does not want to tell them about her real life, the cobwebs and faded wedding dress. Instead, he tells the pair that he and Miss Havisham enjoyed a day of extravagant treats, wine and cake and merriment. He gets caught up in the lie, and continues to lie, making "up a story about Miss Havisham sitting in a black velvet coach, huge dogs eating veal-cutlets out of a silver basket, and playing with flags."

The "lie" is found in Chapter 9.

In Great Expectations, why does Pip lie to Mrs. Joe and Pumblechook about Miss Havisham?

Pip doesn' want to let on that he didn't have a good time at Miss Havisham's. He also doesn't feel right about telling others about her personal life. Talking about how time has stopped in her house, that she still wears her wedding dress, and that she made him play cards with Estella (who very openly didn't like him) are all things that Pip feels he should keep to himself. He doesn't think its his place to share these things. He starts making up a story that he thinks they will believe, and Pumblechook's eager agreement shows his lack of knowledge about Miss Havisham and her house.