When taking a standardized test like the ACT, you are guaranteed access to official ACTs. These practice tests consist of official questions from past ACTs, as well as answers to check your work. The real test will have questions similar to these previous tests.
Attached are some practice ACTs released by ACT, Inc. Some test years are missing because ACT, Inc. will use one test and duplicate it to use for the next couple of years. These tests, as mentioned above, are official ACT tests with official ACT questions. They are posted below:
ACT, INC. has also released an official ACT prep book, The Official ACT Prep Guide, 2018-19 Edition. This book offers four full length practice tests; however, only one of them is unique to the tests listed above. As a result, we recommend buying the third edition which offers 5 unique ACT practice tests.
We will also offer tips for ACT and would like to mention one more time: these tests will not guarantee a good grade. If there are some things you do not understand, make sure to study and learn them for the test. As mentioned, studying one test is a bad idea. Make sure to expose yourself to as much content as possible and learn the key concepts of that content. That will help you a lot more than just studying one test.