Or else use someone's GPS to give you a speed number to compare to your in dash speedo. I've had some success disassembling a speedo, cleaning with a small paint brush and lubing the pivot points with a drop of wd-40 on a toothpick. No need to break rivets on the face, just wear gloves as the paint on an old speedo face can easily pick up oils from your skin or get smudged. Clean and lube the pivot points for the needle and the rotator mechanism driven off the cable, plus any dirt or dust from the weight assembly attached to the needle. Work carefully here to keep from messing up your dial face as wd 40 can ruin or stain the face paint.