The top speed of a 49cc dirt bike can vary depending on several factors such as the bike's make and model, its weight, the terrain, and the rider's weight and riding style. Generally, a 49cc dirt bike can reach speeds between 25 to 40 miles per hour (40 to 64 kilometers per hour). However, it's important to note that these speeds are estimates and may differ in practice. Additionally, factors such as wind resistance, surface conditions, and mechanical modifications can also affect the bike's top speed.