Petrol or diesel: which used car should you buy?

Is a diesel worth it for the miles I actually do?

A diesel earns its keep when you regularly drive long, settled journeys — a commute of real distance, towing, or motorway miles most weeks. If your week is the school run, the shops and the odd trip out, the extra complication of a later diesel buys you very little. Work out the journeys you genuinely make in a normal week before you decide, not the journeys you might make one day. Read this alongside how many miles is too many on a used car: a higher-mileage diesel that has done steady long runs can be a sounder car than a low-mileage one that never properly warmed up.

What goes wrong on a diesel used only for short trips?

Short, cold trips are the hardest life you can give a diesel. The exhaust filtering on a later diesel needs the car to get properly hot to clear itself, and a car that only ever does ten minutes at a time rarely gets the chance. Over the years that tends to show up as warning lights, lumpy running and a repair that a petrol car of the same age would never have needed. If your driving is mostly local, that is the single biggest reason to look at petrol instead.

Is a petrol car cheaper to run?

Not always — it depends on the car and on your mileage. A petrol engine generally uses more fuel than a diesel of similar size on a long run, so a high-mileage driver often still comes out ahead in the diesel. But petrol cars are usually simpler, with less exhaust after-treatment to go wrong, and simplicity matters more than pence per litre once a car is ten years old. What really decides the cost of keeping a car on the road is set out in what makes a used car cheap to run.

Which is cheaper to tax, petrol or diesel?

The registration date decides it far more than the fuel does. A car first registered between 1 March 2001 and 31 March 2017 is taxed by its CO2 emissions band, which is where the efficient diesels of that era often do well. A car registered from 1 April 2017 is on a standard yearly rate instead, with an extra charge for five years if its list price was over the threshold. So check the band for the exact car rather than ruling out a fuel — what makes a used car cheap to tax explains the bands. Remember the tax does not come with the car: as the new keeper you have to tax it before it goes on a public road, and you can do that straight away online with the V5C reference.

How can I tell how a used car has been driven?

Its MOT history says more than any advert. Anyone can check a car's MOT history free on GOV.UK with the registration number: every test result, the advisories, the failures and the mileage recorded at each test. Big, even jumps in mileage point to long runs; very small yearly mileage on a diesel is the pattern worth asking about. Every one of our car pages old enough to have been tested shows that DVSA history test by test, with the mileage at each test, so you can read the pattern without leaving the page. How to check a used car's MOT history walks through it, and a car in England and Wales needs its first MOT by the third anniversary of registration and then one every year.

What should I check on a used diesel before I buy it?

Check how it behaves cold, and check that it gets hot. A short shuffle round the forecourt tells you almost nothing about a diesel.

• Start it from cold if you can, and listen to the first thirty seconds.
• Watch the dashboard as it starts — note any light that stays on.
• Drive it far enough and fast enough for the engine to reach full temperature.
• Look for a service history that matches the mileage, not just a stack of receipts. What does full service history mean sets out what counts.
• Read the MOT advisories for anything repeated year after year.
• Ask what the car was used for before it came to us.

What to look for on a used car test drive covers the rest of it, petrol or diesel.

What does ONECARS do before a car goes on sale?

Every car is checked on 34 points by ONEFIX, our own workshop on the same site at Plas Acton, Wrexham — the whole list is published at how we check our cars. Where that check leaves work to do, our system will not let the car be handed over until ONEFIX has put it right. Each car page carries the car's photographs, its full spec and its MOT history, so you can do most of your homework before you drive over. And there is no admin fee: the price shown is the price paid.

Can I hold a car while I make up my mind?

Yes — a reservation holds the car for fourteen days. Within those fourteen days you can cancel for any reason and the fee comes back to you in full, and if you go ahead it comes off the price instead. That is long enough to get an insurance quote on the exact car and to think about the petrol-or-diesel question properly. How reserving works has the detail.

What if the car is delivered and I change my mind?

A car bought from us online and delivered is a distance sale, so you have fourteen days from delivery to change your mind and return it. We deliver free within 20 miles of Plas Acton on weekdays in hourly slots, and the car is checked, valeted and fuelled before it leaves us — see delivery and can I return a car I bought online.

What happens after the sale?

Every car comes with a free three-month warranty on the engine and gearbox, up to 2,500 miles, with any repairs done by ONEFIX on the same site, and longer paid cover is available if you want it — what our warranty covers spells it out. After that, ONEFIX next door goes on servicing, repairing and MOTing the car for as long as you own it. For a diesel in particular, having the workshop that checked the car also looking after it is worth having.

Where can I see petrol and diesel side by side?

At Plas Acton Garage, 282 Chester Road, Wrexham LL12 8DU, or at our second forecourt at Ruyton near Shrewsbury. We are open Monday to Friday 9 to 5 and Saturday 10 to 4 for viewings by appointment. A straightforward petrol example on the forecourt now is the 2010 Volkswagen Polo 1.2 at £2,995, and a long-legged diesel one is the 2012 Mercedes-Benz E220 CDI at £3,995. The rest are on used cars, and if you would rather just describe your week's driving to someone, get in touch or ring the office on 01978 356983.