Whether you're planning on turning your hydrostatic lawnmower into a competition mower, or you just want to get your grass mowed at a better clip, it is possible to increase the speed on your mower. Some steps are easier than others, but if speed is really your goal, there are several things you can modify on your riding lawnmower to bring out the muscle car it has deep inside.
Check your air filter and replace it with a new one if it's dirty. Maximum air flow is very important for getting the engine to run at its best, and to get the best speed.
Examine the muffler on the tractor and make sure there aren't any additional holes that weren't there when the muffler was put on. These holes in the muffler can lead to a drain on power and it can sap speed. If there are holes and leakages in the muffler than remove it by unbolting the clamps that hold it in place and install a new, fully functioning muffler.
Remove as much weight as possible from the mower. The casing over the engine isn't strictly necessary, but by removing it you are sacrificing protection of your motor. If you have a side kick for catching grass clippings it is adding weight and you can remove that too. In extreme cases, if you are using the mower for racing rather than yard maintenance, remove the blade assembly to get more speed out of the mower.
Pour a higher octane fuel into the gas tank. Higher octane fuel burns faster and creates more power for the engine to use. It is more expensive, however, so be sure that you really have a need for speed.
Remove the governor from your lawn tractor. The governor is a device that is made of white plastic that limits the amount of power your engine can put out. If you can't readily find it, look in the manual to see where the governor is located. Remove the bolts or screws that hold it in place and unhook it from the engine. Once removed, the cap on your engine's output will be gone, providing more power -- and speed.