One BMW 5 Series 525d SE manual emits 179g/km and boasts 163bhp. Another BMW 5 Series 525d SE manual emits a much lower 132g/km and has a rather more beefy 214 horses. What's the difference? One was on sale in 2000; the other was on sale last year.
That's the very real difference in the running costs, efficiency and 'greenness' (if you take 'green' to mean low CO2) in ten year's worth of R&D at BMW.
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Between 2000 and 2011, the average CO2 emissions output of cars in the executive segment has fallen by as much as 35 per cent according to the Society of Motor Manufacturers and Traders. This is due to a combination of more efficient engines, improved engine technology, lightweight engineering and a number of tweaks to reduce fuel consumption and efficiency.
The result is executive cars that are much cheaper to run than their counterpart from last decade.
2000 BMW 525d versus 2011 BMW 525d
The reduction in CO2 output means a cut in road tax, so the owner of the 2000 5 Series would need to pay £220 annually, yet the 2011 5 Series costs just £115 every year top tax.
Cars registered before 1 March 2001 are now charged one of these two different tax rates depending on whether the engine size is below or above 1549cc.
A car registered after 1 March 2011 is charged a tax rate for the first year, followed then a separate rate every following year. The exact charge depends on what tax band, ranging from A to M, a car falls under based on its CO2 output.
A 2011 BMW 5 Series is placed in band E, whereas the 2000 model would falls under band I under the current system.
So, if the 2000 BMW 5 Series was taxed by its CO2 emissions on the current scale the owner would pay £315 after the first year of ownership, then £210 in every following year. Either way there's a significant annual saving for the 2011 owner over the 2000 owner.
What's more the newer model can return 56mpg while the 2000 model can only manage 42mpg. With petrol at a notional £1.50 per litre, travelling exactly 100 miles will cost £12.18 in fuel if driving the 2011 5 Series, or £16.24 in the 2000 5 Series.
Extract that figure to 10,000 miles per annum and there's a difference of over £400 per annum. For company car drivers there's a difference of… need to do these sums too
The disparities in running costs between these cars is down to engine efficiencies clawed back in engine refinements, improvements in tyres, lighter materials and advanced technology.
More efficient engines
This change in design now sees executive cars use smaller litre petrol or diesel engines, with less valves and cylinders.
To compensate for these reductions, extra performance and efficiency is extracted from the use of turbochargers. Nowadays virtually every executive saloon offers a turbodiesel or turbopetrol engine of some kind. Smaller engines mean lower fuel consumption and carbon emissions.
Some manufacturers are also turning to hybrid technology to achieve large cuts in CO2 for their executive models.
A recent example of the executive hybrid is the Infiniti M, which combines a 302bhp V6 petrol engine with a 67bhp electric motor. With a CO2 output of 159g/km the results are not spectacular but crucially allow companies a higher write-down allowance, making it attractive to fleet managers. What's more, other brands are set to deliver hybrid saloons in the next couple of years with more impressive figures.
Mercedes will launch a hybrid version of the E-Class executive saloon, with a reported CO2 output of just 109g/km.
Manufacturers have also brought elements to execs from eco-signature cars such as stop-start technology, which switches off the engine when the car is stationary in traffic.
Another example is regenerative braking, which captures unused energy that is stored and later used to assist vehicle acceleration.
Manufacturers are also encouraging a more eco-friendly style of driving from their customers with the help of onboard data computers which tell the driver how efficient they are currently driving.
A similar feature is the optimum gear shift indicator, which informs the driver via a display on the instrument cluster which gear is best in the current given driving situation. The feature encourages swift changing of the gears whilst preventing unnecessary high engine speed in the process.
Manufacturers are also cutting down CO2 levels because of changes in the design of cars and the components they use.
Lighter build materials, low-rolling resistance tyres, lowered suspension and optimised aerodynamics have all evolved through every segment including executive cars, and it all contributes to the effort to cut CO2 down.
Rising petrol prices have driven buyers - and therefore manufacturers towards more efficient cars. But in the European Union there's an additional incentive in striving for low-CO2 cars looming on the horizon, with stiff financial penalties for car manufacturers who can't reduce the CO2 output of their ranges.
One reason CO2 levels have fallen so steeply is EU emissions regulations aimed at driving down pollutants such as CO2 and nitrous oxides.
Under upcoming Euro 6 regulations, which define the acceptable conditions for vehicles sold in EU member states, the average CO2 emissions output for new cars across a manufacturer's range will need to fall below 130g/km by 2015.
These rules have required manufacturers to change their approach in making engines for all cars including executive models.
As a result BMW's CO2 output across its entire range fell to an average of just 143.33g/km by 2011, a reduction of 2.17 per cent compared to the year before.
The real-world savings on your fuel bill
The most efficient model in the current 5 series line-up, the 520d Efficient Dynamics specification, can travel up to 967 miles on the 70 litre tank of diesel at a combined fuel efficiency of 62mpg and CO2 output of 119g/km.
That means road tax of £30 annually and free for the first year. Travelling 10,000 miles a year would cost £1,044.85 in fuel.
That compares with the most efficient car in the 5 Series range in 2000, the 525d with manual transmission.
The same equation would mean fuel costs of £1,542.39 for 10,000 miles at today's prices to add to the £220 in road tax.
Over a three-year lease deal that translates to over £2,000 - the very real difference in buying the most efficient car in BMW's range today when compared to its most frugal counterpart over a decade ago.