Time speculation has been widely used to achieve high performance in modern design as it exploits average-case timing optimization instead of worst-case timing optimization focusing on reducing longest path delay which rarely happens. Variable-latency design (VLD) style is one research category of time speculation. Since process and environmental variations are hard to predict, traditional variable-latency units (VLUs) designed at presilicon stage will suffer significant performance loss due to pessimistic assumptions for addressing variations. In this paper, we propose a novel sensor-based, transition-aware VLU (S-VLU) scheme adapting to process-voltage-temperature (PVT) variations by using in situ sensors to obtain real-time transition information in a circuit. Moreover, we also propose a sensor deployment strategy to achieve near-maximal performance gain. On average, the S-VLU achieves a 31.27% performance improvement as compared to a 19.26% improvement by using traditional HL. The area overhead of the S-VLU is 13.48%. To the best of the authors' knowledge, this is the first wok to address PVT variations in VLD style.