Speedometers for bicycles are pretty accurate.
I have a speedometer on my bicycle and it uses a magnet that registers the revolutions of the rim on the bike which tells the speedometer how fast you're going.
They may not be completely accurate but they are pretty accurate.
My speedometer for the bike also has a mileage counter that tells me how many miles I go on the bike and so far I've racked up 500 miles in 3 years of bike riding since I bought and installed the bike speedometer.
I would recommend getting a bike speedometer as it's a very cool thing to see how fast you're going on the bike and how many miles you're riding the bike.