EXECUTIVE motoring is entering a new phase with a tax-busting diesel from BMW.

The latest 5 Series saloon has been given the eco treatment making it the leanest and greenest in its class.

Exhaust emissions have been pared down to 119g/km giving the car tax advantages for private and business users alike.

It is exempt from road tax in its first year, costs £30 a year thereafter and for company drivers it is in the 13 per cent benefit in kind banding.

That is lower than Audi can manage with its family sized A6, Mercedes with the E-Class and Jaguar with its XF diesel.

Even hybrids such as the Lexus GS fail to come close.

BMW's trick has been to give the 520 diesel the Efficient Dynamics treatment slashing the car's CO2 output from 125g/km and taking it below the critical 120g/km threshold where significant tax advantages kick in.

Fuel economy is also boosted to 62.8mpg - almost 10 per cent more than the standard 520 diesel.

Even more impressive is that the improvements have been achieved with little impact on performance, giving the car a brisk 0 to 60 time of 8.7 seconds and a top speed of 144mph.

BMW Efficient Dynamics models have improved aerodynamics, engines which regenerate energy while braking and are fitted with stop/start systems to save fuel in traffic.

It is already deployed by BMW on the smaller 3 Series model and has proved so popular the eco variant makes up almost 50 per cent of sales.

On the 5 Series it works supremely well. Be light on the throttle and it is possible to exceed the official mpg figure without trying too hard.

We managed to see an average of 66 to the gallon on the car's trip computer which now includes what BMW calls and EcoPro readout showing the theoretical additional mileage the car has managed. Over 50 miles we had managed to add 8.8 miles to the range.

Unlike other diesels, which seem to sound more clattery when given the economy treatment, the 520 sounds sublimely refined inside.

It is priced from £30,435 making it almost the cheapest of the 5 Series line up and only £1,000 more than the standard 520 diesel.

The car we drove had sufficient extras to satisfy most corporate demands and still tipped the scales below £40,000.

Leather upholstery and 17-inch alloys are standard and a full blown sat nav system incorporating BMW's latest Real Time Traffic Information system, Internet connectivity  and a head up display were part of the £6,000 worth of options which made the car tick.

If there is a downside it is that the Efficient Dynamics model is manual only and many an executive would expect automatic transmission in a car of this ilk.

Nevertheless, the 520d Efficient Dynamics is classy and the savings are obvious, making this compelling kit in the corporate world.

To get emissions so low in a car of this size is no mean achievement and while the 520d uses a four cylinder two-litre engine BMW is on the way to introducing a three pot 1.5-litre engine next year.

The new engine will make its debut on the 1 Series models and will become the first BMW cars to fall below 100g/km where it will escape vehicle excise duty, congestion charge and drop into the lowest of all business tax brackets.