<p>Well, the SAT is an aptitude test which does just that. Measure scholastic aptitude. The ACT is an achievement which means it tries to measure how much information you have actually gained in school. I scored an 1800 (700 CR 610 M 500 W) on SAT while I received a 31 on the ACT (32 English, 33 Reading, 30 Math, and 29 Science 10 Essay)</p>
<p>Granted, I took the ACT 6 months after the SAT and after ~7 APs, so I may have gained some more knowledge and been in a testing mode that test. </p>
<p>@dragooner I know people who do worse on ACT. It is a two sided coin.</p>