GMAT Strategy

I got 740 and 750 on both of the gmatprep cats, before I really started studying, about 1 month ago. Last week after a lot more practice and studying I got 670 and 640 on two manhattan cats.

Did anybody have a similar experience with the two types of exams? I don't get it..
Was I just lucky with Gmatprep? Or is there some kind of difference in the reasoning between the two that might make manhattan cats harder for some people?

Your second thought is correct. Some people just have a tougher time with the Quant on unofficial tests and the verbal is really tough for practice test writers to get right. Only the GMATPrep really gets it right.

Unless you had already studied the GMATPrep questions and therefore saw lots of questions that you already knew I would take the GMATPrep scores as representative and I would have scheduled the test as soon as I saw those numbers. I have always scored from 760 to 780 on the practice GMATPrep and 760 to 770 on the real GMAT.

I think you should schedule your test for a convenient time that is pretty soon, get in there and you will see. I would bet you get anywhere from 730 to 760. Remember that you can take the GMATPrep tests a second time - you may get a couple of questions repeated but it is better to take the official practice test a couple of times more!

Good luck!