Which is better a Small Petrol or small Diesel Engined Car?

A friend of mine is after a small car for blatting around town with for their job and the school runs. have found a few cars Citreon C3 and Peugeot 206 and 307 in their price range and which they like and fit their requirments and I am basically wondering is it best to go for a small petrol engine or a small diesel engine (1.4)

there driving will be short journeys 7 – 10 miles with a mixture of country roads, Town driving and sitting in traffic.

Busses, Taxis public transport and walking or cycling are not an option.

Many thanks

Economy is less relevent, so really should be looking at ppm vs reliablity/safety.

For me, small petrol.

Right now I think I'd choose a modern small petrol.

If bigger, I'd go diesel.

Diesel doesn't really become cost effective untill your doing 10,000+ miles in a year, and are better on the motorway at a constant speed.

Going petrol will get you a newer car (possibly with more warrenty remaining) and modern engines are so well made they'll last as long as the diesel anyway, but without all the expensive stuff to go wrong on them.

Honda Jazz 🙂 The new ones are brilliant.

Small petrol engines work far better than small diesel engines in small cars.

The extra weight and purchase price of the diesel are bigger proportions of the total in both cases, which means you don't see any benefit.

Petrol all the way. A diesel will be barely warmed up on those sorts of trips which, fuel economy aside, makes for rather uncomfortable winters.

Petrol engines have fewer expensive points of failure and because they get up to temp faster will be no less fuel efficient on short trips. Plus they're cheaper to buy – even if there was a small fuel saving then it would take many years to recoup.

Definitely small petrol. And for anyone doing predominantly short urban runs diesel is best avoided due to the particulate filter not having the time or revs to get hot enough to burn off the particulates, which means it saturates prematurely. Also diesels tend to cost more to buy and to service so if you're not doing enough mileage petrols work out cheaper (as well as cleaner).

Diesels make a lot of sense in big heavy cars that are racking up the miles (or indeed HGVs!)

Small petrol engines work far better than small diesel engines in small cars.

Not true, however petrol is definitely better for town driving and shorter trips as the OP requires. I had a small diesel and loved it. But it did a lot of open country miles.

Diesels don't cost more to service tho.

service, no. Fix when they go wrong, yes. There's just so much more in the way of expensive bits to go wrong – dual mass fly wheels, very high pressure injectors etc… I know some some of this stuff is creeping into petrol cars as well, but there are far far more small petrol engines without all the bells and whistles than small diesels.

Not true

I think you'll find it is. Parkers have a diesel VS petrol calculator. Taking book figures, the average small car will take 30000 miles or so to break even, and that's ignoring the cost of capital for the extra money you spend on the diesel in the first place. Bias things to town driving and it swings even further into petrol's favour.

If you look at properly small cars, a lot of manufacturers don't even bother offering a diesel option. The diesel Fiat 500s have a payback of > 60000 miles.

Diesel doesn't really become cost effective untill your doing 10,000+ miles in a year, and are better on the motorway at a constant speed.

I don't know where this myth comes from

One of the main reasons diesels are more fuel efficient than petrol engines is that they do not suffer from pumping losses. In a petrol engine the engine is sucking in the fuel air mixture against a partial vacuum unless the throttle (that's why its called a throttle) is fully open. However a diesel takes a full charge of air each cycle. Therefore at motorway speeds where the throttle is open a petrol engine starts to get closer to a diesel in fuel economy whereas the diesel will have more advantage at slower speeds.

I don't know where this myth comes from

You don't understand pumping losses. You need very few horsepower to maintain 70mph on the motorway, so petrol cars have a near closed throttle butterfly at that sort of speed.

Because a diesel engine + ancillaries is generally much heavier, they lose out in stop start conditions.

At motorway speeds the thottle isn't open unless you're sitting at top speed on the autobahn! To sustain a 70mph cruise on the flat in a car that isn't shaped like a barn door requires about 25bhp, so less than half power from the most stingily engined modern car.

my 05 plate clio 1.4 dci returns 60mpg if i commute to work 5 miles each way.

does 70-80 on the m way.

60mpg is pretty good going, just thought i'd throw that in.

yes, but what would you get out of a similar petrol, and how many fuel tanks of petrol could you buy for the price differential between the petrol and diesel versions?

I don't know where this myth comes from

It's simple maths. Diesel fuel costs are X per mile, petrol are Y per mile. But diesels cost Z more to buy. So for it to be cost effective, the following needs be true

Z < (X-Y) * N

where N is the number of miles you do in total. I think N usually comes out to be around 30-50k miles, so if you are doing 20k a year it's worth it, if you are doing 5k it might take you 10 years to break even.

Nuffink to do with pumping losses, at least not directly 🙂

The real world varies from official fuel efficiency figures. For a start, they're done on a rolling road according to a set programme. I'd be amazed if there wasn't a little bit of optimisation done.

D's aren't THAT much heavier you know. It really varies from car to car, you can't just lump all into one group.

I've a 2 litre TD that's better on fuel (across the board) than the 1.6 petrol of the same era. Significantly better – like if I'm thrashing the nads off it round town loaded it goes down to the petrol's motorway figures. But likewise I've a petrol 2 litre T that gets half of both of them. You really can't claim one is more expensive than the other, you might as well just look into the cars you want to buy, or base yoru choice on individual car selection, not fuels.

By the book servicing of my TD has so far cost me about £50 a year. About the same as the petrol. though the TD does eat the front tyres a bit more willingly so I'd have to factor that in.

For a start, they're done on a rolling road according to a set programme. I'd be amazed if there wasn't a little bit of optimisation done.

True, but relative to each other they should be accurate as the test conditions are identical, though I do wonder how they factor in vehicle weight if it's on rolling road. Most manufacturers are known to tune their econoboxes to suit the fuel economy testing, it's the only sensible thing TO do if you sell cars. That's partly why plenty of cars can be re-mapped to make huge power gains sacrificing economy/emissions.

The real world varies from official fuel efficiency figures. For a start, they're done on a rolling road according to a set programme. I'd be amazed if there wasn't a little bit of optimisation done.

That's widely known. However they have to have a standardised test. Don't expect to achieve the stated figures – they are only intended to be used as a comparison. Some manufacturers cheat the tests more though, so the tests need a re-think imo.

I've a 2 litre TD that's better on fuel (across the board) than the 1.6 petrol of the same era.

My point was about weight as a proportion to the car. A diesel might be 50-100kg heavier than a little 1000cc petrol, and when you're talking about something that's a tonne or less, that's a lot!

Definitely less of a problem in a bigger hatch or saloon.

thomthumb – Member
my 05 plate clio 1.4 dci returns 60mpg if i commute to work 5 miles each way.
does 70-80 on the m way.
60mpg is pretty good going, just thought i'd throw that in.

thisisnotaspoon – Member
yes, but what would you get out of a similar petrol, and how many fuel tanks of petrol could you buy for the price differential between the petrol and diesel versions?

We had a 1.6 Clio and that would do 39mpg.

I would like to k ow how people think a diesel cost more to service.
You change oil, oil filter, air filter every other service and fuel every so often. So that would be the same as a petrol then with out needing any spark plugs.

One thing I've found with my diesel (Audi a4 1.9tdi) is the first few miles it's doing <20mpg until it's warm then it's into the 50's. Fine for me as I don't do short journeys but school runs suggest short trips so in may end up doing 50% of it's mileage cold and getting worse figures than an equiv petrol. My old Audi 1.8T petrol reported much less of a difference between cold and warm.

You can actually remap modern cars to gain power and drivability AND real world economy, because manufacturers' engine maps are often skewed to perform well on the MPG/CO2 tests and thus less economical in normal use!

The other thing is that a little petrol engine can be a horrible thing, whereas modern diesels, even small ones, have a much more pronounced torque band, and often feel a lot more pleasant to drive.

We had a Citroen ZX 1.4 and a 1.9TD – both had about the same power, but the TD was a far nicer engine, as well as doing 50+ mpg rather than 30+mpg

See cp's post – there's a reason diesels are way more powerful than they used to be, and that's down to some fairly expensive hardware. So it's not basic servicing costs, it's the bigger expenses such as the direct injection system. For the same reason you won't be seeing such epic mileages before death as old diesels and commercial diesels manage, because the new car diesels are so much more highly stressed.

The Diesel VS Petrol longevity thing has been overstated for years now.

Back in the pre-1993 days when stuff had carburettors, and petrol cars suffered from bore wash, they would wear more. Since EFI, it's much of a muchness.

Yeah but diesels drive at lower revs, so the engine's turned over fewer times in a given number of miles. This should mean less wear on rings and bearings no?

In reality once a diesel gets really old a turbo failure or some such would render it un-economical I am guessing.

I think it depends on what ultimately kills a car. Nowadays, what is it? Rust, or a crash most likely, or an expensive repair job. You don't see that many blue smokers any more.

Lower revs but far higher compression ratios. What's killed most cars of late is being worth less than £2k…

Higher compression ratios and over-build blocks and pistons and stuff. Hence extra weight 🙂