Of all the floors in or around your house where level matters the least, the garage has got to be number one. The classic test for how much you can tolerate interior floors that are out of level--"Will the peas roll off the dinner plate?"--hardly applies to the rough and tumble atmosphere of the garage. As long as you can get the car in and it stays in place, you're fine. Even level-critical services such as the water heater and washer/dryer can be leveled individually.
However, an out of kilter garage floor may require leveling if the garage door does not meet it or if pavement water is flowing into the garage. No slab concrete garage floor fixes are easy, though.
1. Apply Leveling Compound For Spots
Leveling compound is the classic "easy" way to fix a floor that is uneven. In fact, for some interior floors, it's just a matter of pouring the compound out and letting it achieve its own level. For garages, it's not that simple. This Old House advocates for troweling on a concrete patch compound across the entire area of a floor that slopes down toward the doors.
While I trust TOH's judgement in most areas, I fail to see how this is possible across the entire expanse of a garage floor. For novices, it's impossible to hand-trowel something like a 2 inch gain over 20 feet.
Instead, apply leveling compound as a spot-fix if you might have a section that has cracked off and is now sloping down.
2. Slab Jacking: Not For DIY, But Most Promising Method of All
Slab jacking is one method that can correct the tilt of an entire concrete slab floor. You cannot do this yourself and it is expensive. But it is still cheaper than demolishing and re-building your garage.
A matrix of holes is drilled into the concrete.
Then, a slurry of sand, gravel, ash, and water is injected in the holes. The technicians do not randomly inject slurry, though; it's a game of making sure that the slab is supported in all areas. If one area gets too much push, it can crack the slab.
3. DIY-Built Concrete Block-Style Floor Repair
This third method does not apply to the typical garage floor, though I am including it for informational purposes. In this case, a reader wrote in about a small garage 20'x16' that is built on concrete blocks. The center has to be lifted up to level it. The reader goes on to say that
The floor is plywood that has been screwed to the floor. The blocks form the entire floor with plywood on top... I believe blocks form the the exterior walls, with rows that are four feet apart through the inside of this building. I believe there are four rows of blocks that the plywood sits on.
This unusual garage floor appears to have joists built out of concrete blocks, and these blocks are embedded in the ground. Strip off all the plywood to determine what is going on with those rows of concrete blocks.
Likely, some of the center "joists" have sunk into the ground over the years and with the weight of vehicles.
You can pull out those sunken blocks and then add enough gravel to the hole, so that when you replace the blocks they are equal level with the other blocks. Then replace with new plywood.
A quick and dirty method is simply to leave those sunken blocks in place and add "shims" on top. Shims can be other plywood, spare composite shingles, or any material that won't rot. Then nail plywood back down again--preferably new, thicker exterior-grade plywood.