US News: “The ACT and the SAT both assess arithmetic, algebra, geometry and trigonometry, so much of the mathematical content will still apply if you switch exams. However, you should note that the SAT includes a distinct subsection for Problem Solving and Data Analysis, while the ACT includes Statistics and Probability instead … The ACT Writing prompt requires you to read a brief text that introduces an issue, followed by three distinctly different perspectives on the issue. In your response, you must draw upon the given perspectives to state and justify your own point of view.The SAT essay is different in that it is an exercise in rhetorical analysis. On the SAT, you must read a text and show which devices the author uses to build his or her argument.”
“Unlike the SAT, the ACT contains a dedicated science section. Despite its name, however, the ACT Science section primarily tests students’ critical thinking and reading skills, as well as their understanding of scientific skills like the scientific method … If you intend to switch from the SAT to the ACT, you should devote study time to reviewing skills from your science classes, but rest assured that you do not need to master every scientific concept and term. Similarly, the SAT Reading portion assesses vocabulary in context more heavily than does ACT Reading. Students transitioning to the SAT should address this difference by adding more vocabulary questions to their review regimen.
“In general, the ACT is a more fast-paced examination. The ACT contains more questions, although the questions are typically more straightforward than those on the SAT. On the ACT, you have approximately 36 seconds per English question, 60 seconds per math question, and 52.5 seconds per reading question. Compare these numbers to the SAT, on which you have about 48 seconds per writing and language question, 75 seconds per reading question, and roughly 83 seconds per math question. For the essay portion, you are allowed 40 minutes on the ACT, versus 50 minutes on the SAT.”