Why Electric Cars Fail the MOT: Tyres, Suspension and the Weight Problem

An electric car arrives at the test station with no exhaust to blow, no emissions to measure, no oil to leak and no cambelt to fray. Whole sections of the MOT inspection simply do not apply to it. On that basis it ought to pass more often than a petrol or diesel car of the same age. It does not. It passes less often, and the reason is not the battery, the motor or the software. It is the tyres and the suspension, and behind both of those sits a single fact: the car is heavy.

The figures come from The MOT Files' analysis of DVSA's anonymised MOT test data, a set of 33 million class 4 tests. Within it are 51 electric models and 489,247 tests on them, which is enough to say something firm rather than anecdotal. Set against petrol and diesel cars of the same age, electric cars fail more often, and every part of the gap can be traced to a mechanism that follows from the physics of the car.

The headline: same age, fewer parts, more failures

At three to five years old, electric cars pass 86.7% of MOT tests. Petrol and diesel cars of the same age pass 89.9%. The gap is 3.2 percentage points. That is not a collapse, and nobody should read it as evidence that electric cars are badly built. It is, however, the wrong way round for a vehicle that has removed an entire category of failure from the test, and it is consistent enough across the data to need an explanation.

The explanation starts with how failures are counted. A failed MOT rarely lists a single defect. A car that fails on one worn tyre often fails on a second, and the tester records each as a separate item, so the natural unit of comparison is not the share of tests that fail but the number of items recorded per 100 failed tests. A figure of 92 does not mean 92% of anything. It means that for every 100 electric cars that failed, testers wrote down 92 tyre defects between them. Per 100 failed tests, the categories compare like this.

Read down the two columns and the shape of the problem is obvious. Electric cars record 1.6 times as many tyre items and about three times as many suspension items. In return they record about a third as many brake items, fewer wiper and screen items, and effectively no engine or emissions items at all. Lamps are level, and corrosion barely registers on either side because neither group of cars is old enough to rust. Everything the electric car gives back on brakes, wipers and emissions, it loses again on tyres and suspension, and then loses a little more.

Why same-age is the only fair comparison

Almost every electric car on British roads is young. The mass-market models arrived in numbers only in the last few years, so the electric cars presenting for their first, second and third MOTs are overwhelmingly three, four and five years old. The petrol and diesel fleet is nothing like that. It includes cars that have been through many winters, cars with corroded brake pipes and exhausts hanging by a bracket, cars whose owners have long since stopped fixing advisories. Put the whole electric fleet against the whole combustion fleet and electric cars would look magnificent, and the comparison would be worthless, because it would be measuring age rather than propulsion.

So the comparison group is drawn from petrol and diesel models that were themselves launched recently and have the same age profile as the electric cars. The Ford Puma and the Toyota Yaris Cross are typical of it: modern, popular, and almost entirely between three and five years old at the point of testing. Both groups are being tested at the same point in their lives, on the same roads, under the same testing standard. Whatever gap remains is a property of the cars.

That framing cuts both ways. It removes the flattering effect that youth would otherwise give the electric car, but it also removes the unflattering comparison a casual reader might draw against the national average pass rate, which is dragged down by old cars of every fuel type. The 3.2-point gap is a real difference between similar cars, not an artefact of the fleet. The test itself is the same for both; the rules that apply to an electric car, and the parts of the inspection that fall away, are set out in the companion guide to the electric car MOT.

Tyres: 92 items per 100 failures

Tyres are the single biggest reason an electric car fails its MOT, and the margin over the combustion comparison group is large. Ninety-two tyre items per 100 failed tests against 57 is a ratio of 1.6, and the individual defects within that total show which kinds of tyre problem are doing the work.

The most frequent single tyre item on a failed electric car is a tyre seriously damaged, recorded 37 times per 100 failed tests against 29 for petrol and diesel. That covers cuts deep enough to reach the cords, bulges in the sidewall, lumps, and tread or sidewall rubber torn or separating. Next is tyre cords visible or damaged, at 29 per 100 against 10, which is nearly three times the combustion rate and the largest proportional gap of any individual tyre item. Third is tyre tread depth not in accordance with the requirements, at 26 per 100 against 16.

Those three items describe two different processes. The tread depth defect is wear: the tyre has been driven until the tread across the central three-quarters of its width has dropped below the legal 1.6mm minimum, which is measured the same way on every car and is covered in detail in the guide to tyre tread depth law. The damage and exposed-cord defects are impacts: the tyre has hit something, usually a kerb or a pothole, hard enough to tear the rubber or break the structure underneath it.

On a petrol or diesel car of the same age, wear and damage sit closer together. On an electric car, the damage items pull well ahead, and cord exposure in particular runs at nearly three times the comparison rate. An electric car does not merely wear its tyres out faster; it breaks them more often. That is the distinction that matters, because a tyre that has simply worn down gives its owner months of warning and a tread gauge will find it. A tyre with cords showing through a sidewall gash may have been fine a week ago.

Weight, torque and the low-profile tyre

Three properties of the electric car explain the tyre figures, and they compound.

The first is mass. The battery pack in a typical electric car weighs several hundred kilograms, and the car as a whole ends up around that much heavier than its petrol or diesel equivalent, because the motor and its electronics are lighter than an engine and gearbox but not by nearly enough to compensate. Every one of those kilograms is carried by four patches of rubber, and the load on each patch governs how fast the tread scrubs away on every corner, every braking event and every acceleration.

The second is torque. An electric motor delivers its maximum twisting force from a standstill, without the build-up that a combustion engine needs. That is what makes electric cars feel so brisk away from junctions, and it is also what puts the tyre carcass under peak load at exactly the moment the tyre has the least rolling speed to help it. The tread surface is being asked to transmit full torque through a heavy car from zero. Some of it slips, microscopically, on every launch, and that slip is wear. The deeper structure of the tyre, the belts and the cords that give it its shape, is loaded harder too, and a structure that is worked harder is closer to its limit when it meets a kerb.

The third is the tyre itself. Many electric cars are sold on large-diameter wheels with low-profile tyres, partly for appearance and partly for the sharper steering response that a short, stiff sidewall gives a heavy car. The sidewall is the part of the tyre that absorbs an impact. Hit a pothole edge or a kerb with a tall, soft sidewall and the rubber flexes and recovers. Hit the same thing with a short, stiff sidewall on a heavy car and there is less material to deform and more energy to deform it. The result is the pinched sidewall, the bulge where the inner cords have snapped, and the gash that shows fabric through the rubber. That is the mechanism behind the near-threefold rate of cord exposure.

Put the three together and the ordering of the tyre defects makes sense. More mass and more torque wear the tread faster, which lifts the tread depth item. More mass concentrated on a stiffer, shorter sidewall makes every kerb strike more consequential, which lifts the damage and cord items further still. None of this is a fault in the car. It is what heavy, quick cars on low-profile tyres do, and it would apply to a heavy, quick petrol car too. There are simply far more heavy, quick electric cars.

One further tyre-related item is worth knowing about. Every electric car in the data is new enough to be fitted with a tyre pressure monitoring system, and on any car first used from January 2012 a system that is obviously not working, or is showing a fault, is a defect in its own right: tyre pressure monitoring inoperative. It matters more on an electric car than the defect alone suggests, because an under-inflated tyre on a heavy car runs hot along its shoulders and wears them fast. The warning lamp is not a nuisance to be ignored until the test. It is the earliest sign of the wear pattern that ends in a tread depth failure.

Suspension: the nine-times problem

Suspension is the second category where electric cars lose ground, and proportionally it is the more dramatic. Twenty-eight suspension items per 100 failed tests against 9 is about three times the combustion rate. Within that, one defect does most of the damage. "Suspension pin, bush or joint excessively worn" is recorded 20 times per 100 failed electric cars and 2 times per 100 failed petrol and diesel cars of the same age. That is about nine times.

The mechanism is the same mass that wears the tyres, arriving by a different route. A suspension bush is a block of rubber, or a rubber-and-metal sandwich, bonded into the end of a suspension arm to let it pivot without metal touching metal. A ball joint does the same job where the movement is in more than one plane. Every time a wheel meets a bump, the arm swings, the bush deflects and the joint articulates, and the force involved is set by the mass being carried over that bump. Several hundred kilograms of additional mass means every bump is a bigger event for every bush and joint on the car, on every mile of every journey. Rubber that is deflected further, more often, fatigues and cracks sooner. Joints that carry more load wear their bearing surfaces faster. A component designed to last a certain number of cycles at a certain load reaches its limit sooner if the load goes up, and on a same-age comparison it reaches that limit inside the MOT window rather than after it.

Torque adds a second load. When an electric car launches, and again when it slows hard on regenerative braking, the arms and bushes of the driven axle take the full reaction to that force, instantly, in both directions. The front bushes of a petrol car feel something similar under acceleration, but not from a standstill, not with the same abruptness, and rarely with the same reverse load under braking, because a petrol car slows through its discs rather than through its driveshafts.

That one defect is the reason the wider suspension category sits at three times the combustion rate. Take it out of the totals and the remaining suspension items, the springs, dampers and mountings, are close to level between the two groups. It is the pivoting rubber and the articulating joints, the components most directly sensitive to mass, that give way. The defect entry for suspension arm bush worn sets out how a tester finds and grades it, which is by levering the component and measuring play, something an owner cannot easily reproduce at home.

Suspension wear and tyre wear also feed each other. A worn bush lets the wheel's alignment wander under load, and a wheel that is no longer pointing exactly where it should scrubs its tyre shoulder on every straight mile. An owner who finds one front tyre worn to the cords on its inner edge while the other is fine is often looking at a suspension defect as much as a tyre defect, and the two frequently appear together on the same refusal certificate.

Brakes: where the electric car wins, and the disc that rusts

Brakes are the electric car's best category, and the margin is as large in its favour as the tyre margin is against it. Ten brake items per 100 failed tests against 29 for same-age petrol and diesel cars is about a third of the rate.

The reason is regenerative braking. When the driver lifts off the accelerator or presses the brake pedal gently, the motor runs as a generator, converting the car's momentum back into charge and slowing it in the process. In ordinary driving most of the slowing an electric car does is done this way, and the friction brakes, the conventional pads and discs, are only called upon for firm stops and the final few miles per hour. A component that is rarely used rarely wears. Brake pads on electric cars routinely last far longer than on a petrol car, and the thin-pad and worn-out-disc failures that fill the brake category on combustion cars are largely absent.

The same mechanism has a downside, and it should be stated plainly. A brake disc stays clean because the pad scrapes it. A disc that is clamped hard several times a day has its surface polished continuously and never has the chance to corrode. A disc that goes days between meaningful applications rusts, first as a surface film that a single firm stop would remove, then as pitting that eats into the metal and no longer wipes off. Testers see this constantly on electric cars, and it appears in the data as a steady stream of advisories for corroded or pitted discs, and occasionally as a failure where the pitting is severe enough to count as brake discs worn or significantly deteriorated. On the Tesla Model 3, 14.5% of all tests, passes included, carry a brake disc advisory. The car has barely used its brakes, and the brakes have suffered for it.

There is a testing consequence too. The MOT brake test is a measurement of the friction brakes, and a disc surface that is corroded and uneven can give a lower or less even reading than the same brake would after a few firm stops. Anyone taking an electric car to its MOT does well to use the brakes properly on the way there, and the guide to brake testing in the MOT explains what the rollers measure and how the results are graded.

What disappears entirely: engines and emissions

The exhaust, fuel and emissions section of the MOT is one of the larger ones on a petrol or diesel car. It covers the exhaust system and its mountings, the silencing, the fuel tank and lines and filler cap, the emissions measured at the tailpipe, and on diesels the smoke test and the integrity of the particulate filter. On an electric car there is nothing in it to inspect. Engine and emissions items appear on essentially none of the failed electric car tests in the data, against four per 100 failed tests on the same-age combustion group.

Four is not a large number, and that is itself a point about the same-age framing. Emissions and exhaust failures on petrol and diesel cars rise steeply with age as catalysts degrade, particulate filters clog and exhausts rust through. A three-to-five-year-old Puma or Yaris Cross rarely fails on emissions either. The electric car's advantage here is real but modest at this age, and it is likely to grow later in life, when the combustion car's exhaust and emissions equipment begins to fail and the electric car still has none.

Lamps, the largest category in the national figures across cars of all ages, are roughly level here at 13 items per 100 failed electric cars against 11. Bulbs blow and headlamps drift out of aim regardless of what turns the wheels. Wipers and the screen run the other way, at 11 against 19, so electric cars do better there too. There is no mechanical reason for an electric car to have better wipers, and the difference probably says more about the models and owners that make up each group than about the propulsion. Corrosion is negligible on both sides, for the obvious reason that a five-year-old car of any kind has not had time to rust in any way that matters to the test.

The net effect is the headline gap. The electric car removes an entire category, wins on brakes and wipers, holds level on lamps, and still fails 3.2 percentage points more often than a combustion car of the same age, because tyres and suspension between them outweigh all of it.

What the Tesla data shows

Tesla's three volume models make the pattern concrete, and the figures for them are expressed differently: as the share of failed tests on which a given item appears, which is a true percentage rather than a count per 100.

On the Tesla Model 3, tread depth appears on 50.5% of failed tests. Half the Model 3s that fail have at least one tyre worn below the limit. Cords visible appears on 34.4% and a tyre seriously damaged on 24.0%. The tread figure is the striking one, and it is backed by the advisory data: 37.1% of all Model 3 tests, passes as well as failures, carry a tyre tread advisory, which is the tester recording a tyre that is legal today and will not be for long. The Model 3 is a relatively low, quick saloon that wears its tyres down rather than breaking them, and a very large share of its owners are being told so a year before the failure.

The Tesla Model Y inverts the order. A tyre seriously damaged appears on 50.9% of its failed tests, cords visible on 47.8%, and tread depth on only 23.9%. The Model Y is taller and heavier than the Model 3, typically sits on larger wheels, and carries the same instant torque, and its failed tests are dominated by damage rather than wear. That is precisely what the weight-and-sidewall mechanism predicts: as mass rises and sidewall shrinks, the kerb and pothole damage items overtake the wear item.

The Tesla Model X has the lowest pass rate of any electric car, as distinct from electric van, in the data at three to five years old, 79.1%, and it adds a defect the others do not: wiper blade problems appear on 16.2% of its failed tests. The Model X has an enormous windscreen that sweeps up over the front seats, which means very long wiper blades working over a very large area of glass, and blades that size wear, tear and lift at speed. It is a reminder that the wipers category is not something electric cars are immune to. It is something most of them happen to do well.

At the other end of the ranking, the Jaguar I-Pace posts the best three-to-five-year pass rate of any electric car at 92.0%, above the 89.9% of the combustion comparison group. It is heavy, quick and rides on large wheels like the rest, so its result is a caution against treating the physics as destiny. How a car is set up, and how it is driven, still matter. Electric vans fare worst of all. The Nissan e-NV200 passes 74.9% of the time and the Maxus e Deliver 3 71.5%, which is about what a vehicle carrying its own battery plus a working load on commercial mileage might be expected to do. The complete ranking of all 51 models, with the failure profile of each, is on the electric cars hub.

What an owner can do before the test

The data points to a short list, in a clear order of priority.

Tyres come first, and they deserve more than a glance. Measure the tread at the inner, centre and outer positions on each tyre and at several points around its circumference, and take the worst reading. On a heavy car with instant torque the shoulders of the driven wheels often go first, and on full steering lock the inner shoulders of the front tyres are visible without tools. Then run a hand around each sidewall and shoulder, feeling for bulges, and look closely at any cut to see whether cord or fabric is showing. On an electric car the damage items are as likely to fail the test as plain wear, and a bulge that has not yet let go will not be found by a tread gauge. Set the pressures to the figures on the door plate and make sure the tyre pressure monitoring lamp is out, because a lit lamp is a defect in itself and a low tyre on a heavy car is how shoulders get worn.

Suspension is harder to check at home, but it announces itself. Knocks and clonks over speed humps, a rattle from the front over broken surfaces, steering that feels vaguer than it did, and above all uneven wear across a tyre are the signs of a bush or joint with play in it. On a car that is three to five years old and several hundred kilograms heavier than the norm, a pre-test inspection with the car on a lift, where a mechanic can lever each joint as the tester will, is worth having, particularly if last year's certificate mentioned suspension at all.

Brakes need the opposite of the usual advice. The discs are not worn; they are neglected. A few firm, deliberate stops from a reasonable speed on a quiet road in the weeks before the test will clear surface rust and let the pads bed against the disc again. Some cars allow the strength of regenerative braking to be reduced, which brings the friction brakes into use more often. Look through the wheel spokes at the disc face. An even, bright surface is what the tester wants to see, and a dull, orange, pockmarked one is an advisory waiting to be written.

Wipers and the screen matter more on a car with a large windscreen, and the Model X figure makes the point. Wiper blades are quick to check and quick to change, and a blade that smears or judders on a wet screen should be replaced before the test rather than explained away at it.

Finally, read last year's certificate. The tread advisory on more than a third of all Model 3 tests is the clearest possible example of a tester telling an owner, in writing and a year in advance, exactly what the car will fail on next time. Advisories for tyres close to the limit, for corroded discs and for slight play in a suspension joint are the electric car's characteristic warnings, and acting on them is the difference between 86.7% and something rather better. How the individual models compare, and which items each one is most likely to be pulled up on, is set out in full on the electric cars hub.