Diesel vs Petrol

As headstrong as I am when it comes to what cars I’m considering, I am by no means above asking for advice from both friends and the online community…most of whom probably have more experience than me when it comes to purchasing and running a car, if not doing background research! Some of the feedback from car forums has centered around the choice between getting a petrol and diesel vehicle and the associated costs of both. It’s not just me who gets this feedback either, as alot of the other advice forums I have read surrounding used luxury SUV’s tend to at least bring up this argument in passing.

On the face of it there seems little to argue about, at least for people buying brand new; diesel engines are usually the only option and produce significantly better mpg and residual versions than petrols…in fact the sales figures of petrol models are so low that in future it’s likely an argument that will cease to exist (more or less anyway). Of course in other sectors of the market this is by no means the case, but for 2 ton+ machines the fact remains that for the time being a diesel is by far the best choice.

But what about for used buyers? Well things get a lot more sticky when you take a closer look at SUV’s aged over 5 years…out of warranty and usually outside of the main dealer network, these highly complex machines bring up lots of questions for experienced drivers, let alone relative novices like myself. So what are the areas of comparison? Well I’ll aim to cover the main ones to the best of my ability now:

Purchase Price:

OK so I’ve already stated that the diesel versions of these cars tend to have excellent residual values, but what about the petrols? Well where there is scope for direct comparison between petrol and diesel versions of the same cars, the diesels are usually alot more expensive than the petrols…which tend to have lesser mileages and higher specifications! As not many new petrols are sold this is harder to compare with the cars I’m looking at, but a 2004 BMW X5 3.0d will probably be worth around £7k, compared to around £5k for a 4.4 V8 petrol! At that price it’s hard to justify spending over 50% more to save a few mpgs. Petrol 1 Diesel 0

Condition:

The powerful V8 versions of these vehicles tend to sit atop of the pile when it comes to price, so tend to be bought by money-no-object types who can afford the fuel and upkeep of such a vehicle. Top spec versions also come with better standard equipment and are usually loaded with options, plus those thirsty engines mean that they are unlikely to be driven the length of the country and end up with intergalactic mileages…so petrols should then usually be better specced, more cared for and have a lower mileage? Petrol 2 Diesel 0

Reliability:

This is a bit of a hard one to talk about definitively as there can be horror stories from both sides of the fence, not just related to the engines themselves but also the different gearboxes used to transmit this power to the roads. Petrols usually have the better reputation thanks to their simpler technology (at least in earlier models), but some V8’s can be a bit troublesome compared to the usually stellar 6 cylinder options. However there are also many tales of diesel longevity too, and the numerous high milers are testament to that fact. It’s a hard choice but on the whole I would be more concerned with the turbos and swirl flaps of a diesel example of the cars I am looking at…but given the use of turbos in many petrol engines nowadays this advantage could be lost for future generations of models. Petrol 3 Diesel 0

Maintenance:

I’ve already mentioned how diesels are more complex in terms of reliability and to a degree this carries across to maintenance; none of these vehicles would be easy to maintain at home and to be honest I am not mechanically minded so will likely take any purchase to an independent garage (a specialist if I can). What is probably more a worry to me is all the advice about ensuring oil is changed every 6 months or flushing automatic gearboxes named ‘sealed for life’ etc. As long as the car I buy has a comprehensive service history I think I will just have to take my chances lest I will end up spending thousands more for a dealership car which might not have been treated any better, so it’s a draw. Petrol 4 Diesel 1

Real Life MPG:

Here are where things start to get a little sticky as there is little way to know how well my new car will do mpg-wise until I actually buy it. It is undeniable that diesels use less fuel than comparable petrol models, but with fuel costing 5p-6p more per litre this advantage can fade pretty quickly in certain types of driving. Let’s take a look at one of my current favourites, the Range Rover Sport; the Sport comes with 4 engine options in my price range, a 2.7 TDV6, 3.6 TDV8, 4.4 V8 and 4.2 Supercharged. That is in order of efficiency but reports can vary greatly as to how well they do, the TDV6 is pig slow but I’ve heard reports of up to 30mpg combined (similar to the official figure). The TDV8 however can struggle to match 21 mpg vs its 26 combined official figure…and the less said about the petrols the better. I’m not stupid and know that my choices are all going to be expensive to fuel, but I’d like to see around 30mpg in town and 35mpg on the motorway…possible only in a diesel (driven very lightly!) Petrol 4 Diesel 2

Driving Style:

Linked to MPG is driving style, both the way in which I drive the car and the types of journeys I would complete in it. Many people state that only people doing over 12,000 miles a year have a reason for paying the initial diesel premium, and at the moment I do around half that. Add in the issue that I tend to do short journeys (ie under 7 miles) and the argument against diesel mounts; more modern cars with DPF’s (diesel particulate filters) can develop problems if only driven short distances, and the turbos really need sufficient time to warm up –  something that can take around 7 miles! My driving style is something that I can change but equally I can’t really classify how I drive into any one category; my car can be pretty nippy and on the motorway I dislike travelling under 70mph, but I am no boy racer and try to keep in as high a gear as possible. I guess that makes me pretty average but I don’t doubt that I would be willing to make slower progress to use less fuel. Add in the facts that I sometimes make long motorway journeys and that my mileage could easily skyrocket (or fall) with a new job and we have another draw. Petrol 5 Diesel 3

Performance:

A hard one to judge as this greatly depends on the engine variants chosen. Older diesel options tended to be a lot slower despite decent torque figures, but most modern ones are competitive against comparably sized petrol variants and offer good pull from a low speed. The problem with SUV’s is that they are heavy beasts, so the entry level engines in any guise can struggle a little. Obviously V8 petrol engines are supremely better and have a great growl to them, but then they have lots of negatives in the other categories here. Petrol 6 Diesel 4

Resale Value:

Although I am not overly concerned with residuals, it would be a nasty surprise to find that my car is worthless – which might well be the case if I bought a 10 year old V8. Diesel cars will hold their values noticeably better…although this is admittedly offset somewhat by the higher initial purchase price. Fuel prices have a big impact on these types of vehicles and with them unlikely to drop much the best I can do is invest in the more fuel efficient option. Petrol 6 Diesel 5

There are of course many other factors that can be used to assess which fuel option would be better, but in reality it is going to be a matter of comparing certain models and what’s available on the market. There is also the issue of certain vehicles which are only available in petrol (or only at a realistic price point). The Cayenne is probably the best example and is actually a really tempting option; here we have a car which trumps everything else in terms of driving dynamics and arguably image, in a fast vehicle which can be bought for more than £5k less than some of my other options. 17mpg mixed is not great though, and whilst £5k buys an awful lot of petrol the car would likely lose £5k more than a similarly aged diesel X5…or so I’d like to think. Of course I could go down the LPG route as some people have suggested, but given my bad history with dual fuel vehicles I am keen on following that route.

Giving a quick look at my current three favourites and another interesting trend reveals itself…in two of them at least. Petrol versions of both the X5 and ML have sold very poorly in these second generation models, to the extent that there is little price difference between the two types. 6 cylinder petrol versions are likely to return the better mpg but even these are priced higher than similar diesels-presumably because of their rarity. V8 X5’s are a lot more common than the inline 6 and undoubtedly the cars at £15k are a lot better equipped than diesels at that level, but then they are also more to insure. Nobody seems to have bought any ML500’s so the majority of V8 ML’s are the AMG, which are out of this world in terms of performance and running costs. Only the Range Rover Sport offers any real difference between petrol and diesel pricing, but with the more common Supercharged petrol often cited as returning single-digit mpg on spirited runs it is not something I can really consider…even if the TDV6 is worryingly slow.

Despite all that I have read and said, it seems likely I will end up in a diesel simply because of the vast choice, lower running costs and higher resale values…I mean of course I could chose a V8 with all the toys but I know in my heart that I would probably avoid using the car as much as I could in order to save fuel, and then what is the point in owning a car?