This paper presents realistic traffic simulations in four representative regions in Chicago, using real map data and historical traffic statistics, to estimate the amount of fuel that can be saved by two types of systems, namely, a Green Light Optimal Speed Advisory (GLOSA) system and an Eco-Routing system. In particular, two previous systems called GreenDrive and GreenRoute are selected to estimate how much fuel they can save in a year, assuming they enjoy the same popularity as Google Maps. Evaluating intelligent transportation systems in simulation has attracted significant research efforts as large scale real-world experiments are often too expensive to carry out. However, simulation-based evaluations pose serious questions on how real the simulated environments are. In this paper, we resort to several data sources from the city of Chicago to quantify the approximation in simulated traffic, so as to convincingly derive the amount of fuel savings on a larger scale. Specifically, we create SUMO simulations of four real regions of Chicago, and utilize traffic counts data, road speed data, and taxi trips data to validate each simulation scenario. In the end, we show that by using our previously proposed systems, the estimated fuel saved in a year can be 17.6 million gallons for the entire Chicago area.