TYRED GEARBOX

 I have an experience that I feel will enthral you as much as it did me when the ‘eureka’ moment occurred. I have a 2010 Citroën C5 Exclusive Tourer as my daily wheels. It has all the usual Citroën hydraulic bells and whistles, as well as a 2.0-litre HDI engine and six-speed automatic gearbox courtesy of Aisin Warner. I bought it 18 months ago as a one-owner, 40,000-mile car, nearly good-as-new.

A few months ago I noticed a fault with the gearchange, and after thorough examination I concluded that the fault only occurred above 50mph and only if I applied enough right pedal to cause a change down from 6 to 5 or 4. The box wouldn’t change back up again, regardless of speed, unless I did it manually. I plugged in diagnostics, throwing up all manner of faults (U1205 steering wheel angle sensor and U1213 ESP communication fault), which all boiled down to an anomaly within the wheel speed detection system.

Having passed O-level physics more than 50 years ago I deduced that I must have odd-sized wheels! I then remembered that the Michelin spare tyre had been fitted to the offside rear some months prior to my noticing the problem. All five tyres are 225/55 R17. Two new Michelins were on the front, the spare was a Michelin, and the two rears were cheapies from the Far East. I grabbed a tape measure and measured the circumference of all five tyres. OS front 2150mm, NS front 2155mm, OS rear 2170mm, NS rear 2155mm. The new spare was 2150mm.

It seemed odd that the Michelin that had been the spare was 15mm larger than the largest of the two new Michelins on the front. In an effort to convince myself that I was going to have to bite the bullet and have the gearbox rebuilt I fitted the new cheapo spare, just to be sure. Eureka! The gearbox now works faultlessly.

Who would imagined that half-an-inch variation in more than seven feet of circumference would have such major consequences? I would love to get my hands around the throat of the anorak who programmed that into the system. Be warned!

Robert Cox