Whenever you’re having new tyres fitted, you should always use replacements that have a speed rating equivalent to, or higher than, the load index stated on your vehicle’s placard. The speed rating of your vehicle’s tyres can be found on the sidewall of the tyre. It’s represented by a letter.
Tyre Speed rating on Tyre Diagram

Like load ratings, speed ratings are regulated by law. You can’t drive with tyres that are lower than the recommended speed rating. Aside from being illegal, it can cause poor handling and unpredictable steering of your vehicle. You should look at your speed rating and your load rating together when buying tyres, as the two work hand-in-hand (like Vegemite on toast).
Speed ratings are determined by indoor lab tests, under controlled conditions. They measure high-speed durability. A tyre’s speed rating is based on minimum standards for reaching and sustaining a specified speed. In general, a higher speed rating gives you better car handling. When replacing tyres, the new tyres should have a speed rating equal to or greater than that of the original tyres. You can choose tyres with a higher speed rating - this can improve your vehicle’s cornering response.
This table shows you what speed a tyre is rated for by it's alphabetical speed rating symbol:

Take note, that the speed rating only applies to tyres that haven’t been damaged, altered, under inflated or overloaded. Lastly, there’s one other speed rating you might come across, which is ZR. Tyres with a ZR rating have a speed capability of over 240 km/h. If this applies to your tyres than you’ll need to contact the tyre manufacturer for it’s speed capability.