Having bad tyres on your vehicle can be costly, as your car will consume more petrol. The rolling resistance will increase and that directly impacts your fuel consumption. The same goes if your tyres are low on air, then the fuel consumption will increase.
When you purchase new tyres, the rolling resistance is highlighted for each tyre, so you can see how well they perform on that rating and you can select one that will yield low consumption by having low rolling resistance. With high fuel prices the savings can be substantial. So when you buy your next set of summeror winter tyres, make sure you get a good set.