And I'm not just talking about writing this article. You see, I couldn't decide what to write about this month, as I had so much from which to choose.
I also wanted to avoid anything too complicated like last month's column on car body styles, which was a huge topic to try to tackle in one go, so this month I decided to be more specific.
Whilst reading Road Test Editor, Dave Humphreys', review of the new Audi Q7 I read something that I had known already - bigger wheels can push a car into a higher tax band due to higher emissions. However, I had never really fully investigated why this is. Therefore, I chose this as my study topic for this month and, thus, the subject of this car class 101.
Going from 20- to 21-inch wheels on an Audi Q7 will cost an extra €180 a year in motor tax, due to the extra emissions generated by larger wheels. But, what causes that extra inch to burn so much more fuel? Ok, pay attention, now for the science bit: petrol fuels consist of compounds of carbon and hydrogen and when they burn with oxygen in an engine, they release carbon dioxide. I use the term 'petrol fuels' because this includes diesel, as diesel is a derivative of petroleum via a special distilling process and is therefore also made of carbon and hydrogen compounds. Simply put, the more fuel used the more carbon dioxide (CO2) created. The more CO2 emitted the greater motor tax paid.
Fuel efficiency is the measure of an engine's ability to get energy from fuel. The more energy it can get the better. This contributes significantly to a car's mpg rating - though there are other factors. So in summary, the more fuel a car burns, the less efficient it is, the more CO2 it emit, the higher the motor tax. Got it? Easy so far, right? Ok, back to the tyres.
The main way tyres affect fuel efficiency and CO2 production is through rolling resistance. What's rolling resistance? Well, the clue is in the name; it is how much resistance a tyre gives to rolling. The more something resists the more energy is needed to force it to move. Low resistance tyres resist less and therefore need less energy (in the form of fuel) to force them to roll. High-resistance tyres need a lot more fuel to get them moving. Have you ever seen a car tyre under pressure as it moves? It is in a constant cycle of deformation and reformation, known as hysteresis. A tyre uses more energy when during deformation than reformation. This energy loss converts into heat and dissipates from the tyre and more fuel is expended to deform and reform the tyre time and again.
You still with me? It's physics baby! The less hysteresis the less energy loss the less fuel needed. Oh, I do love a good simple relationship like that. Now, obviously tyre surface will affect how much a tyre deforms and recovers, as will tyre material, tread depth and tyre pressure. For example, low profile tyres, those with an aspect ratio of 45 and 50, have a shorter sidewall, are therefore stiffer and exhibit less hysteresis. However, they are a wider tyre, which means more surface contact, which creates high resistance. However, you can offset this somewhat by pairing low-profile tyres with lightweight alloy wheels.
But tyre size? Well, yes, of course size matters (snigger). In general, the bigger a tyre the more it weighs, and the more weight something has the more energy is needed to move and stop it. Energy in cars equals fuel. So therefore, larger wheels need more fuel to move and stop, using more fuel creates more CO2 emissions, which increases motor tax premiums. Simples!
But, a word of advice, it's not a good idea to change wheel size outside of manufacturers' recommendations, as bigger or smaller wheels could interfere with a car's transmission and gearing.
So let's recap, we were examining what causes that extra inch in wheel size to burn so much extra fuel? Well, the bigger a wheel is the heavier it is. Heavier objects need more energy to get them moving and stopping. In cars, energy comes in the form of fuel; therefore, bigger wheels need more fuel. Bigger wheels also have higher rolling resistance due to more hysteresis energy created and lost, and the engine needs more fuel to create this energy. The more fuel that is burnt, the more CO2 created and the more motor tax paid. Furthermore, the more fuel burnt by a car the less energy efficient it is therefore the lower the mpg figure.
So, what should you do when your car comes with massive 21-inch alloys? Well, tyre manufacturers are now producing energy efficient tyres that could cut a car's energy consumption by five to seven per cent and save 4,000km of fuel over the life of four tyres. Not bad, eh?
Now, how's that for a motoring and economics lesson all in one?
Class dismissed.