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Problem with a digital meter speed |
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iowtheme
 
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  Member No: #59299
Joined: Jan 09 2022Location: I.O.W |
Hi all I’ve just recently got the car I’ve now got used to driving it I’ve now find it’s got a problem I’ve been to Citroen and asked them if they can help sort the issue out they said they can’t do anything for me but they are willing to take money off me to check but that is all they can do. So the problem is when I am doing 30mph The car is showing 2mph faster I have used a standard GPS monitor shows me my speed and I’ve drove past a meter showing your speed as you’re travelling close when I spoke to Citroen they said it could be my size of tire they said the tires what are on the car are correct 205/60/R17 I read in the book they did it in two types 205/55/R16 does anyone have any ideas what I could try to do to correct this problem | ||||
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routemaster1
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   Member No: #574
Joined: Jul 08 2007Location: Dorset |
If I understand correctly you are saying that your speedo is reading slightly higher than a GPS. This is entirely normal. None of my cars has ever had the speedo and GPS reading the same.You probably have the correct tyres, don't waste money on diagnostics. EDIT: This is a list of tyre sizes that fit a pre 2013 C4 Picasso. Tyre Size Front & Rear: 215/50R17W Front & Rear: 215/50R17V Front & Rear: 215/55R16W Front & Rear: 215/55R16V Front & Rear: 215/50R17H Front & Rear: 215/45R18H Front & Rear: 215/45R18W Front & Rear: 215/55R16H Front & Rear: 215/45R18V Front & Rear: 215/55R16W Front & Rear: 215/45R18V There are no 60 ratio tyres, the tallest being 55%. On re-reading the post I still am unsure of whether the speedo is reading higher or lower than the GPS. If higher that is normal, if lower it is not normal, and is actually illegal. If the latter is the case, and if your quoted tyre sizes are correct I suggest you visit a tyre specialist. There is a tyresize checker on the site. - Click Here - I suggest you put in one of the sizes as specified in the list above and check against the 60%s fitted. If the difference is more than about 2%, I would recommend getting tyres of the correct size. Further EDIT: I've done the calculations with the sizes provided. The 60% tyres have a rolling radius 6.78% bigger than the 55% ones. The recommended maximum difference is 2%. So I assume that the speedo is showing higher than the GPS, the 6.78% would explain this. If I have surmised correctly, you really need to get rid if the 60% tyres and get something from the list above. This may have implications for your insurance if you have an accident. I'm quite surprised that the bigger tyres fit within the wheel arches. |
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BigJohnD
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  Member No: #82
Joined: Jan 22 2007Location: Hoylake |
Over-reading by 2mph is normal. In fact speedos are required to over-read as a speeding deterrent. | ||||
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vexorg
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 Member No: #48843
Joined: Jul 06 2017Location: Scotland |
I dont think it's just as a deterrent, I think it's a legal [%*^#@!] covering thing from manufacturers, so you cant claim the speedo said 30 when you get done and are actually going faster. | ||||
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Richard_C
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Member No: #46470
Joined: Oct 11 2016Location: Cambridge |
They are not required to over read but the tolerance is plus x minus nothing, so they are not allowed to under read. In practice that means manufacturers err towards overread. Long before GPS whenever I got a new car I would time it between the numbered 100m marker posts on motorways at quiet times. Drive at 120 kph, indicated 74.5 mph, and you woud expect to cover 2Km in one minute. The posts are numbered - km them 100m. Now I have google maps on the phone in a vent clip I just look at that. I have never owned a car that had a wholly accurate speedometer, typically they over-read by 5 to 10%: my current pic shows 73 or 74 at true GPS 70, 32 at true 30. There is a website here which gives you rolling circumference of different tyre sizes if you want to play. - Click Here - As there are no serious consequances of travelling 2mph slower than you think you are, as are the majority of road users, why worry? |
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vexorg
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Member No: #48843
Joined: Jul 06 2017Location: Scotland |
You can verify it, the odometer is usually accurate. Drive at 60mph with cruise control on for an hour and see how many miles you've gone. | ||||
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routemaster1
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Member No: #574
Joined: Jul 08 2007Location: Dorset |
vexorg wrote ... You can verify it, the odometer is usually accurate. Drive at 60mph with cruise control on for an hour and see how many miles you've gone. This is assuming that the odometer is accurate. This is probably difficult to verify. |
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vexorg
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Member No: #48843
Joined: Jul 06 2017Location: Scotland |
It's pretty easy to check on the motorway, the service signs or distance to junctions will be accurate | ||||
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DeuxChevaux
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 Member No: #16472
Joined: Nov 21 2010Location: North Norfolk UK |
In the BSI settings you can select the different tyre sizes that will fit that model of car, if it reads 2mph over that will be correct but if its 2mph under then it may be the wrong tyre sizes are set in the BSI. You will need Lexia diagbox type of OBD reader to reset to correct size. | ||||
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Richard_C
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Member No: #46470
Joined: Oct 11 2016Location: Cambridge |
It's pretty easy to check on the motorway, the service signs or distance to junctions will be accurate Which bit of the junction do they measure to? Where on the services slip road? The numbered 100m posts are spot on and much more accurate. |
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vexorg
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Member No: #48843
Joined: Jul 06 2017Location: Scotland |
If it says services 28miles at the sign, then you get to the services 28 miles later then the on 0.1 or 0.2 mile will be a very small error for the exact position. However if it says 30 miles then you know there's an issue | ||||
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BigJohnD
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Member No: #82
Joined: Jan 22 2007Location: Hoylake |
- Click Here - These posts are accurately placed but metric, as per EU. | ||||
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