Loaded for Málaga: a sidewall bulge, low pressures and 2.3 mm of tread
You are loading the car for a drive from Gibraltar to Málaga - about 130 km, mostly motorway, forecast 34°C. Four adults, the boot full to the parcel shelf, and a roof box carrying about 40 kg. You plan to leave at 13:00, the hottest part of the day. Before leaving you check the tyres. The offside front has 2.3 mm of tread at one point on the outer edge but 4 mm across the middle. The nearside rear has a small bulge in the sidewall about the size of a 10p coin. All four are inflated to the pressures in the handbook for two people and no luggage. The car is four years old, has done 48,000 km, and the spare is a spacesaver rated to 80 km/h.
1. Rank the three tyre problems by what should stop the car leaving, and explain the reasoning for each.
Read the worked answer
1. The sidewall bulge - the car should not go on that wheel. A bulge means the internal structure of the sidewall has been damaged; it cannot be repaired and it does not fail gradually. It fails suddenly, and the conditions here are the worst possible ones for it: full load, roof box, 34°C, motorway speeds for an hour and a half. This is the item you cannot manage by driving gently.
2. The pressures. They are set for two people and no luggage, and the car will be at or near its maximum load. Under-inflation makes the sidewall flex more, and flex generates heat, which is exactly what destroys a marginal tyre at sustained speed. This one you can fix in ten minutes.
3. The tread at 2.3 mm. The legal minimum is 1.6 mm around the central three-quarters of the tread, so this tyre is legal - but 2.3 mm at one point is close, and wet grip and resistance to aquaplaning fall away sharply below about 3 mm. Check it at several points round the circumference; if any point is below 1.6 mm it is illegal and must be replaced. Otherwise it is a reason to slow down in rain, not a reason to cancel.
2. What pressures should the car be running, and why does that matter more on this trip than on a short local drive?
Read the worked answer
Use the handbook figures for maximum load / sustained high speed, which are normally higher at the rear than the solo figures - typically by 0.2 to 0.4 bar. Check them cold, before the car has been driven, and do not let air out of hot tyres: a hot reading is naturally higher and deflating to match the cold figure leaves them under-inflated.
The reason it matters here is the combination of load, heat and duration. Extra weight and a roof box both increase the load on each tyre and the aerodynamic drag the engine has to overcome; a hot road and 34°C air add to the tyre's own temperature. An under-inflated tyre at motorway speed with four adults aboard is being asked to do more work while being given less support, and the failure mode is a blowout at the worst possible moment. Correct pressures also keep the handling and braking predictable when the car is heavy, and the roof box will already have raised the centre of gravity and reduced the speed at which the car should be driven.
3. The spacesaver in the boot: what are the constraints on using it, and does it change your plan for this journey?
Read the worked answer
A spacesaver is a get-you-home device, not a tyre. It is restricted to around 80 km/h, it has a much smaller contact patch so grip, braking and steering response are all reduced, it is often inflated to a far higher pressure than the road tyres (check the sidewall - 4.2 bar is not unusual), and it should only be used on one wheel at a time and for a short distance. ABS and stability systems may also behave differently because one wheel is turning at a different rate.
So it does change the plan. "If anything happens I'll put the spacesaver on and carry on to Málaga" is not a plan: 130 km of motorway at 80 km/h on a spacesaver, with a full load, is not a sensible use of it. Resolve the tyre before you leave - replace or properly repair it, or take a second vehicle - and treat the spacesaver as what it is: enough to reach a garage a few kilometres away.