New Car verses Old Car

As the economic climate collapses around us, many people are putting off their purchase of a new car and making do with what they already have. Old cars are bad and polluting, new cars are clean and efficient right? So this is a bad thing?

Not necessarily. The SMMT say that the average new car generates 1 tonne of CO2 to build (lets ignore the raw material / recycling arguments for the moment). We also know from the DfT that the average car in the UK travels about 10,000 miles per year.

Let’s say that you haven’t had to sell your children into slavery as a result of the credit crunch, and in fact you’re on the market for a new Mini. Well, let’s just imagine that those people still exist.

BMW as a company has become one of the leaders in the field of fuel efficiency improvement with their so called “mild hybrid” systems. The calculation is shown below:

Carbon Credentials of Minis

Carbon Credentials of Minis

The conclusion then is that to make the CO2 Savings for the base model MINI you have to drive more than 62,000 miles. So after 6 and a bit years you will have made a positive impact on the environment by buying a cleaner car.

So what about the fuel savings? The calculation for the mpg (fuel economy figures) are below as well:

Fuel Credentials of Minis

Fuel Credentials of Minis

Given that the average spend on fuel per year per car is about £1,000, how long would it take to save 10% of that, or £100? Well, 16,000 miles on the base model MINI.

Considering that a base model MINI retailing at about £11,000 and will lose just 15% of it’s value (£1,650) in the first year (it’s the slowest depreciating car you can buy currently), then saving £60 in fuel is pretty minimal.

2007 Mini One

2007 Mini One

Obviously if you decide to replace your 30 mpg petrol car with a similarly priced second hand 40 mpg petrol car, then you can expect to save about £340 and 880kg of CO2. Which is not to be sniffed at, especially when you consider the further savings in tax, insurance and so on.

So should you buy a new car for environmental reasons? Probably not. Buy a smaller second hand car instead. Or learn to drive more efficiently.

2008 Mini One

2008 Mini One

Should you buy a new car to save money on fuel? Definitely not. You’d have to madder than a box of frogs.

Should you buy a new car because you want to? Of course, that’s one the delights of living in a free country. In fact at the moment you’ll probably get a very good deal indeed as most car markers are looking down the barrel of bankruptcy. But just don’t try to justify it for money saving or environmental reasons…

Speed-bump, road hump, speed ramp or sleeping policemen. Let’s face it by any name they are the bane of urban driving (unless of course you make your living selling replacement shock absorbers). They’re a hassle, damage cars and cause urbanites to drive otherwise unnecessary 4x4s (in my experience anyway). Yes, average speeds are reduced, but so is your attention of what’s up ahead as you’re trying not to clout your exhaust on that unnecessary piece of road calming. Other complaints made against them are that they slow down emergency services (endangering lives), they are particularly unhealthy for people with back or neck pain and create noise for local residents.

Speed Humps

Speed Humps

But I can add another one, that they’re giving the poor Polar Bears an early bath. Yup, that speed bump round the corner on Mornington Close is directly responsible for the increasing popularity of bear-centric swimming lessons at the North Pole. So let me tell you how….

The AA (Automobile Association) did a bit of research on fuel consumption for cars at the Millbrook Proving Ground and found that a typical mid-size car running at a constant 30 mph did 58 mpg. However a car slowing down and speeding up for speed bumps only did 31 mpg. Put that into carbon figures* for a petrol car** and you get 113 g/km of CO2 at a constant 30mph, or 211 g/km over speed bumps. So a kilometer of road with speed bumps creates an extra 0.1kg of CO2 for every single car that travels it.

For argument’s sake, lets say this km of road has on a average a car going each way down it every minute during the day and night***. In a year, that means a single stretch of road creates an extra 103 tonnes of unnecessary CO2 per year. We’re making estimates here, but if that km of road has 10 of the UK’s estimated 100,000 speed humps, then speed bumps in Britain are responsible for a colossal 1.03 million tonnes of CO2 per year. That’s about the same as the total carbon footprint of the 830,000 residents of Fiji, or almost twice that of Greenland. Ouch.



You can console yourself that as a UK taxpayer, based on the estimate that 50 standard humps on three or four connecting residential streets costs about £150,000. So the UK speed hump population cost us £300 million pounds. Which is about what Newcastle United would cost you if you wondered up to St James’ Park with a really big chequebook this morning.

*Clear research

** A diesel car is actually worse with 128 v 240 g/km

*** More in the day and less at night of course

More information here.

A Toyota Prius looking smug yesterday

A Toyota Prius looking smug yesterday

Hollywood loves the Toyota Prius for the green image it purveys to celebrities who have a carbon footprint several times that of the average citizen. Middle England loves the Toyota Prius because over a dinner party they can tell their neighbours they drive a hybrid car. I even like the Toyota Prius because it has raised awareness of hybrid technologies to the world, and brought them into the public consciousness. But is it as green as Toyota claim?

Well, the official CO2 figure for the Prius is 104 g/km, which is equivalent to 62.8 mpg (although Toyota actually claim 65.7 – someone in Japan has their maths wrong there). Now DEFRA in their guidelines to the carbon offsetting quality standard (which is soon to be released) says that reported g/km “factors must be uplifted by 15% to take into account ‘real-world’ driving conditions”. This is not Prius-specific, but does admit that even the government doesn’t believe the CO2 figures they use to tax us on.

With this uplift, the Prius would now be at 119.6g/km (54.6 mpg) which on the face of it seems more reasonable. In independent tests the Toyota has also failed to live up to its claimed efficiency, with What Car (1) recording 52mpg, The Sunday Times (2) recorded 48.1 mpg on a long journey and owners typically reporting between 53 & 54mpg. This gives us a worst of 48.1mpg and a best of 54mpg. Translating that into CO2, that’s 121 to 136 g/km of CO2. So not bad, but still 16-31% higher than Toyota claim (104 g/km) and very misleading.

It’s also worth remembering that a hybrid engine is only beneficial for lots of stop start driving. Running at constant speed there is no benefit over a normal engine, and in fact a disadvantage because you are carrying the extra weight of unused batteries and motor. It’s also worth noting that fuel consumption (and hence CO2) is all about your driving style and not what you drive. 91mpg was achieved by a chap called Will from Warwick Uni (3) in a VW polo Bluemotion, in excess of the claimed 74.3mpg and that included traffic on the M25. A Prius will also cost you more than an equivalent non-hybrid car, and the extra money you spend will take a long time to recoup (if ever). And if you were thinking about buying one to replace your old car, remember that means that somewhere another vehicle needs to be manufactured with all the raw materials that entails.

So, if you want to show people you care about the environment, enjoy looking smug at dinner parties or star in Hollywood blockbusters, then the Prius is the car for you. If you simply want to save fuel & money, lower your carbon footprint and actually do something for the environment, then improve your driving style and follow the example of Will from Warwick.




Source: Clear analysis

Most manufacturersare now obliged to report figures now for CO2 as well as MPG. Its pretty obvious thatthe more fuel you burn for any given distance, the higher the CO2 and the lower the mpg. What most people don’t realise is that there is an exact formula exactly linking the two figures.

It’s far more accurate to measure how much CO2 your car produces using the MPG, because manufacturer figures are always very unreliable. For example a vicar who bought a car rated at 40mpg and does lots of long journeys at 50 mph is likely to get 50+ mpg, whereas a boy racer who spends most of his time with the front wheels spinning is closer to 30 mpg:

  • 30mpg is equivalent to 218g/km (Petrol) & 252 g/km (Diesel)
  • 40mpg is equivalent to 163g/km (Petrol) & 189 g/km (Diesel)
  • 50mpg is equivalent to 130g/km (Petrol) & 151 g/km (Diesel)

But hang on – doesn’t this mean Diesel is less environmentally friendly than petrol? Surely that can’t be true?

Well it is and it isn’t. Every litre of Petrol you burn creates 2.31 kg of CO2, whereas every litre of Diesel produces 2.68kg. However diesel cars tend to be more efficient (because they can run a higher compression ratio mostly) and this offset’s the difference because less fuel is burnt in the first place.

So there you go. And to make it a little bit easier, below is a handy graph which allows you to translate easily between mpg and co2 for any vehicle.

MPG to CO2

MPG to CO2

As a final note, I should point out that for a car to be able to compete with the train as a clean form of transport, it would have to do about 109mpg (~60g/km). Now that is a challenge!

Source: VCA data / Clear analysis