What to Do When the Speedometer Doesn't Match the Actual Speed?
4 Answers
Speedometer not matching the actual speed is a normal situation. Here is a detailed explanation of the reasons why the speedometer may not match the actual speed: 1. For safety considerations: To prevent drivers from speeding and overtaking, and because the vehicle's speed measurement device may have a delay, the displayed value is often higher than the actual value to prevent sudden acceleration and potential danger. 2. Calculation errors: 0<= indicated speed - actual speed <= actual speed/10 + 4km/h. This means the displayed speed cannot be less than the actual speed, and it cannot exceed the actual speed by more than 10% plus 4km/h.
Last time I was driving on the highway, something felt off—the speedometer shot up to 100 mph, but the GPS showed only 85, which scared me into slowing down immediately. This issue is quite common. First, check your tire condition—uneven tire pressure or changes in wheel size can cause the speed reading to be faster or slower. I usually use a tire pressure gauge to check all around and see if the wheels have been fitted with aftermarket parts. If the tires are fine, try comparing the actual speed with a mobile GPS app. A significant discrepancy likely means the vehicle speed sensor is dirty or the wheel speed sensor is malfunctioning. Don’t delay—safety comes first. Large deviations can easily lead to accidents or speeding tickets. It’s best to visit a reputable auto repair shop as soon as possible to calibrate the instrument with professional equipment. During routine maintenance, remind the technician to inspect this part—spending a little now can save big trouble later.
As a car enthusiast, I have considerable experience with instrument errors. Speed inaccuracy often relates to tire modifications: if you install larger rims or tires with different aspect ratios, changes in wheel circumference can cause sensor misjudgment; or it could be due to dust accumulation on wheel speed sensors or aging wiring, leading to data deviations. During diagnostics, I use an OBD scanner to check the speed signal and rule out ABS interference. For practical verification, I recommend comparing with a reliable GPS device on highways—if the error exceeds 5%, it should be addressed. Daily maintenance should adhere to the original tire specifications, and unauthorized rim modifications should be avoided. Calibration at a 4S shop is usually quick and inexpensive, and neglecting repairs may increase fuel consumption and wear.
Speedometer deviation is too dangerous. I noticed the displayed speed is slower than actual while driving, potentially leading to unknowingly speeding. The most common cause is tire issues: low pressure or excessive wear affecting measurement accuracy. It's recommended to immediately pull over to a safe area and verify actual speed using a mobile navigation app. If the deviation is significant, contact maintenance as soon as possible. Don't ignore it, as it can cause safety accidents or fines. Develop a habit of checking tires monthly and testing the speed system during maintenance.
