I would be wary of some of the online free tests to do, I got my CDL in New York and for a month prior I studied and did the online tests over and over until I was getting 90+ results, but when it came to the actual NY DMW test, almost NONE of the questions I had been asked were actually on the exam. Thank God I had actually read the DMV book a few days in advance (we had a few copies lying around) and I still passed with only a few wrong. I passed general knowledge, airbrake and tanker all on the first try....a few of my co workers took the tests and failed either part, and havent bothered to go back and re try.
My department insists I go for my CDL within the first 6 months of getting in, which I did with zero issues, but for some reason they slacked on my co workers...guess they like riding around in the truck more than doing actual driving.