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Car clinic: Will a spare tyre's sensor affect my car's computer?

Your motoring problems solved


BMW 3-series

Q. I recently suffered a puncture on my new BMW 3- series Touring and am now thinking of buying a proper, full-size spare wheel and tyre, as the boot is usually half empty. Would the presence of a fifth wireless tyre-pressure monitor cause a problem with the car’s computer?
IN, Paignton, Devon

A. There are two types of tyre-pressure monitoring system on current BMWs. One works by using the antilock braking system and speed sensors on the car (basically it detects when the diameter of the tyre is not what it should be) and does not require a sensor in the wheel.

The second system uses a sensor behind the valve in the wheel to send air-pressure information wirelessly to the car’s computer. In both cases the system logs pressures from only the four tyres in use, so keeping a spare wheel in the boot would not affect the computer.


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Many drivers miss having a full-size spare tyre as car companies opt instead for puncture-repair kits or, as in your case, run-flat tyres. However, BMW says that the latter can be driven on for up to 94 miles at a speed of up to 50mph, which should be enough to get you to a suitable garage. And research suggests that the average time between punctures is 4-5 years.

So, on the off chance that you suffer a second puncture in a distant location, you are considering spending upwards of £800 on a spare wheel, tyre-pressure sensor, wheel brace and jack. If you’re still not swayed by the cost implications of doing so, then consider the impact of the extra weight of the full-sized spare wheel on your fuel economy.

Sunday Times Driving car clinic expert: Dave Pollard

INSPECTOR GADGET
Dave Pollard has written several Haynes manuals and has tested just about every car-related accessory – read more from Dave here.

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