Let's start the topic of diesel economy by covering the most obvious question:
Is Diesel Cheaper than Petrol?
Yes. Well… actually, it's not quite that simple, which is why there's often a lot of debate on the subject, but yes. To answer this question thoroughly and to prove once and for all that modern diesels run FAR more economically than their petrol counterparts, we'll need a couple of real-world examples.
Let's take a small, realistic car with which we're all familiar as a case in point: the Volkswagen Polo. In constant production now for a frankly surprising 32 years and counting, the Polo is a pretty good place to start looking at and comparing the diesel/petrol argument.
Playing Polo
For this experiment, we'll take the petrol-propelled Polo SE 1.4 80ps, with its full five door complement, and the identically doored SE 1.4 TDI 80ps which is bowled along by diesel. Since there's a load of numbers and figures coming up, we'll refer to them as 'the petrol one' and 'the diesel one', and variations of those phrases, to save confusion.
Let's start with one of the most important factors that most people consider when they're buying a car: the price. We're assuming that the two cars we're buying are brand new, so the both have an on the road cash price of £8,227. They both produce 78.9 horsepower. The diesel's engine is 10ccs bigger than the petrol's, and it gets to 62mph 0.6 seconds slower, at 12.8 seconds. The diesel also loses out on top speed… by 1mph (the petrol will amble up to 109mph, the diesel up to 108mph). And the petrol can be found in insurance group 4, while the TDI is in 5. That 'T' makes all the difference, apparently, and we'll come back to the insurance in a moment.
Perfect Petrol & Its Diesel Double
But that's pretty much all the advantages that the petrol Polo has over its diesel doppelgänger. One of the most interesting advantages the diesel has over the petrol is its noise: contrary to earlier stereotypes, the diesel is a whole decibel quieter than the petrol. More importantly, for environmental (and, therefore, taxation) purposes, the diesel chucks up just 119g of carbon dioxide per kilometer to the petrol's 150g.
Now fuel economy. If you're driving your Polo around town, which is pretty likely, then your petrol will afford you 34 miles per gallon. Put another way, 8.3 liters will get you 100 kilometers further away from where you started in your petrol Polo. Not bad by most standards. But that figure withers with embarrassment when you stand it alongside the diesel's mighty 51.4mpg. That's 5.5l of diesel per 100km. Through town. And it gets better; if you're sticking to the motorways or inter-city roads for extended periods of time, providing you can avoid serious traffic hold-ups, you can squeeze 72.4 miles from every gallon you feed that excitable little engine. The petrol will manage just 54.3. On average then, the petrol Polo will travel 44.8 miles per gallon, and the diesel will travel 62.8. That's 18 miles further than the petrol. It beggars belief that anyone buys the petrol at all!
So what is the difference in cost of running the two cars? Well, at the time Nu-Car is writing this, the average national price of unleaded petrol is hovering around £1.01 per litre. There are 4.55 litres in a gallon, and the petrol Polo requires 245.5 gallons of petrol to cover its average 11,000 miles per year. Since we know that the average mileage of the petrol Polo is 44.8 miles per gallon, we can figure out that it will cost £1,129.30 per year to fuel it. Using the same equation, but factoring in the diesel engine's far superior mileage, and the fact that the current national average for diesel is £1.05, fuelling the diesel Polo for a year will cost £837.26. That's a saving of £292.02 per year for the diesel! Even though diesel costs more at the pump!
Insuring a Diesel
But there are other factors involved in running a car. One of those is the insurance. For the two cars we're comparing, Nu-Car went to one of the leading insurance providers for a quote, for which Nu-Car claimed it was a 26 year old professional living in the south of England who had been driving for more than three years, had not made a claim in over five years and had no convictions. The diesel was quoted at £271.95 per year, and the petrol was quoted at £247.95, a saving of £24.15 per year for the petrol Polo. So the petrol's insurance is cheaper.
Save your Road Tax
But the saving on insurance simply evaporates when you consider road tax. As we mentioned a bit earlier, 150 grams of the stuff coming out of the back of the petrol Polo per kilometer is carbon dioxide, compared to just 115 grams of the planet-stopping gas from the diesel Polo. That means that your petrol Polo will cost £115 per year, where the diesel Polo will cost you just £35. That's another £80 saved right there.
So, to bring the Polo petrol/diesel debate to a close, let's summarise by looking at the total price of running each car for a full year, including the buying price. The unleaded petrol will cost you £9,719.25 a year, once all of the insurance and tax is added up and including its on the road price, and then £1,492.25 every year to run it after that. The diesel will cost significantly less, at £9,371.21 in its first year, and then just £1,144.21 in the following years. Volkswagen's diesel Polo will save you a whopping £348.04 a year over its petrol twin.
Nu-Car chose the Polo completely arbitrarily for that little test; we didn't choose it based on any prior knowledge of the Polo's economy. Genuinely. All of that was fresh information to us as well. We're learning as we're teaching. But while researching and doing the maths for this guide, Nu-Car learned that the entire Volkswagen range, and most German cars, in fact virtually all other European manufacturers generally, have similar savings: diesel is more efficient across models, ranges, and entire manufacturers than petrol.
Another Example
Let's run through the experiment again, really briefly, except this time using a completely different type of car from the Polo. And to make it even more realistic, let's pick a second-hand car that is for sale on this website right now, as this is being written. We've settled for the BMW 3 Series Diesel Touring 320d SE 5dr Auto estate. Its petrol equivalent is the 320i estate. We won't go into as much detail regarding performance as we did with the Polo, we'll just look at the running costs this time. Needless to say, the diesel can match, if not outperform, its petrol-powered brother on every factor. Nu-Car is advertising the BMW car at £11,225. For the sake of argument we'll say that the equivalent 320i costs the same (the real-life example Nu-Car is using, found on another website, is actually a couple of hundred pounds more expensive. However, we want to highlight only the running costs in this example, and as such the prices of the cars are not relevant here).
Starting with fuel consumption, the diesel BMW's average mileage is 48.7mpg, and we'll conjecture that the average miles per year are a little higher in this example than the last; let's say 20,000 miles per year. So, it takes 410.68 gallons per year which, at current average costs in the UK, would come to £1,943.34. The same car in the same amount of time and the same amount of miles, but with a petrol engine, will cost you £2,899.35.
Taxing the diesel BMW will cost £165 per year, where the petrol model will cost £205. Even the insurance is cheaper for the diesel. Nu-Car, this time in the guise of a 30 year old professional, was quoted £501.91 for the diesel and £514.50 for the petrol from the same leading insurer as we used for the polo.
So the BMW 320i estate, the petrol one, will cost you around £3,618.85 per year to run. The BMW 320d SE, the diesel one, will cost you £2,610.25. That's a difference of £1,008.60 between the diesel and the petrol, a truly startling saving. Nu-Car rests its case.
So, to answer the question with which we began:
Is Diesel Cheaper than Petrol?
The answer is no. At the pump, petrol is cheaper. But is diesel cheaper in practice? Is it cheaper in the real world? That's an obvious and resounding YES.