This document discusses, among other things, systems, devices, and methods for detecting or classifying tachyarrhythmias or making a therapy decision. In one example, a rate-dependent threshold is used for comparing atrial and ventricular rates for classifying a tachyarrhythmia as a ventricular tachyarrhythmia (VT) or a supraventricular tachyarrhythmia (SVT). In another example, the classification uses an atrial rate cutoff value, a ventricular rate cutoff value, or both. In another example, a tachyarrhythmia detection is tested over a time window with a duration that is automatically adjusted as a substantially continuously monotonically decreasing function of duration vs. rate. These techniques improve the specificity of arrhythmia detection or classification, allow anti-tachyarrhythmia therapy to be better tailored to the particular tachyarrhythmia, or provide more automatic operation making it easier for a physician to use an implantable device.