GMAT Practice Test
GMAT practice tests can be one of the most important tools in your GMAT prep arsenal. Official GMAT practice tests are best, but due to their limited quantity it is fine for the most part to use unofficial practice tests.

If you do use an unofficial GMAT practice test, there are a few things you should know. First, don't get hung up on the small details of each problem. Many will include concepts that never appear on the GMAT simply because GMAT has determined that they are too confusing to test.
Second, remember that many practice tests sadly have errors in them. If you are sure you are right, then don't fret over a couple problems that your practice test insists are incorrect. The main benefit of GMAT practice tests is the ability to test your skills, get mentally ready for test day, and get a rough approximation of your expected performance so you can improve it before test day if desired.
Just like a football team's practice isn't exactly like a football game, your GMAT practice test does not have to be exactly like the real test. The common element is the coach- to get a good coach for your prep, be sure and get a good study guide.
Best of luck on your GMAT, and remember that it's the overall effort that counts, not the details of your prep or GMAT practice test.