This technical note relates to a paper by Lam (4), “Estimating Fuel Consumption from Engine Size”, which recently appeared in this journal. Using the average travel speed model and data primarily from official fuel consumption tests, Lam investigated the relationship between engine capacity and fuel consumption. However, it is difficult to isolate the effects of individual vehicle characteristics using a very aggregate model such as the average travel speed model. The approach used by Lam could be described as a correlation/regression approach. This type of approach can cause inconsistencies and anomalies when estimating fuel consumption, and may lead to errors in the interpretation of parameters. To determine the actual effect of any one vehicle characteristic, it is necessary to include all relevant characteristics in the fuel consumption model. Regression estimation of these effects, especially in aggregate models, may not result in reliable estimates due to multicollinearity caused by high correlation between the vehicle parameters.
AKÇELIK, R. and BIGGS, D.C. (1987). Estimating Effect of Vehicle Characteristics on Fuel Consumption. ASCE Transportation Engineering (Jan. 1987) Vol. 113 No.1, pp 101-106.