HELP DESK: MICHAEL McALEERanswers all your motoring queries
From From G Reynolds, Co Wicklow: There is a lot of mystery about CO2 emissions which goes right down to the motor tax bands and vehicle registration tax (VRT). This is just simple Leaving Cert chemistry. There is a simple relation between fuel consumption and emissions.
For a petrol car to get a good approximation of CO2 emissions in grammes per kilometre:
a) divide 6,600 by mpg (eg a car doing 30 mpg equates to an emission of 220 g/km) or
b) multiply the consumption in litres/100km by 24 (this comes from the “2.39” in the second line)
For example, a Renault Clio quoted as 64.2mpg gives 103g/km with the formula, but the published figure is 117g/km. This is because the fuel consumption tests are carried out under controlled conditions by expert drivers but the CO2 figures are averages for typical driving conditions.
The variations between quoted figures are normally described as experimental errors and are not significant for typical car drivers and driving.
So emissions are not some mysterious function of the car’s “eco efficiency” as the Greens would have us think, just a proxy for the plain old fuel consumption.
Then why does a car with twice the fuel consumption attract almost four times the tax? Furthermore, we are already paying the carbon tax through the petrol tax: if we drive more we produce more CO2. If we stay at home, we produce no CO2 from petrol but still have to pay the motor tax, so it’s actually an incentive to use the car.
We’ve shown your ready reckoner to an industry expert with far more knowledge of such matters than ourselves and, while he says it is a useful rule of thumb, he still believes the industry figures are more accurate representations.
For a start, while burning a litre of fuel will produce a given amount of CO2, emission figures must also take into account the efficiency of the engine and vehicle tested and the distance travelled to produce the CO2/km figure.
This figure is produced as a result of a controlled test drive on a chassis dyno following strict preparation and using a set driving pattern commonly referred to as the NEDC.
The fact modern vehicles operate fuel cut on overrun means even though the vehicle is covering distance, it will not always be consuming fuel and therefore it’s CO2 output will vary depending on the type of driving.
It’s true that all these figures don’t stand up to real-life scrutiny, but the conditions of the tests represent a level playing field for all manufacturers. “All things being equal” is a common approach adopted by the industry. There are plans to review the test procedures.
In terms of taxation the reality is the bands applied for tax purposes in Ireland are entirely arbitrary and set to protect tax revenue as much as promote efficient motoring. Trying to find some logical reasoning in tax policy may be a surefire way to insanity.
For example, our emissions bands don’t match those of much larger markets. Ultimately car firms will spend millions tuning their engines to get the best results for a market like Germany or Britain before they try to maximise their potential in an Irish market that might peak at 55,000 new cars this year.
From Michael Pegum: In my present car (a petrol-engined Mondeo), even at city speeds, I sometimes have difficulty hearing what my passenger is saying above the noise of the wheels on the road. Theres no problem when at rest, so it’s not the engine. The problem may be partly due to my hearing, but I would like to get a car with better noise insulation.
This issue is coming to the fore and new rules on tyre noise have been introduced. They are due to come into effect in Ireland at the end of the month. Bascially all new tyres sold from then should carry an S-mark, to sybolise a low-noise tyre.
The problem has been that you can reduce noise levels, and lower resistance to improve fuel consumption, but early versions of these tyres were never as good under heavy braking on wet roads. This has largely been rectified and the major brands already sell S-marked tyres. The mark may not be that easy to spot but you should ask for S-mark tyres when buying them.
Send your queries to Motors, The Irish Times, Tara St., Dublin 2 or email motorshelp@irishtimes.com