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Dear SmartPlanet, Are 'miles per gallon' the key indicator of how green a car is? If not, what is? Sudeshna, Worcester, UK.
If you're considering a particular type of fuel and car, and want to limit yourself to one simple indicator, 'miles per gallon' (mpg) is a very good start, as mpg is closely linked to tailpipe emissions of carbon dioxide (CO2). But a more in-depth answer requires looking at other factors as well.
The difficulty arises if you want to compare the emissions from different vehicle types. For example, diesel cars generally have better fuel economy (mpg) than petrol, resulting in around 15 per cent less CO2 per mile. But diesel cars emit more nitrogen oxides (NOx) and particulates. Balancing the environmental impacts of lower CO2 (a climate change gas) versus more local pollutants is a complex issue.
The carbon content of the fuel is also a factor. Over 100,000 cars in the UK can run on liquefied petroleum gas or LPG. Although an LPG car uses around 30 per cent more fuel (by volume) than its petrol equivalent, CO2 emissions are reduced by around ten per cent due to the gas's very low-carbon content. As such, if you only consider mpg, you overlook the benefits of LPG.
To complicate matters further, so-called 'up-stream' emissions associated with fuel production also need to be assessed. In the case of some biofuels (such as bioethanol made from sugarcane or wheat), emissions from growing the fuel can be carbon neutral -- much of the CO2 emitted while you're driving is balanced by the crop absorbing carbon during growth. But like LPG, a bioethanol car's mpg figure doesn't look good at all.
How to compare different cars, then? Well, if you only consider one fuel type, then mpg is a very good indicator of a car's CO2 emissions and environmental impact. Miles per gallon will also give you some measure of other pollutants such as NOx and particulates -- though it won't be as good an indicator as it is for CO2, as these emissions are strongly dependent on engine build and manufacturer. Check out the VCA website for basic raw mpg and emission data for all new UK cars.
If you want to consider more than one vehicle type, then mpg is a less useful indicator for all the reasons above. In this case, the only real solution is to apply some form of 'external costing' analysis, which puts a monetary value on all tailpipe and up-stream emissions so that they can be compared. One online tool that does just that is WhatGreenCar?, which gives an overall environmental impact rating for UK cars registered since 2001.
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