The maximum speed of a bike with a 125cc engine can vary depending on several factors, such as the bike's design, weight, aerodynamics, gearing, and engine performance. Typically, most standard 125cc motorcycles have a top speed ranging from 55 to 70 miles per hour (88 to 113 kilometers per hour). However, some sportier or more advanced models might have slightly higher top speeds, reaching up to 75 to 80 miles per hour (120 to 128 kilometers per hour). It's important to note that these figures are approximate and can vary between different bike models and manufacturers. Additionally, factors such as road conditions, rider weight, and wind resistance can also affect the actual top speed achieved in real-world riding situations.