How we test
How we test cars
By Adrian Porter
Article 1 of 2
Every car we review is subjected to hundreds of tests in our lab and on the road. Find out how our tests help you choose the best car, and avoid the worst.
Our thorough lab and road tests allow us to assess all the most important aspects of a car - ranging from how it handles to how accurate (or not) manufacturer fuel efficiency or electric car range claims really are.
Plus our annual reliability survey allows us to incorporate feedback from tens of thousands of car owners. Which means we can tell you what a car's like to live with and whether it's reliable, or not.
Whatever your needs, our unique independent testing will tell you everything you need to know to choose your next car.
Our reviews answer the most crucial questions about cars:
- How safe is it? Watch our video of a Jaguar badly handling our avoidance and stability test
- How reliable is it?
- What are its true fuel economy and emissions levels?
- How easy is it to drive?
- How comfortable is it?
- How practical and spacious is the car?
- How Which? scores cars.
Find out which cars excelled in our tests by heading to our round-up of the top cars for 2019.
How safe is it?
We make an assessment on how well adult and child occupants, and pedestrians, are protected in a crash by looking at what active safety systems a car has. This includes lane-keeping assist (should keep you in lane), blind-spot assist (lets you know when someone is in your blind spot) and autonomous emergency braking.
We also look at passive safety systems, such as how many airbags there are.
Euro NCAP is a car safety organisation that we helped set up in 1997, and we continue to use its crash results as part of our evaluation.
All cars tested by Euro NCAP are given an overall star rating out of five stars. If a car that has been tested by Euro NCAP gets three stars or less, we make it a Don't Buy car as we believe modern cars should earn four stars or more to be considered safe.
Since 1997, Euro NCAP has used crash-test dummies to record how well a car's occupants are protected in a crash, should the worst happen. But in more recent years, as safety systems become more advanced, it also looks at the effectiveness of active safety systems.
Active safety systems actively monitor the environment around the car and are designed to take premeditative action to avoid an incident altogether, or mitigate the worst effects of an impact.
Autonomous emergency braking (AEB) is a key safety system that Euro NCAP assesses. It's a system that should detect if you're about have a collision and, if you haven't already hit the brakes, will apply the brakes for you.
The latest addition to the tests in 2018 saw the AEB system being assessed for how well it detects vulnerable road users, like pedestrians and cyclists. Happily, some modern cars are up to the challenge. The second generation Nissan Leaf was the first car to go through the updated tests in 2018 - and it aced them.
But don't think that all modern cars are up to scratch just because they're new - as proved by the Jeep Wrangler being awarded a single star for safety.
Some cars have optional safety kit that you can purchase as an optional add-on, or is available with a higher trim version of the car. In these cases, Euro NCAP awards the vehicle two separate star ratings - one with the optional safety kit, and one without.
When two star ratings are available for the car, we always use the lower safety rating from the version of the car without the additional safety equipment. This is because we believe that good car safety is not an optional extra.
Which? avoidance and stability test
Hopefully it will never happen, but if you find yourself in a situation where you have to swerve around something, or someone, at the last moment, you need to be sure your car won’t spin out of control.
We conduct a test travelling at speeds up to 56mph where we swerve around an object and rejoin the original lane. This determines how stable and controllable a car remains, and whether it is likely to understeer (lose grip from the front, and continue in a straight line) or oversteer (where the rear axle loses traction, potentially resulting in a spin).
The assessment takes into account the behaviour of a car's ESC (electronic stability control) system, and how likely it is to keep the car on its intended line, should you need to make a sudden manoeuvre.
Video: watch how badly this Jaguar handles our avoidance and stability test
The video below shows the Jaguar F-Pace SUV, which handled the course badly and locked its wheels, compared with the rival Audi Q5 which handled the course very well. In real life, the Jaguar could leave you in an oncoming lane of traffic for longer.
How reliable is it?
While our lab tests are good, they can’t predict the future – that’s where the annual Which? Car Survey comes in.
Every year, tens of thousands of people tell us about the cars they own. Through this survey we learn about a car’s common faults, annual repair bills and owner satisfaction.
Simply put, if a car is unreliable, it won’t be a Best Buy.
The information you give us also helps us track any issues, from potential problems with specific models to identifying areas of research we can follow up on for future reports and investigations.
You can also use our online car reliability checker to find out what common ailments your car suffers from.
What are the car's true fuel economy and emissions levels?
With diesel-fuelled cars falling out of favour and hybrid and electric cars on the rise, we're all more aware than ever of poor air quality. Which is why our fuel economy and emission tests have never been so relevant.
The vast majority of cars can't meet their official fuel economy (mpg) figures when they’re faced with our more realistic tests, which include a motorway element to truly assess fuel consumption at high speeds.
We also measure emissions in the same stringent tests and have found a large number of cars that, although they pass official tests, emit big quantities of toxic gasses such as NOx and CO.
We've also found a few that are really clean; there is such as thing as a diesel car that produces low amounts of NOx.
This is such an important part of our tests that we've created a dedicated guide to how we test mpg and emissions.
Our tests reveal the cleanest and dirtiest cars
We keep a running tally of the dirtiest and cleanest diesel cars, where we also reveal how much of a difference AdBlue (an additive for reducing NOx emissions) makes.
We have a similar guide to the dirtiest and cleanest petrol cars.
How easy is it to drive the car?
You'd expect modern cars to all be a piece of cake to drive. But we know that's not the case. To save you from buying a car you later regret, you can use the results of our tough tests to choose the best car for your budget.
Engine smoothness and gearbox
We rate how smoothly the engine delivers its power.
- A good rating will see the car pull away in a smooth, controllable manner
- A car with a poor rating will struggle to pull away or match national speed limits, jerking upon acceleration or changing gear.
We also look at in-gear acceleration and see how well a car accelerates from 37mph to 62mph. This simulates moving to a faster lane on a dual carriageway or motorway, or overtaking a slow-moving vehicle on a country road.
In an emergency, you need to know if you’ll be able to stop quickly and without swerving. To assess this, we run repeat tests of braking from 62mph to standstill, seeing if braking distance is consistent and doesn’t increase with repeat runs.
We also brake around bends to test the car’s directional stability.
Driven on UK roads
We aim to drive every car that we test in the lab on UK roads.
Occasionally, we will present a first drive review written by one of our researchers. This is purely subjective but is a way for us to present our initial impressions of certain cars while we wait for them to be tested in the lab.
How comfortable is the car?
How comfortable you’ll be depends on how well the car handles the lumps, bumps and gaping potholes that make up the DNA of UK roads. Our test track has all those features and more.
We drive over it repeatedly to assess a car’s suspension.
We also look at how well padded and supportive the seats are, making expert assessments of lumbar, thigh and head support. Our tests reveal which cars will keep you comfortable on long road trips and which won’t.
We both measure and subjectively assess the trio of road, wind and engine noise in each of our tested models. If a car makes a massive racket the entire time you’re driving, you won’t want to be inside it for long.
How practical and spacious is the car?
We use a dummy to work out how much head, leg and knee space there is for the driver and all passengers. We don’t just get inside the car and comment on the amount of room inside.
Ease of entry
We assess every car on how effortless it is to enter and exit. This is particularly important if you or any of your regular passengers have mobility issues or are particularly tall.
So you‘ll know if it’s a case of just stepping in, or whether you’ll end up doing an awkward side shimmy or bending over double just to get yourself into the vehicle.
Rather than subjectively saying visibility is good or bad, we take measurements from all round the car using a rotating camera. We then measure the 360-degree view from the driver’s perspective, which determines how much all-round visibility there really is.
Visibility has become worse as manufacturers build safer cars with thicker window pillars, so it’s no surprise that parking sensors have become the most in-demand car feature.
Car manufacturers vary in the way they measure their claimed boot space, and can include removing carpets or counting spaces that would normally be filled with a spare tyre.
We load the boot with foam blocks to figure out the usable amount of space, ignoring useless nooks and crannies.
As well a providing you with the exact measurement of each boot, we give each car a boot space rating out of five, so you can easily compare cars at a glance.
Our boot space star ratings use to be class specific, but we changed our test programme in 2017. So cars tested after this date are comparable across classes.
How do we score different cars?
The overall score in each of our car reviews combines all of our test results with data from our car reliability survey. Each Which? test score is comprised of the following:
- 50% of the total test score comes from our vigorous lab tests. This 50% is comprised from the three main areas of our testing: performance, ease of driving, practicality and comfort, and fuel economy and emissions.
- 40% comes from reliability – brand reliability is used when we don’t have sufficient reliability information for that particular model.
- 10% comes from our safety assessments. If the car has been tested by Euro NCAP, we also consider its rating.
How much each facet of our lab tests contributes toward a car's overall score changes across the different car classes. The graphic below reveals how we weight the different classes of car, what owners have told us matter the most (and least) in the latest Which? Car Survey for each class, and the scores needed to be a Best Buy.
Ready to find your next car? Use our expert, independent new and used car reviews.