17.4 Comparing fuel efficiency of individual models over time
Car models do not stay the same for long. Manufacturers continually develop and change their cars. By grouping on the characteristics of each model in the dataset, including number of gears and displacement (engine size in litres), those that have been tested in most years can be selected. Two have been chosen here in both manual and automatic versions, the Toyota Corolla (1993-2016) and the Mazda MX-5 (2006-2021). They are two very successful models in their respective classes. Figure 17.8 shows the estimated city, highway, and combined mpg figures for the Toyota. Note that the estimates are only reported in rounded integer values. Unrounded values for all models are provided from 2011 on but have not been used here.
Figure 17.8: Fuel efficiency of Toyota Corolla models for manual transmission (lines with points) and automatic (lines) transmission
As expected, the highway mpg figures are better than the city figures. Manual transmission is generally slightly more efficient than automatic transmission. There were improvements in the first few years, then a level performance and then, surprisingly, a decline. Perhaps the cars became heavier. Unfortunately there is no information on car weights in the dataset. Unsurprisingly, there is some variability in the EPA’s estimates. The introduction of new model versions in 1995, 2000, 2006, 2012 does not appear to have had immediate effects. Additional changes may have been introduced in other years.
The estimates for the Mazda MX-5 are shown in Figure 17.9.
Figure 17.9: Fuel efficiency of Mazda MX-5 models for manual transmission (lines with points) and automatic (lines) transmission
The MX-5 estimates are less variable than the Corolla ones. The large jump in 2016 was due to the introduction of a new model. The manual and automatic transmission models appear to achieve very similar fuel efficiency.
Answers Small cars get higher mileage in general. Hybrids and newer cars get better mileage than other cars. Sector classifications give some surprising results for luxury cars. SUVs have mostly replaced station wagons.
Further questions How does fuel efficiency vary between city and highway driving? How does displacement influence fuel efficiency?