GMAT Club quant tests are highly representative. If you are consistently scoring 45-50, be assured of a 49-51 in real GMAT. These tests are tough, in fact very tough, but as you sail through them learn to finish in time. The art of leaving a question when you cannot solve it is very critical for real GMAT. IMO, before you start taking GMAT Club quant tests go through the GMAT Club Math Book, which is one of the best resources available for basics. Take notes and go through them every day. Additionally, follow Bunuel and learn the way he tackles questions. After you have followed this curriculum for atleast two weeks, start taking these tests, you ll feel a lot more confident.
Hope it helps.