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
- 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 2017.
How safe is it?
Safety: 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, such as 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: We helped set up euro NCAP in 1997, and continue to use their crash results as part of our evaluation. If a car that has been tested by Euro NCAP gets three stars or less, we make it a Don't Buy.
Which? avoidance test
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 the car won’t just 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, to determine 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.
The video below shows the Jaguar F-Pace SUV, which handled the course badly and locked its wheels, compared to 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 its true fuel economy and emissions levels?
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 know that a lot of cars pump out a lot more emissions, such as NOx and CO (CO2 measured separately) than is permitted, to meet official emissions legislation.
And if you're going to buy an electric car, you're going to want to know just how far a single charge will really take you.
This is such an important part of our tests that we've created a dedicated guide to how we test mpg and emissions.
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.
Brakes: 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 it?
Ride quality: 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.
Comfort: 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.
Noise: A car that’s comfortable is all well and good. But if it makes a massive racket the entire time you’re driving you won’t want to be inside it for long. We both measure and subjectively assess the trio of road, wind and engine noise in each of our tested models.
How practical and spacious is the car?
Seat space: We don’t just get inside the car and comment on the amount of room inside. We use a dummy to work out how much head, leg and knee space there is for the driver and all passengers.
Ease of entry: We assess every car on how effortless it is to enter and exit. 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.
Visibility: 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.
Boot space: Car manufacturers vary in the way they measure their claimed boot space, often 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. Boot ratings use to be class specific, but since changing to a new test programme for 2017, cars tested after this date are comparable across classes.
How do we score different cars?
The overall score in our review combines all our various test results with information from the reliability survey. Each Which? test score is comprised from the following:
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.
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.
But how much each facet of our lab tests contributes toward the overall score changes across the different car classes, and is based on what car owners told us in our last Which? Car Survey.
The graphic below reveals how we weight the different classes of car, what owners have told us matter the most (and least), and the scores needed to be a Best Buy.