Normally, digital speedometers are found only in luxury cars and high-end motorbikes. Even if your motorbike has a mechanical speedometer, what will you do when it gets damaged? First, you need to replace the mechanical worm gear and then the cable. Anyway, we describe here how to build a digital speedometer cum odometer for your motorbike. The circuit uses a micro controller, an LCD display and some commonly available components. It is a better alternative to the mechanical speedometer and even a beginner with minimal skill level can assemble it. The features of the digital speedometer-cum-odometer are:1. Digital readout 2. Speed displayed in km/hour 3. Distance traveled displayed in kilometers 4. Readings saved in non-volatile memory (EEPROM) 5. Reliability due to use of the micro controller 6. No mechanical wear and tear 7. Home-brewed speed transducer/sensor 8. Self reset to zero after completion of 99,999.9 km 9. Easy to build and fix onto the bike Calculations: You first need to know the radius of the bike’s front wheel. The calculations here are based on Hero Honda’s Splendor model. The radius of the front wheel is 30 cm. (This can vary with the brand or model.) Circumference of the wheel= 2πr (where ‘r’ is in cm) = 2×3.14×30 = 188.4 cm or 1.884 meters Speed. Let’s assume that in 1 second the wheel completes one revolution. In other words, in one second, the bike has covered 1.88 meters. Therefore the speed in km/hour: N×1.88×3600/1000 = N×6.784 or N×6.8 where ‘N’ is the number of revolutions per second. ‘6.8’ is a constant and only ‘N’ varies; for example, if ‘N’ is 5, the speed equals 5×6.8= 34 km/hour. Distance. The odometer is updated every 100 meters. To cover 100 meters, the wheel is required to make approximately 53 revolutions (100/1.88). The micro controller takes care of the tasks of revolutions counting, speed calculation, conversion and display of results.