Here is a list of small pets and whether wheels are safe for them or not:
First! Always remember any wheel used should be solid, not made of wire mesh or rungs. Why? Little critter tails and feet can easily get caught in the mesh or rungs. The pet could end up injured, and could even lose a tail or limb!
Here is the list:
Hamsters (and mice):
Hamsters should always have a wheel. In the wild they travel miles in search of food, so they get a lot of exercise. A cage doesn't provide the same opportunity for exercise, so they need a wheel. Especially dwarf hamsters! Most dwarf hamsters have ENERGY!
Do gerbils need wheels or not? Some people say yes, some say no. I say it depends on the gerbil, because I have met some that completely ignore it or just pack it full of bedding. But most gerbils seem to love them and gerbils are quite hyperactive in my opinion so if they like the wheel definitely give them one. If your gerbil doesn't use the wheel you can take it out of the cage.
I have talked about wheels with rats before, but I will again. Whether a rat needs a wheel or not depends on the rat. Some rats completely ignore them, some hate them. Provide a wheel and if they like it then keep it in the cage. If they ignore it take it out. If you adopt you can also ask the rescue or shelter if the rats like their wheel. Make sure the wheel is large enough (at least 11 inches)
Note: If you have multiple rats and some like the wheel, while others do not, keep the wheel in the cage so that the rats that do want to use it can. If they all don't like it then you can take it out.
Giving guinea pigs wheels is an absolute NO-NO. Here's why: Guinea pigs backs aren't meant to bend and curve, so when they are in the round wheel, it causes spinal damage, and can result in injuries and even paralysis of the back legs. I have seen this before. Guinea pigs spines are very sensitive and easily injured. For exercise give them floor-time in a pet proof room or pen.
Don't use wheels with rabbits. First of all, rabbits hop, they do not "run" in the way that other animals do, so they can't use a wheel properly. And anyway, you won't find a wheel big enough for most rabbits, and most likely the rabbit would not use it as a wheel. They would probably just throw it around and play with it. So no wheels for rabbits. Instead let them have free-range floor time in a rabbit proofed area for a couple hours each day for exercise.
Ferrets are very long, so I have never ever seen a wheel big enough for one. And like rabbits, ferrets do not really run they way animals that use the wheels do. They kind of "bounce". Instead of wheels give ferrets floor time in a ferret-proofed area. You can also provide tunnels which ferrets love to play in.
Chinchillas should have a wheel but make sure it is the BIGGEST wheel you can find and that it is solid not mesh.
Wheels are great for degus, and in my degu experience, they LOVE them! But it is EXTRA important that with degus you do not use wheels with wire mesh or rungs. Degus tails are extremely sensitive and can easily break off. (or the skin of the tail may come off leaving the rest behind). So definitely use a SOLID wheel with degus.