?How accuracy During usage, if need assistance, pls contact with me anytime. As usual, it’s 2-5MPH/KMH off from the car engine speed. The display speed could be calibrated by hand also; First time usage, it takes 2-5 minutes to receive GPS signal; the device doesn't have rechargeable battery, it need plug to car adaptor; the device don't have on/off button, it can be on/off automatically. ?What other functions Over-speed alert. Default setting alert value is 120. Adjust the over-speed setting, once over value, it will give a warning. If want to off speed warning, just adjust the over-speed value to a very impossible high value. ?What’s the speed unit KM/H and MPH. Long press the Enter Button, if you see number 100 shown on the screen, which means the speed unit is KM/h, if you see number 161 shown on the screen, which means the speed unit is MPH. ?What does the GPS HUD display Car driving Speed, Single Driving Time and Mileage. Single Driving Mileage is the mileage from ACC is on to ACC is off. If you want total Driving Mileage - Odometer, please choose model C80 or C90. ?What’s GPS Speedometers Collecting speed from GPS satellites. No relation with the car engine, therefore, it’s 100% universal for all vehicles include car, truck, pick-up, SUV, scooter, train, motorcycle, bike, boat, golf cart..