The break even point is around 15 MPH. That's the speed at which aero drag passes all other factors combined that are typically holding you back. As you accelerate beyond 15 MPH, aero drag becomes progressively more and more significant.
If you typically ride 10 MPH or less, you might be able to sense a speed difference in low drag tires. If you are able to ride in the middle to upper 20's on the flat, the rolling resistance factor, while still present, will become such a small fraction of the drag factors holding you back as to become insignificant.
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