buying a car - does it matter diesel or petrol?

(8 Posts)
wildpoppy Thu 21-May-15 21:21:00

Just that really - we need an automatic that can fit three child seats for 5k or under. most in our price range appear to be diesel - does this matter?

LexLoofah Thu 21-May-15 21:44:15

wild sorry don't know the answer, read something recently about diesels becoming unpopular due to fuel duty changes, hoping someone else knows something more specific as we are in the same boat - looking for a new car and don't want to pick the wrong type both in terms of running costs and future re-sale

caroldecker Thu 21-May-15 21:55:37

some diesels will be banned from London and are having a higher parking charge in residents bays (again in London). Other nig cities are considering this - outside this, it makes little odds.

WMittens Fri 22-May-15 08:55:21

In addition to what caroldecker posted, there is a lot of rumour that tax will be increased on diesel vehicles, as the EU is going to fine us over missed pollution reduction targets.

Maintenance-wise: if you mostly do short, slow journeys (urban driving, stop start traffic) it's going to knacker some of the parts quicker - the Diesel Particulate Filter (DPF) needs to regenerate every now and again to burn off the crud it's accumulated; that needs sustained high engine temperatures, e.g. 70mph on a motorway for at least 10 minutes.

This is why diesels are better suited to high mileages, sales-rep-type driving. A ballpark estimate is under 16,000 miles a year you won't see much (if any) saving in fuel costs by using a diesel, and possibly increased maintenance costs.

lljkk Fri 22-May-15 09:35:37

Wow, 16k is the highest figure I've read yet, for the threshold when diesel is better than petrol. More like 7k or 9k in other sources.

I understand that even with petrol cars it knackers them faster to use for short journeys, so that's something to avoid as much as possible either way.

Our other impression is that diesels are more reliable but they cost more to fix when they do go wrong.

Bunbaker Fri 22-May-15 09:43:39

When we changed OH's car last month we went back to petrol as the cities near us are talking about banning diesel vehicles.

WMittens Fri 22-May-15 10:40:45

Wow, 16k is the highest figure I've read yet, for the threshold when diesel is better than petrol. More like 7k or 9k in other sources.

Those figures probably don't take into account the increased maintenance - if your DPF goes that going to be a few hundred quid to fix; DMF will probably be £600-£1000 depending on the car. That buys a lot of petrol.

It ultimately depends on the type of journey - if you do a journey every day, 7k miles per year is less than 2 miles on average - the engine wouldn't even be warm. If it's two journeys a month each of 290ish miles, that's a completely different matter - it's all down to how the car is being used.

Commuting 15 miles a working day is about 5000 miles a year, plus another couple of thousand for domestic purposes to take you up to 7000 - this is not diesel territory.

Diesels take longer to get up to temperature and won't be fully fuel efficient until they are; petrols warm up quicker and get their maximum fuel efficiency (for a given situation) sooner, even if that efficiency is lower on average than a diesel. By the time you've worked out pence per mile (petrol also being cheaper than diesel) on short journeys, there's probably nothing in it.

I understand that even with petrol cars it knackers them faster to use for short journeys, so that's something to avoid as much as possible either way.

It's probably not great, but nowhere near to the extent of a diesel - the flywheel torque of a diesel reaches its peak at about 1750-2000 (and may continue on a wide band); a petrol engine may not reach its peak torque until 3000-4500, or even higher. Plus the diesel peak could be 4 or 5 times that of the petrol. Given that in everyday driving people are using maybe 1500-2500 in a diesel, or 2000-4000 in a petrol, the diesel spends a huge amount of time bashing the DMF just as hard as it can - the petrol on the other hand is bashing it maybe a tenth as hard (because it's not near peak torque, and peak torque is much lower anyway).

Our other impression is that diesels are more reliable but they cost more to fix when they do go wrong.

15 or 20 years ago, sure, but modern diesels have tried to keep apace of demand for big performance with low fuel consumption, their popularity has been driven by tax bias. Now we have high power diesels that are complicated and fragile. BMW's problems with swirl flaps could destroy the engine if they broke off and were ingested (which happened more often that would have been liked).

Motortrader Fri 22-May-15 13:43:32

^ Good advice from Mittens above.

Around the �5k mark, DPFs are still going to be rare (because the car will be too old to have one), but there are plenty of other sources of trouble. We are seeing a lot of problems with blocked EGR valves and sticking turbo vanes on middle aged diesels, especially Fords and VWs that are used for short trips.

DMF isn't relevant here (the OP wants an auto).

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