The distance is pretty much one of, if not, the most important stat you want about your runs. It's easy on the road because you can use an app to measure the distance you've run and you can be sure of it's accuracy. But on a treadmill you rely totally on the console giving you the right reading. You may have a race you're training for or some interval training you're doing.
Distance accuracy is something you want to be confident is right. You need it accurate to the meter or yard hopefully. If you are doing some interval training by distance you need to know when that half mile or 500 meters is up and you can do a recovery again. For race distances you need to know when you've achieved it. So here I look at how accurate treadmills are on distance.
How the distance is measured
The treadmill knows how long the belt is, every time it passes a sensor near the motor it measures one revolution. This way it knows how far you've walked or run. Treadmills are quite sophisticated and you can usually choose between kilometers and miles. You can trust that the distance on treadmills is accurate.
Measuring the distance is not difficult for the treadmill and is quite rudimentary when you think about it. That's what makes it so accurate is because it does measure the actual distance you're covering by using the length of the belt. Other measurements especially the calories burned may not be very accurate at all.
This way you can be confident in training on a treadmill if it's for a race while the weather is bad or to do your speed work. The time is obviously accurate as there are very little inaccuracies in modern electronics when it comes to time. If you're running for 3 miles you will actually be running for 3 miles on a treadmill.
In order to keep the accuracy it is important that you maintain the treadmill. This is especially the case for the belt as this is what the distance is measured from. You need to lubricate the belt every 3 months or so and is quite easy to do. If you're worried about the speed then that can be calibrated as well.