On-road real driving emissions (RDE) testing was used to study the effects of many parameters not taken into account by laboratory testing but which can greatly influence vehicle pollutant emissions. Among these, road grade plays a key role. This paper investigates the effects of positive and negative road grades on the exhaust emissions of two Euro 5 diesel vehicles. Both vehicles were tested in the laboratory on the chassis dynamometer and on the road along a mixed route in Naples (Italy). The circuit includes urban, extra-urban and motorway driving in order to comply with Euro 6 RDE requirements. Laboratory testing aimed to assess emissions during current type approval driving cycles; on-road tests using portable analysers were used to determine real emissions as a function of road gradient. Vehicle Specific Power (VSP) analysis revealed that with respect to positive road grades, negative ones explain a higher share of total VSP. For −4% to +5% road gradients, the percentage variation of carbon dioxide emissions is linearly correlated with variations in slope. The same correlation for nitrogen oxides is vehicle dependent and well explained by a second-order polynomial function.