Compass work needs to meet the following 3 conditions: 1. GPS positioning successfully. 2. The compass can only work normally while the vehicle is in motion. 3. Driving your car, the compass direction is the car driving direction. Seller's reply. ?UNIVERSAL FOR ALL VEHICLE?Collecting speed from GPS satellites. No relation with the car engine, therefore, it's 100% compatible for all vehicles include car, truck, pick-up, suv, scooter, train, motorcycle, bike, boat.. ?ALARM FUNCTION? Over-speed Alarm and fatigue driving alarm to ensure your safe trip. Default setting is 120KM/H or 120MPH. You can adjust it according to your needs. When the product detects speeding, it will beep to remind the driver. ?BIG AND GREEN COLOR FONTS? big screen size display make more easily and comfortable to read. Big and green color fonts, green light will let the driver more compfortable and protect the drivers' eyes, specially for long distance driving. ?MORE SAFETY?1. The shell is made of high temperature resistant material, which will not deform at 80 ? for 3 hours. 2. No built-in battery, no explosion ,at high temperature. ?TIMELY AND ACCURATE ?Speed and compass data is from satellites, timely and accurate.In particular, the direction of the compass is not affected by the magnetic field. ?MULTI-FUNCTIONS? This GPS speedometer HUD not only displays the vehicle's driving speed in time, but also displays the driving direction by compass, Single Driving Time, Single Driving mileage, Let you know the key informations for your trip.