Normally a person running an 8 minute mile on a treadmill at 0% of incline will run a slower outdoor mile. This is what we have found from testing some people during events that run at our YMCA. Not everyone will have the same thing, but most of our people say that they are slower outdoors than on a treadmill. But, it is tough from me to feel an 8 minute mile pace compared to a 8:30 mile pace outside. So, if I setup the treadmill for that pace, I run it.
You may also have a sensor going out of the treadmill. We have that problem from time to time, but we replace that sensor as soon as we know it is failing.
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2007 Jamis Ventura Comp
2006 Jamis Explorer 2.0
2000 Specialized Hardrock (bought used)
Swim, Bike, Run and sounds like fun