Speed variations are considered as an alternative for reducing greenhouse gases emissions during the use phase of passenger cars. In this paper, we present a real driving evaluation of two conditions of speed variations: 50 to 70 km/h and 90 to 110 km/h. The experimental evaluation shows that a reduction in fuel consumption, consequently in greenhouse gases emissions, is achievable with a conventional gasoline passenger car, with no adaptations for realizing speed variations. Numerical simulations based on a backward quasi-static powertrain modeling are used to evaluate the potential of speed variations for reducing fuel consumption in other speed variation conditions. When deceleration is performed with gear in neutral position, speed variations are always correlated to a lower fuel consumption. This was suspected through previous numerical tests or evaluation on test bench but not in real driving conditions.