After finishing the main story, you'll be at rank 21. From the garage (via the 'career overview' menu), you can then replay the earlier assignments that you didn't choose the first time around. Each one of these is worth one rank. There's a further 40 ranks to be gained here, so you should hit rank 60 with one assignment to spare.
If you're also going for the trophy for 100 cumulative gold medals across both cop and racer careers, this is a good opportunity to knock off a bunch of these since various assignments will require you to get gold medals in certain events. I got to 62/68 gold medals in the cop career without too much difficulty. Most events are pretty easy with the Hennessy, except some of the Hard Hot Pursuits.