Motoring Discussion > Diesel v petrol Miscellaneous
Thread Author: Crankcase Replies: 17
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New Tue 15 Nov 16 09:24 
 Diesel v petrol - Crankcase
In another thread there is talk of "lots of miles means diesel is better".

Is this based on running costs? Because it seems to me the is a general perception that "once you do about 18k a year, diesel is going to be cheaper".

But based on my diesel car giving me 44 mpg, as I suspect many do these days, and my old petrol Mondeo also giving me 44mpg, the fact that diesel itself generally costs a bit more than petrol, and that diesel engines have "more expensive stuff to go wrong" I can't see why you'd pay the premium to buy a diesel car at all on a purely cost basis.

I wonder if this all stems from an old calculation years ago, when diesel cars were giving 60mpg and petrol about 25mpg - the world has moved on and the differences now are probably pretty titchy, aren't they?

The driving experience is a different matter, but not what I'm asking about.

How do the sums actually work out?
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New Tue 15 Nov 16 10:21 
 Diesel v petrol - movilogo
Petrol vs Diesel argument is somewhat like Brexit - everyone will say his opinion is the correct one :o)

For a true comparison, same person needs to buy similar spec petrol and diesel cars and use both on alternate days on same routes and after 12 months compare the overall cost.

Techically speaking, a diesel engine is better for constant low RPM which is why these are used in railways, marine crafts, HGVs etc.

Petrol engines are good for revving up and down in quick succession - so these have been more suited to passenger cars.

The introduction of DPF has made matters complicated for diesel cars.

My personal opinion:

1. Diesel is still good for high mileage (18k pa or above).
2. Petrol is better if mileage is lower or variable commuting (life)style (my work situation changed and mileage dropped from 18k/yr to just going to stations daily now)
3. Diesel cars likely to face higher tax in future - thus lower resale value.
4. Some cars are only available as either petrol or diesel (but this may change due to #3) e.g. I have a Kia Ceed petrol automatic but Kia now only makes Ceed diesel automatic! Automatic SUVs are hard to come by in petrol mode.
5. Outside warranty diesel has more things to go wrong and thus likely to be expensive to maintain compared to similar spec petrol cars.


Last edited by: movilogo on Tue 15 Nov 16 at 10:22
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New Tue 15 Nov 16 10:45 
 Diesel v petrol - Crankcase
Thanks, movi.

Your answer number 1 is the nub of my question really. Why 18k?

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New Tue 15 Nov 16 11:14 
 Diesel v petrol - Mapmaker
I run an 11-year-old 2.2 diesel Accord that I've had for five years. It was no more expensive than a petrol when I bought it; in fact at a smidgen under 3k (from a small dealer) it was a complete bargain. I have - thus far - not spent a penny on diesel-specific problems.

It does 45mpg; a petrol would be under 30mpg (for a slower 0-60 time). Moreover, the petrol would be £100 more annually for road tax than the diesel is.

It has therefore saved me money since the day I first drove it. The clutch is beginning to slip though, and that might be £500 of diesel-related expenditure (particularly as it has only lasted the 29k miles I've driven it - a mal-adjusted cruise control switch also being brought into the equation).

Across those miles I have used 650 gallons, against 966 for petrol. (Probably more as there's a fair chunk of town driving and I pay no attention to the insignificant quantity of fuel I use monthly.) So 300 gallons at £6 per gallon has saved me £1800 at least.

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New Tue 15 Nov 16 13:12 
 Diesel v petrol - movilogo
>> Your answer number 1 is the nub of my question really. Why 18k?

Open a new Excel worksheet.

X = number of miles
Y = fixed cost (purchase price, tax etc.) + variable cost (mainly fuel but also servicing, tyre etc.)

Calculate values of Y when X = 1000, 1500, ...., 19500, 20000 etc.

Plot 2 curves - one for petrol another for diesel

This is the only scientific way of answering this question. You can then see yourself.
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New Tue 15 Nov 16 13:18 
 Diesel v petrol - movilogo
Simpler option.

X = extra cost for buying a diesel over similar spec petrol car
Y = fuel cost saved per mile

number of miles for you to make profit on diesel = X/Y
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New Tue 15 Nov 16 13:19 
 Diesel v petrol - Mapmaker
X = extra cost for buying AND RUNNING a diesel over similar spec petrol car
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New Tue 15 Nov 16 13:42 
 Diesel v petrol - CGNorwich
Compared with the overall cost of running a car any difference between petrol and diesel is fairly marginal. Most people don't really buy cars on predominantly considerations anyway - if they did we'd all be running around in basic Fiestas.

Just buy whatever fuel type gives you the drive you like. I prefer diesel for motorway driving which represents most of my mileage although I only do around 11,000 miles per annum.
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New Tue 15 Nov 16 11:02 
 Diesel v petrol - Bill Payer
>> But based on my diesel car giving me 44 mpg, as I suspect many do
>> these days, and my old petrol Mondeo also giving me 44mpg,

Is that based on the same kind of use? 44MPG is typical for a 2 litre diesel in mixed use, but 44MPG for a petrol is remarkable.
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New Tue 15 Nov 16 11:18 
 Diesel v petrol - zippy
I am a diesel fan.

Recently had a new style Vauxhall Astra 1.4T petrol on hire that returned 49 mpg tank to tank in a week (about 600 miles) of motorway and town driving and motorway queues as well. I though that was pretty good.

It had a touch screen satnav - these are not good - you have to look at them to change settings when a button can be felt for!
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New Tue 15 Nov 16 11:18 
 Diesel v petrol - Crankcase
Yes, that was a true figure, mixed use, annual average.

But it was a 1999 Mondeo. My father in law had an identical Mondeo, albeit a 2000 model, and he was always miffed he seemed to get just 42mpg where I consistently got 44, so I remember it well.

My last experience of a petrol car was a 2008 Aygo, and that gave me mid fifties mpg, so I was assuming a current small to medium model would still be at least mid forties. Not so?


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New Tue 15 Nov 16 11:40 
 Diesel v petrol - madf
Our Jazz has averaged 43mpg over 4 years. 90% town usage and 5% extreme very hilly one track roads , frequent stops for horses/cars and tractors..

Our 2003 Toyota Yaris averaged 48mpg on the same diet. It is unsuited to purely short journey town use as the catalyst clogs with carbon. Shell V Power makes a difference (No DPF)
Last edited by: madf on Tue 15 Nov 16 at 11:41
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New Tue 15 Nov 16 12:33 
 Diesel v petrol - PR
There is no right and wrong answer. The answer will be different for everyones different needs / desires.

There are some occasions where one is obviously not suited, ie diesel engines for short journeys that then clog up DPFs etc.

Apart from that it comes down to personal preference and priorities.

I chose the diesel version of mine mainly due to the economy. The petrol version in real life gets around 16-19mpg and I get around 40mpg. I have had it 20 months and have done just under 40k miles, so the saving is considerable, so the diesel was for me.
I could have got the petrol and paid more in fuel had that not been an issue, sadly it was lol
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New Tue 15 Nov 16 17:00 
 Diesel v petrol - sajid
i have a honda civic 2.2 59 reg diesel, i do about 4500 miles per year, it cheaper to run than the petrol jazz i had previously,

It is roughly 25 percent cheaper to run than the jazz, in fuel costs, i do a mixture of stop start driving and once a month drive on the motorway,

The honda diesel doesnt have a dpf, and compared to the jazz it got a lot of low down torque and is has the midrange acceleration to keep up with the traffic on the motorway.

I think that the initial cost o f buying the deisel is offset by its low depreciation cost of running and higher resale value.

There also because deisel burn to a higher fuel air ratio that it has to be more durable and that why deisel engines are lot heavier.

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New Tue 15 Nov 16 18:29 
 Diesel v petrol - Cliff Pope
>
>>
>> I think that the initial cost o f buying the deisel is offset by its
>> low depreciation cost of running and higher resale value.
>>


Is that still true, with rumours of impending restrictions because of pollution scares?
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New Tue 15 Nov 16 21:58 
 Diesel v petrol - Bobby
I am still staggered by the efficiency of my 1.6 diesel Civic.
Zero road tax (another thing to factor into the equation ) and NEVER less than 60mpg in any fillup and that is from my spreadsheet - the car computer usually has it around 65.

My typical commute consists of
first 3 miles, stop start, rarely out of first gear.
Then 3 miles of steady 30mph
Then a mile of stop/start
Then 5 miles on motorway which will be steady 50 with occasional slowing down to second gear.

Todays trip computer returned 69mpg.
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New Wed 16 Nov 16 00:00 
 Diesel v petrol - Auntie Lockbrakes
Impressive Bobby.

Mind you, I was astonished to see 60mpg on the trip computer of a Hyundai Elantra 2.0 petrol last month in Australia. Admittedly I was cruising along at around 55-60mph on the inside lane of a motorway, but Wow! I was amazed. This reading over 40km journey too, not just on a quick downhill stretch!
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New Wed 16 Nov 16 00:30 
 Diesel v petrol - Stuu
I cant say ive ever had a diesel I really liked, although I do have a soft spot for the Rover 75 oil burner, it suited the car better than the petrol engines.
My old Rover 420 was the best, not for refinement but that lovely torque from idle that modern diesels seem to lack. Only 85 bhp or so but seemed enough to get around and never less than 50 mpg.

My Charade was cheap to buy, 30 quid a year to tax and reliably gets north of 60 mpg in mixed driving, running on Tesco Momentum which seems to make a difference. The two other Charades I have owned both struggled to get north of 55 mpg, but had 30k and 50k on the clock, rather than the near 70k my current example has, so perhaps the mileage helps.

I may be in the market for a brand new car next year, if a few things fall into place, but will only be looking at petrol.
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