Hi,
While we do try to make our tests reflect the real GMAT as accurately as possible, I have heard that ours are often a little harder. Since the GMATprep exams use the same algorithm as the official GMAT and also use questions that have actually appeared on the exam, they are a better judge of your ability than our exams. Our exams can define your ability within a 50 point margin, whereas the official GMAT and the GMATprep tests can define your ability within a 30 point margin.
That being said, since you used our free practice test, be sure to analyze where your strengths and weaknesses were and go over the questions you got wrong to make sure you understand them. There's a lot of value you can get out of the explanations and analysis we give beyond the actual practice of sitting down for four hours to take the exam.
Best wishes,
Caitlin Clay
Student Services Associate
Manhattan GMAT