Tom Sawyer is an example of a liar that Huck considers honest.
Huck admires Tom Sawyer because of his imagination and sense of fun, but does not consider him dishonest until he finds out that Tom kept Jim’s freedom from him.
Huck appreciates Tom’s sense of adventure, but he often takes things literally. Everything is a game to Tom, but serious to Huck. A perfect example is when Tom tries to raid the Sunday School picnic for Arab treasure, when in fact it is nothing more than a picnic.
I didn't see no di'monds, and I told Tom Sawyer so. He said there was loads of them there, anyway; and he said there was A-rabs there, too, and elephants and things. I said, why couldn't we see them, then? (ch 3)
The same basic situation happens at the end of the book, but it is much more serious. Tom knows that Jim has been freed, but he concocts a plan to help him escape anyway. He risks his own life, and Huck and Jim’s, just for an adventure.
Although Tom Sawyer is somewhat of a role model for Huck, he is far less honest and moral. Huck tries to do what is right, and Tom just wants to have fun. Tom has been sheltered, and does not see the reality of serious situations like Huck does. He simply does not see the danger of his lies.