If you’re a regular user of premium diesel fuels, or are sometimes tempted by their appeal, then you really need to see a tangible return on the extra £5 to £10 you will pay for every fill up.

What do oil companies say about their diesel fuels? Shell’s standard priced FuelSave Diesel is, in their own words “designed to help improve your engine’s performance, and enable your vehicle to run more efficiently.

“It contains a powerful detergent, designed to help clean up and control harmful fuel system deposits in fuel injection equipment in modern diesel cars, helping to restore engine performance. FuelSave is high in Cetane, which can… lead to more power, better fuel economy and lower emissions, (although) in our experience, it’s ill-advised, and potentially misleading, to make specific quantified claims.”

That sounds pretty honest. But if FuelSave is that good, why spend another 5p to 7p a litre on Shell’s premium V-Power Diesel? It’s “designed to remove existing deposits, to help restore and maintain performance so that your engine can operate at its best, help to deliver more power… helping your car achieve its full performance potential.”

What’s any different about that from FuelSave? Well, we do know that V-Power Diesel contains sophisticated synthetic GTL components that are claimed to aid combustion, and that V-Power is marketed primarily as a performance fuel, but Shell data sheets show FuelSave to actually have higher Cetane values than V-Power, which rather knocks the claimed benefits of the higher Cetane values stated for FuelSave. It all leaves us rather puzzled.

Let’s take a look at BP now, whose standard diesel is “specially designed to help keep your engine fit and running efficiently; lower engine efficiency can lead to higher fuel consumption, higher emissions, and reduced performance. BP regular diesel has been formulated with advanced cleaning properties, and won’t leave harmful deposits in your engine.”

Sounds good. Is it worth paying more then for BP’s significantly more costly Ultimate Diesel which, in their words, is “an advanced, unique formulation with exceptional engine cleaning properties – five times the cleaning strength of ordinary fuels, to protect against deposits forming, (and) remove existing harmful deposits… restoring and maintaining the health of your engine; the Cetane quality of BP Ultimate Diesel means that it burns better than ordinary fuels – more smoothly, more completely… so energy is released more efficiently, reducing exhaust emissions etc.”

Isn’t that pretty much what they say about their standard diesel?

That’s what the two major UK branded suppliers say. Total is abandoning the UK market, so Excellium Diesel will disappear, whilst Esso’s Energy Supreme appears to claim little other than that it has twice the detergent additives of its standard diesel.

So do these words about premium fuels present a convincing case in favour? Is there a measurable payback on the extra few pounds every fill-up?

Many drivers do feel that premium diesel gives them better engine response, power and fuel economy, yet many tests have proved to the contrary.

Back in 2005 we reported on some rolling road tests, run in conjunction with tuning company Tunit. Using three different cars, it matched the BP Ultimate Diesel then available against standard Texaco diesel; the tests found no significant differences in power output and, at best, a small benefit in low-down torque with one of the three cars tested.

Other independent tests (by What Car?) concluded that “premium fuels are an unnecessary expense with no major fuel economy benefit.”

Regarding the benefits of high Cetane values, European Standard EN 590, established in conjunction with the major engine manufacturers, sets a minimum value of 51 for Cetane Number, and there’s little evidence (as with high Octane petrols) that exceeding this value offers gains in most modern engines.

Some may respond differently from others, and possibly your engine has expensive tastes. But short-term tests are unable to replicate the long-term benefits claimed for the detergent properties of most premium (and many standard!) fuels, or any possible fuel economy benefits, so the verdict on this aspect is open.

Some drivers use premium fuel every few fill-ups, which is possibly a justifiable compromise. But don’t spend money blindly on premium diesel if you can’t identify any measurable differences.

If you don’t make some comparisons, by checking your fuel consumption to test any economy claims, and honestly asking yourself if the engine feels livelier, then you’re very likely wasting money and paying for expensive advertising rather than proven benefits.

Either way, you might be better off using an aftermarket fuel additive – something we’ll discuss in a future issue.

Victor Harman