I live in North London, where 30mph speed limits have been cut to 20mph in recent years.

I've owned the same petrol Vauxhall Astra since 2015, and I'm convinced that I'm filling up with fuel more often now than I was before.

This must be because my car uses more fuel at 20mph than at 30mph and I heard someone claim this on the radio the other day too.

I've read online that engines are most fuel efficient when driving at speeds between 55mph and 60mph, and that they become increasingly less economical at speeds higher and lower than this.

I find it very difficult to drive at 20mph; I constantly have to change gear because the engine revs too high in second, and I'm almost stalling in third.

Is changing gear more often, and generally driving 10mph slower, costing me more money and using more fuel, which is bad for the environment? James B, Islington, via email

Rob Hull, Daily Mail & This is Money motoring editor, replies: This is a question many drivers will want to know the answer to, given the wider rollout of 20mph zones.

Some 62 out of 153 local authorities in England have already reduced their 30mph limits. Wales has 20mph as its default in built–up areas, and the Scottish Government imposes 20mph on 'those roads where it is appropriate to do so'.

As such, around a third of roads in Britain's towns and cities are subject to 20mph limits, and it could soon be much more.

Government advisers recently recommended a blanket 20mph limit across all urban areas, because it would save money on signs. The independent Parliamentary Advisory Council for Transport Safety also says making 20mph the new 'standard' urban limit removes the need for councils to update paperwork.

First and foremost, it's important to acknowledge the thinking and logic behind the switch to 20mph limits.

Councils – and Transport for London – have been slashing 30mph zones largely for two reasons: to reduce road casualties and to make walking and cycling more appealing. The latter has a secondary impact of reducing local road transport emissions.

But what is certainly not being taken into consideration is the impact of 20mph zones on the fuel efficiency of cars driven in them.

Various studies over the years have shown that cruising at 30mph is more fuel–efficient than at 20mph.

For petrol engines, this is because motorists typically have to use a lower gear, with higher engine revolutions, at 20mph.

To get technical, driving with a partially open throttle to modulate this lower speed also acts as a flow restriction on the air intake during the induction stroke, which wastes energy and lowers the car's miles–per–gallon performance compared with driving at 30mph.

However, this is only the case when cruising at 20mph and 30mph. Cruising is something you are unlikely to be doing all the time while adhering to speed limits in dense urban areas – like North London – where congestion levels are high and there are significantly more junctions and traffic lights to provoke stop–start driving.

Future Transport London (FTL) – a completely independent organisation that campaigns for sustainable improvements across the capital – in 2023 extensively analysed the impact of 20mph zones v 30mph limits in Britain.

On a simulation model based on a typical 2.5–mile (4km) journey through London, four different cars – a petrol and diesel Ford Focus family hatchback, a large petrol Mazda CX–9 SUV, and a diesel BMW X5 – were tested at 20mph and 30mph, repeating the same route 50 times.

It also analysed the impact depending on whether the car ran on petrol or diesel, vehicle size, and both conservative and aggressive driving styles when completing the same course.

Gear changes – both up and down – were made at the same revolutions per minute (RPM) for the two petrol cars and the two diesels for consistency.

The report concluded that the additional energy required for the engine to repeatedly accelerate up to a 30mph speed limit in stop–start driving means 20mph zones are generally more fuel–efficient.

It explained: 'The energy required to accelerate vehicles to the speed limit was the largest contributor to fuel consumption in urban traffic conditions. Vehicles are often more efficient at higher speeds, but this does not account for the energy required to reach these speeds.'

So how long would you need to cruise for to get back to 30mph being more efficient?

For the petrol Ford Focus, FTL found that it would need to be driven at a consistent speed of 30mph for 0.91km (0.56 miles) before it makes up for the additional energy required to accelerate to 30mph compared with 20mph and then become more fuel–efficient.

However, the study says: 'In urban areas, particularly with other traffic on the road, it is typically not possible to travel even close to 0.91km without having to slow down or come to a stop due to junctions, pedestrian crossings, or other vehicles, and in these cases, it is always more efficient for the Ford Focus to travel at 20mph than at 30mph.'

The average fuel efficiency for the petrol Ford Focus to complete the 4km test route at a simulated speed limit of 30mph was 27.4mpg, while in a 20mph zone it was 37mpg – an increase of 35 per cent.

For the larger Mazda CX–9 petrol and BMW X5 diesel SUVs, this efficiency crossover at 30mph does not occur until 1.3km (0.8 miles) and 3.08km (1.91 miles), respectively, of consistent driving without slowing down and accelerating.

The study also found that the average time to complete the 4km route was just 7 per cent longer on average at 20mph compared to 30. 

To measure the fuel economy impact of reduced 20mph zones for petrol and diesel engines, the study compared the results for the Mazda and BMW SUVs.

To accelerate to 20mph, the report found that the diesel BMW X5 required 5.2 per cent more energy than the Mazda, but to reach 30mph it required 9 per cent more.

However, because diesel fuel contains around 15 per cent more energy per unit, their fuel consumption is almost the same at 20–to-30mph speeds.

The FTL report says that while driving style does have an impact on consumption at lower speeds, it is nominal in comparison to the fuel energy required to accelerate up to specific speed limits.

As your question rightly says, driving at 20mph can require constant gear changes. But this might not have a dramatic impact.

The report found that driving 'conservatively' – which means using a higher gear with lower RPM – will save you fuel, but not as much as you might think.

In the Ford Focus petrol – comparable to your Vauxhall Astra – measurements were taken for 'balanced' driving (with gear changes at the same RPM as the other models tested) but also for 'conservative' (higher gears with lower RPM, resulting in slower acceleration) and 'aggressive' (lower gears with higher RPM for faster acceleration) styles.

It found that using a conservative style uses only 4.7 per cent less fuel than balanced driving in 30mph limits, and 8.5 per cent less fuel than driving in an aggressive manner. In 20mph zones, the difference is even smaller.

Nick Molden, founder and CEO of independent testing specialist Emissions Analytics, responds: Combustion engine cars inherently have lower emissions, both carbon dioxide and pollutant emissions, at intermediate compared to slower speeds. 

This would suggest that imposing 20mph speed limits would increase emissions but this is not always true.

While this is correct for over 80 per cent of driving distance in the UK, the opposite is true for the other 20 per cent, which takes place in town and city centres. 

The difference is down to how much stopping, starting and congestion there is to start with.

Taking London, average speeds are 7.4mph, so reducing the speed limit from 30mph to 20mph reduces emissions by avoiding cars speeding up to 30mph, only then to have to slow down again soon. The process of speeding up takes a lot of energy and creates emissions. 

So, if you have congestion, or junctions and crossings at regular intervals, a slower speed limit is preferable. Even better, the lower limit hardly increases journey times. 

While so much focus is on speed limits, cities tend to miss the rising trend of vehicle size. Bigger and heavier vehicles inherently create more emissions, but they also increase journey times significantly, as fewer vehicles can pass through a green light, for example. 

So, cities must consider the effects of vehicle size, including electric vehicles, to avoid undoing the benefits of reduced speed limits.

Rob Hull responds: FTL suggests 'environmentally conscious' authorities and councils should review their own areas because all roads are not the same.

Any stretches of road with no junctions or road furniture - such as crossings and speed–reducing humps or chicanes - over distances between 0.5 and 0.8 miles could see an increase in localised traffic pollution if 20mph zones were introduced, because cars cruising at 30mph are more economical and produce lower emissions.

However, I can't imagine there will be many roads though towns - and certainly not cities - where this will be the case.

Jack Cousens, head of road policy at the AA, responds: From an insurance perspective, a lower speed limit is preferred, as collisions tend to fall and, when there are claims, they tend to have lower values compared with those at 30mph. That’s something all drivers could benefit from, as claims costs are the biggest driver of premiums.

On the other hand, a blanket 20mph limit could weaken the streets immediately surrounding areas of high community benefit and footfall (e.g. schools, GP or dental surgeries, local shops, etc.). Many people understand and expect the limit to be lower on these roads, but support and compliance might lessen if roads 20 to 30 streets away are also set at 20mph.

The biggest issue in all of this is enforcement. Whatever limit is set, it needs to be policed - and certainly in urban areas, this is hardly done. The ring roads and main roads around estates might get a camera van or a police officer with a hand-held speed camera once or twice a year, but other than that, drivers are left to it.

I’m not saying we need a mass installation of cameras, but we certainly need more police officers in cars to enforce the roads.

Oliver Lord, UK director of the Clean Cities campaign group, responds: We mustn’t forget the main reason for 20mph limits is fewer deaths and serious injuries, less noise, and a better environment for everyone to get around.

When introduced with good street design and traffic reduction measures, a 20mph limit is fundamental to creating a cleaner, safer and more liveable city.