BMW Petrol vs Diesel vs Electric, Which Suits You?

There's a lot of nonsense spoken on both sides of the diesel vs petrol debate, this page aims to get at some of the facts and help you decide which style of BMW is best for you.

Fuel Costs

The most common reason for someone to consider a diesel is the alleged lower running costs. But are they lower? For a start diesel is a few pence a litre more than petrol in the UK because UK refineries were designed to produce a very different diesel / petrol ratio than is now the case.

The big deciding factor is annual mileage, 12000 is often quoted as the point where diesel makes sense. Let's have a look at some figures, we'll assume petrol costs £1.10 a litre and diesel £1.15.

Car	MPG	6000	12000	18000	24000
330d	40	759	1518	2277	3036
330i	32	908	1815	2723	3630
	Saving	149	297	446	594
118d	50	607	1214	1822	2429
120i	36	807	1613	2420	3227
	Saving	199	399	598	798

 

Now let's mention the MPG column first. All manufacturers claim amazing figures for their cars, you're only going to get that if you're sitting on the motorway with the cruise control on at 60mph all the time. In practice you'll do some (or a lot) of short journeys with a cold engine, you'll sometimes drive the car hard and on a diesel the mpg can vary enormously if the engine is clogged up with soot. You didn't get a 330d for economy but because you wanted a fast car, otherwise you'd have bought a 320d. I think the figures I've put here (which are for Imperial not US gallons by the way) are reasonable.

For the driver with the 330i/d we can see that annual savings are likely to depend on driving style as much as anything else. If you thrash your 330d you'll likely get worse mpg than someone who drives their 330i gently. The defining factor is the driver. Of course you can get excellent mpg from a 330d, over 40 easily. But whether that's your average depends on your daily driving pattern and style. Even then the financial savings seem pretty small, under 300 a year for our average 12,000 mile a year driver.

Now the more frugal cars, the 118d and the 120i. A 118d can get as much as 60mpg, maybe more. But even to get a 50 average you're going to have do mainly out of town long distance driving with a lighter foot and as before a badly serviced car will give less. But this time we can see that a high mileage driver can make savings, but at 12,000 a year it's not a big number.

But the fuel economy isn't the only cost difference.

Clogged EGR valve from an E46 320d.

Vehicle Costs, Diesel Problems and Reliability

In recent years diesel cars have held their value slightly better than petrol cars and been more expensive second hand, this alone can wipe out much of the annual fuel savings. In addition they can be more expensive to service and are more prone to problems requiring the intervention of a mechanic.

Diesel cars have problems petrol cars don't, this is common to all of them and not just BMW. For one thing older diesels take longer to warm up than a petrol car. This is because there's no throttle plate so they take a big gulp of air every revolution, on a petrol car the throttle limits this. BMW and others solved this by using swirl flaps from 1999 onwards, but on some engines they were faulty and caused a lot of problems (details here). The issue is solved on newer cars.

Diesels burn less cleanly than a petrol engine and create a lot more soot. In the old days this was just blown out, that's illegal now. So it's recirculated and can gum up engine parts clogging things like the EGR (exhaust gas recirculation) valve and the backs of the inlet valves. This lowers performance and reduces economy. The only fix is to strip off engines parts and manually clean them.

In later cars you may find you have a diesel particulate filter (DPF) which collects the soot. The idea is that it'll burn it off once it warms up. But it you only do short journeys it never gets warm, clogs solid and costs an unpleasantly large sum of money to replace. Short journeys really are the enemy of all engines, but especially diesels.

From 2014 even tougher emissions laws mean that on many diesels (especially larger ones) we now have in effect a second catalytic converter, but in order to function it needs a tank of dilute ammonia (often call AdBlue) to further react with the exhaust gases and reduce emissions. If the tank runs dry the car will refuse to start, that's the law.

Most new petrol BMWs have a turbocharger, so they're on a level playing field with diesel in that regard. But personally I'd prefer a non-turbo petrol engine, it's one less thing to fail. Most pre-2011 petrol BMWs are normally aspirated.

Environmental Considerations

Even with all this technology a diesel is still putting out more nitrous oxides than a petrol car and that causes people to have breathing problems.

Diesels became popular in Europe and the UK because they can emit less CO2 per mile than a petrol car, at the time politicians focused on this with good intentions to help target global warming. The down side was increased particulate emissions and greatly increased nitrous oxides. It's gotten to the stage where cities such as Paris are trying to ban diesels on certain days of the week in order to improve air quality.

The USA was spared this because until about 2008 American diesel was too high in sulphur for European engines to work there. Even on petrol engines BMW sold the N43 engine in Europe with gasoline direct injection (GDI) and the N46 in the US and Australia without because the sulphur content in petrol was different.

Driving Differences

Pre-2011 most BMW petrol engines didn't have a turbo. This meant that in order to get peak power for overtaking or spirited driving you had to drop a gear and rev them. This was good fun, it was a skill to be practiced and polished. Diesels aren't like that, they're all about lower revs and torque. Turbo diesels in the mid-90s helped a lot, if you've never driven a non-turbo diesel you're lucky.

Diesel cars don't rev so much, even a good BMW diesel. They're more suited to an automatic gearbox and deliver lots of torque and power across a wide rev range. A basic 320d can deliver more torque than E46 M3 for a good chunk of the power band. It's not a driving style I like personally although some will prefer it. Sadly with the advent of mostly petrol turbo engines in BMWs it's becoming a thing of the past.

The i3, a much better idea than a diesel.

What About Electric Cars?

Now this is the new curved ball, the electric car. An ideal solution for the commuter, assuming you have a driveway where you can fit a charging point. Not so good it you live in a third floor flat in town. If you mainly drive to work each weekday and your journey is under 40 miles each way the BMW i3 is for you, but get the version with the petrol range extender. You may find a government grant covers the cost of installing your fast charging point, if you're away from home you can charge from any mains socket but it'll take longer.

The i3 drives well and is a real BMW. Only caution I'd make is about the low rolling resistance tyres, these will be useless on snow and ice so if you live somewhere where that's even remotely going to be an issue get winter tyres (excellent advice for all BMWs). I see a lot of i3s every time I go to Norway.

The sensible way to run an i3 is on personal contract hire, in the UK this can be as little as 300 a month - you probably spend that on fuel. You don't want to own the car long term as the battery will need to be replaced one day and that won't be cheap. If you buy a used one be certain you know what's going to happen if the battery starts to fail.

Personally I'm hoping for an i3 coupe before long. With that low centre of gravity due to the battery it'd handle superbly.