Fiat Motorhome speedometer accuracy

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Q   
I own a motorhome based on a Fiat chassis and fitted with a 2.8 turbodiesel engine. The vehicle was first registered in September 2006, but the chassis was previously in the hands of the body builder for at least a year, so production date was probably in 2005. The speedometer on the vehicle shows a 10% error – it reads high, with an indicated 60mph actually being about 54mph. This has been verified and I believe that it has been like that since new. The temperature and fuel gauges are OK. I tried to get assistance from Fiat, but all they will say is to take it to a main dealer. Service managers of local main dealers will only suggest bringing the vehicle to them and they will look at it, charging their normal hourly rates. They will not discuss the problem. Having adjusted speedometers in the past, both at the sender end and the speedo head, I wonder if this is a DIY job?
Hugh Gemmell

A   
I would assume that your motorhome is based on the Fiat Ducato chassis and running gear. From the information I have, your speedometer operation should be cable-driven. On earlier gearboxes it was possible to adjust the speedo calibration by changing the driven gear for a unit with a greater or lesser amount of teeth. However, it seems that modern gearboxes don’t have this flexibility and only one driven gear option is available. Likewise, the speedometer head unit is not easily calibrated. The manufacturer deliberately builds in a tolerance which is around 10%, to enable the speedometer to remain relatively accurate and to prevent it from under-reading during the changes that will occur to the tyres’ diameter over the course of their life. A new tyre will have a larger diameter than a worn tyre, varying the accuracy of the actual speed reading. It may be worth checking with the company that carried out the conversion that no alterations were made to wheel or tyre sizes. In such cases of speedometer mistrust, I generally advise using a sat-nav or GPS device to check your true speed. You should find it remains within 10% of true speed throughout the complete range.

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