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Hi - I've just purchased a C5 2litre petrol (55 reg) with 18k miles on the clock with FSH. It looks as if the speedo has never been changed by the service history.
When driving the analogue speedo is about 10mph slower then the actual speed. This is confirmed by SatNav and the digital Cruise control that both appear to be accurate.
Any ideas how to fix? Dave
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Supreme Being
Group: Forum Members
Last Login: 19/01/2012 17:32:47
Posts: 404,
Visits: 264
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| Hi You can't adjust the speedo and it takes its signal from the ABS sensors. All car speedo's generally read high (ie when they say you're doing 70mph, you're only doing 65 or so). That's because it is illegal for them to be inaccurate the other way (otherwise you could blame them for your speeding ticket). C&U regs say they have to be accurate to within 10%, but must not understate the speed. They also only have to register speeds over 10 mph and since it's quite hard to get accuracy to 10% at very low speeds, that's why lots (but not all) of car speedos only start at 10mph. If yours is over 10% out, especially if it's understating the speed & therefore illegal, check the tyre/wheel combination in case the rolling radius is throwing things out. (eg have you/someone put bigger alloys on without low enough profile tyres?) Otherwise it's a new speedo I'm afraid - try a 2nd hand unit from 1stchoice spares.com or breakeryard.com (there are lots of parts brokers - put it into google) Tim
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| hi, I deliberately put larger tyres on my Fiat Uno I had years ago. Greater fuel economy and free tyres off my sisters old mot failed Japanese datsun. However the speedo of course was out, I realised this would happen so I was prepared for it. I got my friend to drive at a set speed and I followed to check the difference, you have the advantage of a sat nav to do this check. Felt tip pen the right speed on the clock glass at 30 40 60 and 70, thats all you need to do is have a guide, or in fact just remember the difference as you would if your watch is 5 mins fast .
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Hi Dave,
I have exactly the same problem, also on a Citroen C5 2L petrol, also 55 reg, and also showing 10 mph slower than the actual speed, confirmed by SatNav. BTW, my cruise control works fine too, showing the same speed as the SatNav.
It is important to notice that the error is not proportional to the speed (like 5% or 10% off), but constant: it is always 10mph off, at all speeds. It even shows minus 10 when the car is parked, and zero when doing 10mph. So it can't be due to tyres sizes, or any gear ratio between wheels and speedo. It is as if someone had just put the needle in the wrong position!
Does anyone have any suggestion please?
Many thanks
Mark
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