Paper based tests Vs Manhattan GMAT Vs actual GMAT??

[slumdog]

Hi.
I solved 3 paper based old GMATs without doing OG (but I had studied Manhattan Sentence Correction) and got 750, 760, 780.
Then I studied Manhattan quant guides and some Official guide problems for a month and then got 640 (V38, Q34) in Manhattan Computer based GMAT. For Quant, I ran out of time and could not answer the last 10 questions (not even guessing).
I found that math questions in Manhattan CAT were much more difficult than those in the OG or the paper based tests.
How do Manhattan CAT and paper based GMAT tests (or OG questions for that matter) compare to the questions in the actual GMAT (for a score of 700 to 750)?
I'm planning to take GMAT in 3 weeks. But this low score has puzzled me.

[beatthegmat]

I would say that the MGMAT test scores are more indicative of your actual GMAT performance. The reason why is that the paper tests are not computer adaptive, thus the results tend to skew higher (I've observed this only anecdotally).

The best practice CAT you can take to predict your current GMAT performance is GMAT Prep, which features real but retired GMAT questions. You can download the GMAT Prep software for free at MBA.com

Good luck!