Buying a Tow Car - Which Fuel?

Diesel, petrol, hybrid engines and all electric have their advantages for towing.

There have been big changes in the car market over recent years. Following the 'dieselgate' scandal of 2015, in which Volkswagen was found to have cheated emissions tests in the US, diesel cars have declined in popularity. Petrol power now takes the lion's share of new car sales, and electric vehicles and plug-in hybrids are taking a growing share of the market.

Although petrol cars account for around 65 per cent of UK new car sales (2019), tow car drivers shouldn't dismiss diesel. This type of engine still has many advantages for towing, and the latest generation of diesels emit much less than older diesels.

In the longer term, both petrol and diesel fuels look set to be replaced by battery electric cars (and possibly hydrogen fuel cell vehicles), with a move to hybrids and plugging hybrids bridging the gap between the internal combustion engine and the fossil-fuel free future.

Growing numbers of hybrids are homologated for towing, and even some pure electric vehicles.

So, how do you separate anti-diesel fact from fiction? Are modern petrols just as capable of towing as their diesel counterparts? And are hybrids and electric vehicles now capable tow cars?

Which type of fuel is best for towing?

Diesel versus petrol

Although alternative technologies are gaining popularity, most tow car drivers are choosing between diesel and petrol. Here is a look at the pros and cons of each.

Diesels are almost always more fuel-efficient than the equivalent petrol car, and that can lead to big savings at the pumps. We have taken a basket of three popular cars (the BMW 5 Series Touring, the Kia Sportage, and the Skoda Karoq) and compared the fuel bills of a petrol and diesel model. Our calculations are based on the official combined figure, under the new Worldwide Harmonised Light Vehicle Test Procedure (WLTP) standard. The results of these new tests are much closer to the economy drivers are likely to achieve in real-world driving than the old NEDC (New European Driving Cycle) tests.

The diesel versions saved between £209.51 and £411.42 for a driver covering 10,000 miles per year, although this must be set against the higher cost of diesel cars: expect to pay a premium of around £1,000 to £2,000.

It will take many miles to make up for the higher price in fuel savings, but the greater your towing mileage the more you are likely to save. In 2013, The Camping and Caravanning Club compared fuel economy when towing with petrol and diesel cars, working with Practical Caravan and What Car?. The research showed that economy worsened far more for petrol cars than diesels when towing a caravan.

It is not just that diesels will cost less to run than petrols. The way the engines respond when the driver presses on the accelerator is also important. Diesels generally have more torque than petrols, and this is delivered at lower engine revolutions. This means that diesel engines respond more strongly in the middle of the rev range than petrols, and are better able to pull in a high gear when towing.

By contrast, petrol engines often have more power at the top of the rev range. This means the driver needs to hold on to a lower gear for longer when accelerating to get the best from the engine. To maintain speed on hills or overtake when towing, a driver will need to change gear more often when driving a petrol car.

Although this generalisation still holds true in most cases, there is a strong trend in petrol engine design for small capacity turbocharged units. By using turbocharging to force air into the combustion chamber these engines are more efficient and have torque characteristics that make them better suited to towing than most non-turbo petrol units.

For example, the 1.5 TSI 150PS engine in the Skoda Karoq and many other VW Group cars has 250Nm of torque. That's more than Mazda's non-turbo Skyactiv-X engine in the 3 hatchback and the CX-30 SUV, which has 224Nm.

It's not just that the turbocharged petrol has more torque, it's also delivered lower in the rev range. The Skoda's TSI puts out its maximum from 1,500-3,500rpm, whereas the Mazda engine delivers peak torque at 3,000rpm.

However, even an efficient and punchy turbocharged petrol struggles to pull as well as a turbodiesel. The equivalent diesel in the Skoda Karoq range has 340Nm of torque. That's a 36 per cent increase over the turbocharged petrol, enough to be clearly noticeable while towing.

Diesel versus petrol tow cars: which weigh more?

As well as having more torque, diesel engines tend to be heavier than their petrol equivalents. This extra weight helps with stability when towing.

Take the BMW 5 Series Touring from our fuel cost comparison. The 520d Touring has a kerbweight of 1,735kg, whereas the 520i Touring has a kerbweight of 1,705kg. That gives an 85% match figure of 1,475kg for the diesel (see Matching Car and Caravan ) , and 1,449kg for the petrol.

Often, diesels will have a higher legal towing limit than the equivalent petrol. The Kia Sportage diesel in our comparison has a 1,900kg limit, whereas the petrol has a 1,600kg maximum. In the case of the Skoda Karoq, the diesel's maximum is 1,800kg, the petrol is 1,500kg. The BMW is the only car of the three with identical towing limits for both diesel and petrol models (2,000kg).

Reasons to choose a petrol tow car

On the face of it, that makes quite an open and shut case for a diesel engine, but petrol models have their advantages. The most obvious is price. As mentioned above, the difference in cost can be as much as £2,000, which will take many miles to make up in fuel savings.That is one reason why petrol cars can make more sense for low mileage drivers.

In the past, diesels have tended to hold their value better than petrol cars. However, as the popularity of diesel has waned, that pattern has changed. According to What Car?, the Kia Sportage petrol in our fuel economy comparison retains 51% of its original price after three years and 36,000 miles, some 8% more than the diesel In the case of the Skoda, the petrol hangs on to 51%, the diesel 46%.

If you are buying a new car, that means you benefit twice from choosing a petrol – not only will you pay less up front, depending on the make and model you may hang on to more of what you paid when you eventually sell the car on.

For used car buyers, it makes diesel a more affordable choice, as the price premium for a new diesel is eroded over time.

Although service intervals on many modern cars are identical whether you choose petrol or diesel, servicing bills are generally cheaper if you own a petrol car. Then there is noise to consider. Petrol engines are generally quieter and smoother than diesels.

Which is more realible petrol or diesel?

Research by the consumer group Which? suggests that - contrary to popular belief - diesel engines are less reliable than petrols. In particular there is one common problem with today’s diesels: they do not like short journeys. Modern diesel engines are fitted with Diesel Particulate Filters (DPFs) that remove sooty particles from the exhaust. Once in a while the DPF burns off the particulate matter it has trapped, but it needs to reach a high operating temperature to do so. If your driving mostly consists of brief town journeys, the DPF can become clogged. If this is not covered under warranty it may cost a four-figure sum to put right.

Manufacturers have been working to get round this problem, but it is a drawback that has not been eliminated completely.

Petrol cars also have some environmental advantages over diesels. While diesels produce less carbon dioxide (CO2), one of the gases linked to climate change, they do not burn as cleanly as petrols.

Diesels produce more sooty emissions, which DPFs are designed to trap. If inhaled, these particulates can be harmful to the lungs and heart.

Older diesels also produce more nitrogen oxide (NOx), which are also harmful to human health. But the best brand new diesels now produce very little NOx.

Cleaner diesels

All engines must meet certain European regulations, but diesels are allowed to emit more of some pollutants than petrols. However, the Euro 6 standard, which came into force for new models in September 2014 (and existing cars from September 2015), has narrowed the gap in what’s acceptable for the two fuel types. Under Euro 5 rules, diesels could emit three times as much NOx as a petrol car. Under Euro 6, the NOx limits have stayed the same for petrol engines but diesel limits have dropped to just a third more than for petrol cars.

Updates to the Euro 6 standard have also pushed manufacturers to make cleaner petrol and diesel engines. Since September 2017 cars have been tested to the tougher and more realistic WLTP standard, rather than the old NEDC system. So while the permitted level of emissions hasn't changed, the test method has.

The WLTP tests are supplemented by RDE (Real Driving Emissions) tests on the road. These are designed to check that cars are not producing significantly more pollution in real-world conditions than in the laboratory, and to make it more difficult for a manufacturer to cheat the tests.

To help achieve Euro 6 under more stringent test conditions, manufacturers have looked at a variety of new technologies. One of the most important is Selective Catalytic Reduction (SCR), which uses a catalyst and a urea solution called AdBlue (or Diesel Exhaust Fluid) to remove NOx emissions from the exhaust.

The AdBlue is stored in a separate tank and will need topping up from time to time. Volkswagen’s Passat uses AdBlue in some models and VW says a full 13-litre AdBlue tank should last for approximately 5,600 miles.The driver receives a warning message on the dashboard to indicate the tank is running low. Replacement AdBlue is available through car dealers and many filling stations. Prices vary, but expect to pay around £1.50-£2 per litre.

Whichever technology is employed, the new latest generation of diesels is much cleaner than earlier engines. However, independent testing by companies like Emissions Analytics suggest there are still significant difference between the cleanest and dirtiest cars, even those which meet the Euro 6 standard.

Petrol versus diesel: which is best for towing?

For the right driver, LPG makes a sensible alternative fuel and hybrid and electric cars also have their place. Hydrogen power is also on the horizon, although for the next few years the restricted refuelling network will limit its appeal. For most tow car drivers, the choice still comes down to petrol or diesel.

So, there are clearly pros and cons to both of the major fuel types. Which is best depends largely on how many miles you drive, what you need to tow and how often you tow it.

The lower your annual mileage and the lighter your caravan or trailer, the more sense petrol makes. However, for regular towing – especially if you holiday in a substantial caravan – the extra pulling power of a diesel engine makes it a better choice for most drivers. And the best new diesels emit far less than their 'dirty' reputation would have you believe.

Should I convert my tow car to LPG?

A few years ago LPG (liquified petroleum gas) was touted as the next big thing. Less expensive to buy than petrol, LPG seemed like the ideal fuel for owners of petrol cars – especially big 4x4s – looking to keep a lid on their running costs. The reality is a little more complicated, although for the right tow car and driver LPG still has a lot going for it.

Most petrol cars can be converted to run on LPG. This involves installing a second fuel tank, often in the spare wheel well. The car can then run on LPG where available, or on petrol if you struggle to find an LPG filling station. However, according to the dilllpg.co.uk website, there are 1,873 refuelling sitesin the UK, so it should not be too much of chore to find one.

The benefit of LPG is not improved fuel economy – even LPG’s keenest advocates admit that fuel economy is likely to worsen by around 20 per cent – but because the price of LPG is so much lower than petrol, your fuel bills should fall dramatically.

For example, a petrol-powered 4x4 that returns 25mpg fuelled by unleaded petrol might return 20mpg using LPG. Travelling 10,000 miles fuelled by unleaded would mean a bill of £2,302 (based on a fuel cost of £1.266 pence per litre). Driving the same distance using LPG would cost £1,452.50 (based on 63.9ppl). That is a saving of £849.50., or just over 8p for every mile driven.

Of course, there is the cost of the conversion itself to consider. This varies depending on the make and model and the company doing the conversion, but is likely to be around £1,200 (take a look at the DriveLPG (www.drivelpg.co.uk) website for a list of approved installers). Using that figure and the estimated fuel bills above it would take 14,126 miles to make back the cost of the conversion in fuel savings. That is if you are able to drive exclusively on LPG, so in practice the true mileage is likely to be higher.

So, if you plan to keep a car for a while an LPG conversion should pay for itself and ultimately leave you better off. However, there are some disadvantages to be aware of. Just as mpg worsens when using LPG, so power is also reduced. Many owners say they can’t tell the difference, but it is there.

Also, if your car is still under the original warranty an LPG conversion may invalidate it for the related components, although this is not a concern with an older vehicle. You will also not be able to take your car through the Channel Tunnel (although it can be taken on cross-Channel ferries.)

The loss of the spare wheel well is arguably a bigger issue. Drivers can carry a spare secured in the boot, but this leaves less room for luggage so carrying the spare over the axle in the caravan may be an option.

Overall, though, if you own an older petrol car and plan to hang on to it for a good few years, an LPG conversion could save you hundreds of pounds in the long run.

Time to switch on to a hybrid or electric?

Hybrids and electric cars are no longer niche vehicles, and now account for around three per cent of cars sold. An increasing number of hybrids – and even some pure electric vehicles – are approved for towing.

Broadly speaking, there are two kinds of hybrids. Conventional hybrids convert kinetic energy into electricity through regenerative braking systems and can also top up their batteries using the petrol or diesel engine as a generator. This does away with the need for plugging the car in to charge the batteries, but the distances such cars can travel on electric power alone are relatively small. Hybrids of this type homologated for towing include the Lexus NX and RX hybrids,and the Range Rover and Range Rover Sport Hybrids.

Increasingly, manufacturers are turning away from conventional hybrids in favour of plug-in hybrids that can be recharged from domestic sockets or special charging points, as well as using regenerative braking or taking power from the engine. These travel further on electric power alone than regular hybrids (more than 30 miles in ideal conditions, depending on the model). Cars of this type that have high enough towing limits to pull a caravan include the Mitsubishi Outlander PHEV and the Volkswagen Passat GTE.

Both kinds promise good fuel economy and very low official carbon dioxide outputs. Because of the way the official fuel and emissions figures are calculated, plug-in hybrids in particular achieve exceptional numbers in the lab, although manufacturers admit the real-world economy is unlikely to be quite so good.

These cars are especially appealing if you have a company car, since Benefit-In-Kind (BIK) tax bills depend on a vehicle’s price and CO2 output.

As tow cars, plug-in hybrids are at their best when the battery is fully charged. In tests carried out by The Camping and Caravanning Club in association with Practical Caravan and What Car?, the Volvo V60 Plug-In Hybrid achieved 44.9mpg towing a twin-axle tourer with a full charge, dropping to 29.6mpg running with a low battery.

Petrol-electric plug-in hybrids are more common than diesel-electric hybrids like the Volvo, which is no longer on sale. These tend to deliver poor economy once the batteries are low.

So, these cars make most sense for company car drivers with short commutes, who should be able to complete their daily driving on electricity alone and benefit from low (BIK) tax bills.

Electric cars approved for towing include the Audi e-tron (legal towing limit: 1,800kg), the Jaguar I-Pace (750kg), the Mercedes-Benz EQC (1,800kg), the Tesla Model X (2,250kg) and the Tesla 3 (910kg).

However, although the range of electric cars is steadily improving, the extra weight and drag of a caravan reduces the distance these cars can travel on a charge. Our own tests suggested a range of just over 100 miles for the I-Pace while towing.

The difficulty of recharging on a long journey is another limitation. As recharging spaces are sized for cars, not cars and caravans, drivers would need to find somewhere to securely park their caravan before driving to the charging station.

The process will eventually become less onerous as more ultra-rapid chargers are introduced, which can achieve an 80 per cent charge in as little as 30 minutes. In the long-term, as and when electric cars become more common than petrols and diesels, we'd expect charging points to be added to caravan parking areas at motorway service stations.

How to save fuel when towing

• Drive more slowly. Towing at 55mph rather than 60mph on the motorway will reduce your fuel bill
• Always make sure the tyres on your car and caravan are inflated to the correct pressure for towing
• Keep windows closed for better aerodynamics
• Leave the air conditioning switched off whenever possible
• Anticipate gaps in traffic at roundabouts and junctions to reduce the need to stop and then accelerate from standstill.