Durometer for measuring tire hardness. Hardness may be defined as a material's resistance to permanent indentation. Tire hardness has a huge effect on your car's performance. A tire that is too hard won't bite and a tire that is too soft may wear quickly or even ball up. Also, tire hardness changes during its life cycle. To run your best, you must keep track of all your tire's hardnesses. This easy-to-use tool will help you do just that - quickly, accurately & easily.