Recent legislations across the globe have introduced tire labeling requirements to include a rating of tire rolling resistance. While the tests used for rolling resistance measurements are standardized, there is considerable debate whether they represent true on-road driving conditions. Studies have shown that factors such as outdated drum curvature correction, and inconsistencies in drum surface material, among others, can result in the lack of more physically meaningful results. In this work, on-road data are collected using a fully instrumented vehicle and then used for estimating tire rolling resistance using first principles. The results are then compared to indoor rolling resistance measurements obtained from a conventional drum machine as well as a flat belt machine. This comparative analysis of indoor and outdoor testing highlights the differences between laboratory and real-life measurements as well as puts the current legislation in perspective.