To achieve high-volume code capacity for SAW-based radio frequency identification(RFID) tags, it is very important to improve the delay time resolution. An efficient encoding method is to use the phase delay of the carrier wave in the pulses, but one has to solve the issues of the phase ambiguity at unknown temperatures and the location of reflectors to exact positions. In this paper, a method is proposed to obtain a high-phase delay resolution by measuring group delays and constructing a certain restriction on the exact positions of reflectors. To define the restriction parameter for a SAW RFID system with large code capacity, it is imperative to have a priori knowledge of the errors of the phases and group delays. The experimental and simulation errors for both the phases and the group delays, originated from the design procedure, the temperature effect, the fabrication process and the measurement, are presented. The error probability distribution curves in simulation and experiments are plotted. The maximum error of phase delay is about +/- 14 degrees, and the maximum error of group delay is about +/- 4 periods. The temperature range in investigation is from -5 to 45 degrees C.