Any - Petrol or Diesel? - Johnfrog

HJ's latest Newsletter compares the cost of buying a sample of new cars with either petrol or diesel. He doesn't give his calculations but I suspect they are flawed. Why? Well because he bases his argument only on how long it takes to recover the extra cost of a diesel engine but he ignores the fact that, after 3 years, the diesel engined car will be worth more than the petrol version. So, the calculations need to take into account the overall cost of ownership and assume you sell the car at the end of the period. This should make the diesel more attractive. HJ's method seems OK if you intend running the car into the ground but most people don't do that with a new car. Am I right or have I missed something in the logic?!

Any - Petrol or Diesel? - RobJP

No, you haven't missed something out.

Another flaw is that the REAL mpg figures for some cars are calculated, but not for others. In (I think) every single case where the real figures have been used, diesel becomes a far more attractive option than using the 'official' mpg.

I strongly suspect that, for those cars where the 'official' figures are still used, using the 'real' MPG would result in an article recommending diesel. Which would be as fashionable as supporting Tyson Fury in SPOTY. So those numbers are conveniently ignored.

That's the problem with having a bunch of engineers, mechanics, scientists and generally interested (and thus observant) people on a forum. They tend to spot things.

Of course, if (and it's a huge 'IF') diesel cars are banned from various city centres in the next few years, then that may well cause diesel values to drop more than they currently do, negating the argument. However, whether that will remain a political aspiration or become reality nobody can tell.

Any - Petrol or Diesel? - gordonbennet

Nail head hit, the big assumption is retained values.

We're a funny old bunch of Hectors here, even the young Hectors, but i don't suppose we are alone in taking a step back from the increasing complexities of modern Diesels.

When Joe Public catches on, or rather when enough people have been hit in the pocket enough times, coupled with increasing anti sentiments in the press, those cosy Diesel values might not be quite so good in the future.

Any - Petrol or Diesel? - skidpan

My last car was a BMW 118D. When I bought it the cost was £300 more than the 118I and it drove far better. My calcs showed that the diesel would be cheaper to run so that is what I bought. Over 5 1/2 years I did about 40000 miles and I guess I saved about £2000 in fuel and another £400 in VED. When I traded it I got £700 more than the 118i would have theoretically raised. So a saving of about £2900.

The Mrs last car was a Kia Ceed 1.6 CRDi. When bought it was £1000 more than the 1.6 petrol and drove far better. Again, mys calcs showed it would be cheaper to run. Over 5 years and 40,000 miles I guess we saved about £2000 in fuel and another £100 in VED. When we sold it we got about £500 more than the equivalent petrol. So a saving of about £1600.

So would it surprise you to learn we both drive petrols now. With DPF's causing increasing problems for drivers doing similar mileages to us (both previous cars had DPF's and never gave a problem) we decided it was time to step back from diesel and with the more modern turbo/supercharged petrol engines the fuel economy difference is not as great as it was.

When I bought my Seat Leon the diesel equivalent was £2000 more. I calculated using HJ's Real MPG figures that over 40,000 miles the diesel would save me about £800 leaving me still £1200 out of pocket. VED was £10 a year less on the diesel saing me another £50 so still well out of pocket. So even if the diesel retained 1/2 of the extra cost I would still not have recouped the difference. Add to that the petrol is a better drive and has no DPF worries I have no regrets.

The Mrs new petrol car looks like it will probably cost about the same as the diesel over 30,000 miles and 5 years but again no DPF worries and in this case the same £0 VED. The petrol engine is also much smoother and quieter with about the same performance so well happy.

People just need to do their on sums, its not that hard.

Any - Petrol or Diesel? - Johnfrog

RobJP you left out "accountant". We were taught to include ALL relevant financial data when evaluating a project. Sadly, Jo Public will rely on this misinformation. The evaluation also ignores low mileage drivers, for whom petrol may be better and DPF problems for diesel drivers who make lots of short journeys.

I am about to change my own car and hoped this article would give some useful steers (no pun intended) but it hasn't.

Any - Petrol or Diesel? - Andrew-T

RobJP you left out "accountant". We were taught to include ALL relevant financial data when evaluating a project.

For me, something that is always left out is the actual consumption of fuel. By which I mean the accountants only tot up how much is spent in the showroom or at the pumps. A major point about using a diesel engine is that it actually burns less carbon per mile, which means that if everyone drove a diesel there would be more to go round, and it would last a bit longer for everyone. But that's harder to quantify, so the bean-counters just stick to counting beans.

There is the associated issue of nasty emissions, of course, but that's another story.

Any - Petrol or Diesel? - daveyjp

"However, whether that will remain a political aspiration or become reality nobody can tell."

Just last week Leeds Council announced that by 2020 it will have an inner city congestion/high polluting vehicles zone.

Whilst this is couched in terms pointed at the 'highest polluting vehicles' (namely HGVs and buses) being required to pay, once the cameras are in place and they aren't earning enough to cover their annual running costs 'mission creep' sets in and charges will be imposed on a larger number of vehicles.

Upcoming UK taxation may make diesels less desirable and there will be changes to EU regs on emissions testing which may also affect the desirability of the diesel car.

Any - Petrol or Diesel? - Engineer Andy

The 'calculations' are really only a approximate guide, as journey type can make a huge difference to the mpg and reliability of a car, e.g.

Car 1 does 10000 miles pa but mainly on frequent short journeys to the shops/friends and one long trip on holiday in jig-jog traffic (main school holiday season);

Car 2 does the same miles but via decent length trips every 1-2 weeks and one long trip on holiday in reasonable traffic (out-of-season).

Each does the same miles, but car 1's journey profile is far more suited to petrol than diesel (mpg being nowhere near as good as the car doesn't get warmed up and so isn't working at optimal efficiency as the petrol will be); car 2's is to a lesser extent as the miles go up. Petrol cars (w***el-engined ones aside) suffer far less problems if used mostly on short journeys than diesel ones, the cost of repairs may be quite high and would need to be factored in as well, which many people don't do. Diesel engines in cars are much more complex than they used to be, especially when compared to petrol engines (due to adherence to emmissions regs) and are thus more susceptable to major failures if used in the wrong way.

As HJ says, diesel should be chosen for high mileages (forgetting the current [temporary] small difference in fuel prices), realistically (taking into account the above sorts of scenarios, the requirements to carry loads of any significance on a regular basis and any costs associated with poor reliabilty if used for short trips a lot. Note also that due to the higher list price, the diesel car may also be in a higher insurance group to the similar performing petrol-engined car, though this will likely be offset by the lower (though not as much as in the past) VED group cost. Servicing (given the diesel's higher complexity) will probably also be more expensive than for equivalent petrol cars. How long owners intend to keep the car would also make a difference (not just depreciation, but how long parts lasted for and the cost and frequency of replacements, e.g. turbos, DMFs, timing chains/belts etc, which will likely last different lengths of time on [some makes at least] diesel and petrol cars)

The 'dieselgate' scandal will also have an effect on second-hand values of diesels and the purchase/running costs due to the likely extra components/reduced mpg/increased CO2 emssions/more additives required etc that may well result in higher running costs as well as the new price.

As a rough guide, I would say, for mainly reasonable-length journeys, the changeover would be around 15-20k miles, with factors added from the above increasing or decreasing that figure.

Any - Petrol or Diesel? - Avant

"....because he bases his argument only on how long it takes to recover the extra cost of a diesel engine but he ignores the fact that, after 3 years, the diesel engined car will be worth more than the petrol version."

HJ isn't ignoring it: as you can see from the above posts, there are many of us, including HJ, who would dispute the use of the word 'fact'. It's an opinion, which others share, and at the moment I think it's still true that the extra initial cost of a diesel bought a few years ago is largely recouped by stronger secondhand values. But only for now.

We need to think why those values have been stronger: surely because buyers expect diesels to last longer and be more reliable than the equivalent petrol-powered cars. People's recent experiences, well documented on this forum, suggest that they don't and they aren't.

And also, as Skidpan has shown with figures to back his view up, the financial savings of diesels aren't what they were: and with the coming of the new wave of economical, torquey petrol engines like the VW Group 1.4 and 2.0 TSis, diesels are no longer necessarily better to drive.

Finally, of the five diesel cars in a row that I had until four years ago, only one of them, the V6 in the A4 Avant, was refined in terms of noise, vibration and harshness. Is there a really quiet four-cylinder diesel around?

Any - Petrol or Diesel? - gordonbennet
Finally, of the five diesel cars in a row that I had until four years ago, only one of them, the V6 in the A4 Avant, was refined in terms of noise, vibration and harshness. Is there a really quiet four-cylinder diesel around?

Oddly enough the 4 cyl 3.0 litre lump as found in Hilux/Landcruiser is quieter and smoother than it should be bearing in mind i had its IDI version in my old Landcruiser which made in 93, same basic engine but common rail in the Hilux we had.

Often enough all you can hear is the pleasant sound of the turbo spooling up (which you only ever get where enough fuel is being fed in, so effortless torque at low revs), though to be fair its bolted onto a strong ladder chassis in those vehicles, so remote from the passenger shell, whether it would be as good in a car's engine bay is questionable.

Its always puzzled me why Toyota didn't sling that engine, or the 4.2 out of the Amazon, into their formidable Lexus 300/400 and surround them in padding, coupled with an auto box they would have taken the fight to the Germans on their own terms, with staggering rust resistance and durability to make residuals world beaters, i'd have one a used one like a shot.

Turning that on its head Landrover do it the other way round, stick car engines never quite large or torquey enough in their 4x4's, always thought the two should have combined, LR design the body and chassis and let Toyota build it, gear it and power it.

Edited by gordonbennet on 21/12/2015 at 13:52

Any - Petrol or Diesel? - RobJP

See, this is what I really like here. A sensible debate, well constructed arguments, people always willing to take on board other points of view.

Johnfrog, my apologies for missing out accountants earlier !

Avant, no, I don't think there are any really smooth 4-pot diesels out there. The BMW 20d in the 5 series is pretty good, though that's probably due to the additional soundproofing, better isolation for engine mounts, etc., as it certainly has a level of coa***ness in the 3 series. I'd imagine the same would apply to the 4-pot diesels in the Jag XF (over the XE), E class (over C class) and A6 (over A4).

Also, I think the market still prices diesels as they do because of the perceived fuel savings, rather than any anticipated longevity / reliability. For someone doing 3-400 miles a week, they see the amount of fuel they put in the tank as their big expense, because they do that every week.

As an example, a 320d doing 53mpg, or a 320i doing 32mpg (both figures from realmpg part of the site not fantasy 'official' figures). The fuel savings in that case for 300mpw are roughly £16.50 per week, or in excess of £800 a year. Go up to 20k miles a year, and it's in excess of £20 every single week.

Much as skidpan, we had a 118d. At that time, the diesels were only slightly more expensive than the petrol variants. It does seem that the manufacturers have decided to take considerably more profit margin for themselves, pricing up the diesels to suit the demand for them. Do bear in mind that the 118d's being mentioned (2007/2008) were DPF equipped too, so no real argument for later ones having the additional emissions gear fitted, and that pushing the price up.


Value my car