Have you ever passed one of those auto-radar signs in a construction zone that displays how fast you're going? Usually, it'll display a slower speed than what's showing in your car. Ever wonder, what's up with that? Here's the low-down.
The US federal regulation on lcd speedometer accuracy reads: Each bus, truck, and truck-tractor must be equipped with a speedometer indicating vehicle speed in miles per hour and/or kilometers per hour. The speedometer must be accurate to within plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph).
So according to this you could be actually going faster than your speedometer is reading, and still be within federal regulations.
HOWEVER, regulations on this in the UK say that vehicles absolutely cannot allow you to go faster than your speedometer is reading. To ensure this regulation is met the UK allows 1-6 mph to be added to a car's displayed speed on it's speedometer.
If we combine these two regulations, you end up with a car manufacturer standard of calibrating speedometers to show you are going faster than you really are, by about 3-4 mph. It's my guess they do it this way so they meet both US and UK regulations with one standard.
So, chances are if you are looking at 70mph on your car's speedometer, your actual speed is probably closer to 67mph. This may explain why that State Trooper didn't pull you over for going 58 in a 45.
The only variable that throws things off a bit is Tire Size. All vehicles are calibrated using the standard factory sized tires for any particular model. The larger the tire you put on a vehicle (larger than the factory standard), the closer you'll get to showing actual speed or possibly going faster than your needle reads.
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