What it Does
Tire Speed Ratings are used by vehicle manufacturers as standards for tires to be put on their vehicles, the speed ratings inform what vehicle speeds the tires should be able to handle, and the speed ratings play inform tire construction such that they impact ride and handling: including what handling the vehicle is capable of and how the vehicle feels to the driver.
Why This Matters
Tires are part of the vehicle’s suspension system. They are literally the only thing between the vehicle and the ground. They bear the vehicle’s weight, they respond to road impacts, they flex as the vehicle accelerates, brakes, and turns, and they impact how the vehicle feels (and even sounds) to the driver. They also apply to more than just their rated speeds. For example, a higher speed-rated tire will likely have a stiffer sidewall construction. This will affect how the vehicle handles and how it feels.
Possible Problems and Concerns
Spec Mismatch
Again, tires are part of the vehicle’s suspension system. If a vehicle is spec’d for H speed rated tires for example, and such tires are required for the suspension to perform adequately, installing lower speed-rated tire (S or T, for example) risks that the suspension will not perform adequately, which means that the vehicle’s handling capability is diminished. Diminished handling capability means aggravated risk of losing control of the vehicle in demanding driving situations.
Disappointment in Ride Quality
Let’s say you’re used to driving around on H-rated tires. But then you have a set of T-rated tires installed. You may actually prefer how your car felt with the older H-rated tires on it! You may note that with the T-rated tires installed, the vehicle feels “softer” (or less firm) and/or that the body rolls/sways under cornering more than it used to. You may note that the vehicle’s steering or suspension systems feel less responsive.
This can hold true even if the vehicle’s manufacturer allows for T-rated tires on that vehicle. In this case, that spec has been met; yet the result is disappointment. Furthemore, installation of H-rated tires (etc.) on vehicles that allow for T (or even S) rated tires can pose an upgrade to the vehicle’s performance and handling capability.
Increased risk of accidents or loss of grip
New tires on the rear. Better tires on the rear. Higher specs on the rear…We have to think about putting better tires at the rear, in general, because most drivers are better able to feel the vehicle beginning to understeer (loss of grip at the front) than oversteer (loss of grip at the back.) When a vehicle has mis-matched speed tire speed ratings, it is desirable for the higher speed-rated tires to be installed at the rear. (without forgetting to account for tire condition, load rating, and (especially) tread depth.)