Diesel Vs Petrol - Help Me Buy A Car Guide

ironclaw

Diesel Vs Petrol - Help Me Buy A Car

It is often debated here on the forum regarding when a person should buy a diesel versus a petrol car. Mileage, driving type and tax are often used as pervasive factors and they all should be considered before making a purchase.

Mileage

One of the key indicators of whether to buy a diesel or petrol car is your annual mileage. Its important not to guess here. Many people have no idea of their daily or annual mileage, and its important you factor this when making a purchase. The industry consensus is generally 20 to 30k miles (50k km) before diesel becomes a viable option i.e. It begins to become economical. To put this in perspective, the average mileage for a car in Ireland in 2005 was 10,500 miles (CarTell data) The overall average has been downwards since 2006.

If you are undertaking low annual mileage and especially if this mileage is in the cities, in most cases a petrol will be far superior for your needs and wallet.

Driving Style

Diesels are more efficient when they have warmed up (Which takes considerably longer than a equivalent petrol engine) and at a sustained cruise speed i.e. Motorway. Diesels are not suitable for short, quick drives in cities. Remember, most diesels are fitted with an additional particle filter in the exhaust called a DPF, this unit will become clogged if the diesel is not regularly exposed to sustained, cruise driving.

Tax

On average, tax for diesel cars is lower owing to their lower Co2 emission. Under the next tax system since 2007 in Ireland, this means a considerable saving. For example, a petrol 2.0L BMW 3-Series from 2006 will cost approximately €7800 to buy plus €906 in motor tax. The equivalent diesel 2.0L from 2007 will cost you approximately €11,623. As can be seen, there is €3000 in the difference between these cars. And whilst the 2006 model has higher tax, you would be spending the equivalent of 3 years motor tax to buy the diesel, and that is before you tax it!

This expensive can also be debated in terms of fuel cost, having spent approximately €8700 on the 2006 model, you have €2900 to spend in terms of fuel. Assuming an average of 7 litres of fuel per 100km, thats close to 12,600km of fuel. And at this point, you have still spent less money than the diesel car.

As you can see, diesel cars are in general more expensive to buy initially. This cost should be factored into any 'saving' you believe you will be making. This extra cost could be spent keeping an equivalent petrol car taxed and putting fuel in its tank. It is always false economy to upgrade a car in an attempt to save motor tax. The upgrade cost will in most instances be equivalent to years of motor tax payments.

Dealers

But if diesels are not suitable for the vast majority of users, why is the market full of them? In opinion this is two fold. First and foremost for a dealer, they generally have a higher sale price. This increases sales figures and bolsters their position. And secondly, many people are obsessed with the cost of motor tax and if given the choice between a petrol and diesel, disregarding the purchase price in most cases, they will be swayed by the price of the tax disc on the windscreen. As such, the dealers and second hand market will respond to this demand.

Maintenance

Diesels can be more expensive to maintain, especially if they misused or incorrectly applied to your driving style. You should fully examine and explore the cost of maintenance for your chosen car before purchase.

Are Diesels All Bad?

No. They have their uses and as you can see above, there are cases when they are warranted and should be used over a petrol car. For example, high mileage or long commutes, they are perfect and should readily be considered.

However, in all instances, your car choice should not be determined by the price of the fuel or the tax disc on the window. These are the cheapest part of any car ownership.


Comment

Just want some input on the above folks, as the threads often pop up here and I think its worth having a go to guide for new buyers and those unsure. I fully understand if some of the info is incorrect. Thats why I posted, I'm looking for input as its easy to overlook or mistype something. I'm also not diesel bashing, I own two :pac:

MuppetCheck

Excellent post. Should form a sticky.

Supergurrier

No more stickys please

We still need a petrol vs diesel an what car should I buy mega thread though

jim-mcdee

you forgot about noise, vibration, throttle lag, and front end weight (nose heavy). Three things that have yet to be sorted out on diesel cars.

R.O.R

2 Points that stand out, in realtion to the OP.

20,000 - 30,000 miles / 50,000km a year is where you need to change to diesel? Sounds vastly over inflated to me, and I'd guestimate 25,000km is around the figure.

Diesels do generally cost more to buy, and therefore have a higher resale value. This needs to be factored in to Whole Life Costs. If you plan to change the car in 2-3 years, there would still be a sizeable difference in the sale price of a Diesel car in comparison to a Petrol car, which softens the blow of paying more for the Diesel in the first place.

i.e. If a diesel costs €3,000 more to buy than the equivalent Petrol, there's a chance it will still be worth €2,000 more in a couple of years time. While this still means higher depreciation of the Diesel, that's offset by the lower fuel and tax costs, so the Diesel could well work out to have lower overall ownership costs.

ironclaw

jim-mcdee wrote: »

you forgot about noise, vibration, throttle lag, and front end weight (nose heavy). Three things that have yet to be sorted out on diesel cars.

Good points but I'm talking pure economics. Those are things that would only appeal to a driver that would actually notice and theres a good chance if you are considering those, then your not to worried about what is powering the actual car.

R.O.R wrote: »

2 Points that stand out, in realtion to the OP.

20,000 - 30,000 miles / 50,000km a year is where you need to change to diesel? Sounds vastly over inflated to me, and I'd guestimate 25,000km is around the figure.

Diesels do generally cost more to buy, and therefore have a higher resale value. This needs to be factored in to Whole Life Costs. If you plan to change the car in 2-3 years, there would still be a sizeable difference in the sale price of a Diesel car in comparison to a Petrol car, which softens the blow of paying more for the Diesel in the first place.

i.e. If a diesel costs €3,000 more to buy than the equivalent Petrol, there's a chance it will still be worth €2,000 more in a couple of years time. While this still means higher depreciation of the Diesel, that's offset by the lower fuel and tax costs, so the Diesel could well work out to have lower overall ownership costs.

The mileage figure I quoted was actually an average of a few different industry opinions. Its probably on the high side but again, I averaged a few different sources. Either way, your talking well north of what most people would consider 'high mileage'

As regards ownership costs, a 2008 2.0L manual diesel 3-Series has a current market average price of €15,300. While the same car in petrol averages €16,230. Taking the 2005 figures, its €6800 (Diesel) and €6319 (Petrol)

So its pretty negligible at that end of the scale in my opinion. The higher resale value of a diesel may also be wiped out by any higher maintenance costs. Swings and roundabouts really!

bazz26

Annual mileage is really only part relevant to how cost effective it is to choose diesel over petrol.

Driving style is more important though. Doing lots of short or stop/start journeys is what eventually causes problems for diesel engines.

R.O.R

ironclaw wrote: »

As regards ownership costs, a 2008 2.0L manual diesel 3-Series has a current market average price of €15,300. While the same car in petrol averages €16,230. Taking the 2005 figures, its €6800 (Diesel) and €6319 (Petrol)

So its pretty negligible at that end of the scale in my opinion. The higher resale value of a diesel may also be wiped out by any higher maintenance costs. Swings and roundabouts really!

Where are you getting the average prices from, and do they relate in anyway to mileage? The price of a diesel 3 Series will not be less than the price of a Petrol 3 Series, when mileage and specification are similar. From seeing what these things actually sell for, you're looking more like a premium of quite a few thousand extra for the Diesel over the Petrol. Gap will narrow in time, but it will always be there.

Also - it's a fairly crappy example to use for higher maintenance costs. As long as the timing chain doesn't go on a 318/320d, it will cost less to maintain than a 316/318/320i where the coils will pop every month or so :pac:


I was pricing a new A4 Avant for a customer, over 4 years / 100,000km. Retail price difference is €2,060 (Petrol cheaper). Estimated difference in the resale value (by an Audi dealer) is €4,000, so you're already on to a loser without taking tax and fuel costs in to consideration.

Didn't bother with the petrol version of the 3 Series touring as the retail difference is €1,490, and the resale difference would be the same as the A4 Avant.