Intelligent cruise control with traffic preview introduces a potential to adjust the vehicle velocity and improve fuel consumption and emissions. This paper presents tradeoffs observed during velocity trajectory optimizations when the objective function varies from fuel-based targets to emissions-based. The scenarios studied consider velocity optimization while following a hypothetical leader executing the federal test procedure (FTP) velocity profile with distance constraint, instead of the classical legislated velocity constraint, to enable the flexibility in optimizing the velocity trajectory. The vehicle model including longitudinal dynamics, fuel consumption and tailpipe NOx emissions is developed for a medium-duty truck with a diesel engine and verified over the FTP. Then, dynamic programming is applied on a reduced-order model to solve the constraint trajectory optimization problem and calculate an optimal vehicle velocity profile over the temperature stabilized phase (Bag 2) of the FTP. Results show 59% less tailpipe NOx emissions with an emission-optimized drive cycle but with 17% more fuel consumption compared to a non-optimized baseline. Whereas, a fuel-optimized cycle improves the fuel efficiency by 18% but with doubled tailpipe NOx emissions. Moreover, it is shown that for a diesel powertrain, including the after-treatment system efficiency associated with the thermal dynamics is crucial to optimize the tailpipe NOx emissions and can not be ignored for problem simplification.