How we test cars
By Adrian Porter
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. 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 practical and spacious is the car?
- How comfortable is it?
- What’s it like to drive?
- What are its true fuel economy and emissions levels?
- How safe is it?
- How reliable is it?
- Should I buy it?
Find out which cars excelled in our tests by heading to our round-up of the best cars.
What’s it like to drive?
Engine and gearbox: Our performance tests replicate real-life situations. While the 0-62mph time and fastest possible speed might be of interest to some, what we find more useful is the 37-62mph time. This simulates moving to a faster lane on a dual carriageway or motorway or when overtaking a slow-moving vehicle on a country road. We repeat the test using different gears to find out which cars can make the gap you’re aiming for and which need a long run-up.
Brakes: In an emergency, you need to know if you’ll be able to stop quickly and without swerving around on the road. 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: Every car we test in the lab is also driven by the researcher who writes the review, to assess their behaviour on our unique 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. We often produce videos for these reviews, so check out our first drive reviews for our initial opinions and video reviews of the latest cars.
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 British 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.
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.
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.
360We measure the 360-degree view from the driver’s perspective, which determines how much all-round visibility there really is.
What are its true fuel economy and emissions levels?
We’ve found that 98% 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.
This is such an important part of our tests that we've created a dedicated guide to how we test mpg and emissions.
How safe is it?
Avoidance and stability test: Hopefully it’ll 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.
We've discovered cars where the front wheels lock, causing the car to lose control. Some will fishtail - where the back end of the car wags from side to side.
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.
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.
Should I buy it?
The overall score in our review combines all our various test results with information from the reliability survey. But we weight the different assessments in line with what you’ve told us matters to you:
50% of the total test score comes from our vigorous lab tests. This 50% is comprised equally from the three main areas of our testing: driving and handling, 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.
Sports cars are rated slightly differently. While 50% of the score still comes from our lab test, a much greater proportion within that 50% comes from driving and handling, and less from practicality and fuel economy. This is because sports cars are more about the driving experience than anything else.