The connection between low rolling resistance tires and fuel consumption is now well established and has been successfully modeled. Previous studies have shown that fuel savings can be predicted by an empirical law tying the difference in rolling resistance force to a difference in fuel consumption while still accounting for the resulting engine efficiency change. However, accurately evaluating fuel savings in real conditions remains a crucial issue, as test uncertainty is generally close to the order of magnitude of the consumption difference. This paper proposes an analytical approach to measuring fuel economy. The test is conducted on an outdoor track using two identical vehicles at a single stabilized speed. The vehicle positions are measured by GPS devices and the absolute fuel consumption of each engine is measured with flowmeters. Specific attention is paid to numerous external parameters that influence fuel consumption, in order to minimize test dispersion. Examples of tests using the approach developed here are given for each vehicle category. Each test situation is then reproduced using AVL CRUISE® to validate the vehicle models. Based on these models, extensive simulations on different usage conditions were completed, the results of which establish that savings observed on stabilized speed test would be close to those seen in real driving conditions, confirming the weak dependency of the tire effect on vehicle use. Finally, when compared to the test results, fuel savings predictions using both the vehicle models and the previously developed empirical law appear to be very consistent.