The maximum speed achievable by a bike with a 125cc engine can vary depending on various factors such as the bike's design, weight, aerodynamics, and engine performance. Generally, a 125cc motorcycle can reach a top speed of around 60 to 70 miles per hour (96 to 113 kilometers per hour). However, some high-performance 125cc motorcycles may have a slightly higher top speed, potentially reaching speeds of up to 75 miles per hour (120 kilometers per hour) or more. It's important to note that these figures are approximate and can vary between different models and manufacturers. Additionally, external factors such as road conditions and rider weight can also influence the actual top speed achieved.