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Discussion Starter · #1 ·
Been thinking for a while that my Speedo isn’t displaying the correct speed - or at least within usual tolerances. Went on a motorway trip last week with a passenger so used a Speedo app and satnav to compare “correct” speed with indicated.
Wow !! Didn’t expect such variation. The worst was travelling at approx 70mph as indicated on the satnav and the app but the Speedo needle actually read 85mph !!

Any ideas what might cause such a vast difference ? Tyres are 245/70/16 which have recently been replaced and I assume these where the original sizes as I can’t see the previous owner from new - a country GP - changing the size for performance/comfort reasons.
Many Thanks in advance.
oh btw, it’s a 2007 2.5 xe
 

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The common speedo error is caused by the sensor drive slipping but that usually reads on the slow side or is irratic . This is cured by dropping the rear propshaft from the gearbox and tightening the nut in the centre of the output flange.
 

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UK law means manufacturers are allowed a 10% plus 6.25 mph error, so in theory you could be travelling at what you believe to be 50 mph, when in reality you are only doing 40 mph. The law also states the speedometer must never under read, i.e. it must never show 40 mph when you are doing 50 mph. The reasoning behind this is to allow for variations in road condition, tyre condition, tyre pressure and also gearbox wear and tear. Generally cars will read 10% over, which means at 30 mph you are probably travelling at 27 mph, at 60 mph you could be travelling at 54 mph.
Your example suggests you are travelling at 70 mph, speedometer is allowed to read 70+10%+6.25 = 83.25 mph.
 

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Hi John
Tyre sizes should be placarded on the door pillar if you want to check.
 

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Discussion Starter · #6 ·
Thanks for your replies everyone. I don’t think I’ll be dropping the rear prop shaft especially as that usually resolves an under reading issue.
10% + 6.25 seems a huge margin of allowable error. I assume that piece of legislation may have been from days long gone by and never updated. However nowadays I’m sure manufacturing tolerances can produce something more accurate.
Tyre sizes are correct as per the sticker on the door pillar and most suppliers agree.
It doesn’t appear there is a solution so if I stick to the indicated speed, there’s no way I’ll be caught by cameras - although anyone travelling behind might not be too happy 😃. For accuracy I might leave the satnav plugged in or maybe buy a separate HUD.
 

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Discussion Starter · #8 ·
Yes, Speedo reads 0mph when at rest. It gets gradually more optimistic as speed increases to the 15mph difference at 70mph.
It is annoying but nothing I can’t live with if there is no common easy(-ish) solution.
 

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These days (as you posted) at least you can cross check speed with satnav John.
 

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The two images are from the Driver & Vehicle Standards Agency for "Individual Vehicle Approval (IVA) inspection manual" section 17, issued in Sept 2020, which identifies the maximum tolerances. Most the cars I have had in recent years have had a 10% discrepency.
 

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Discussion Starter · #11 ·
Thanks for this. So it would appear that my Speedo is at the very top end of the tolerance. So at 70mph indicated on app/satnav I should actually be reading a max of 83.25mph. Maybe I should allow the additional 1.75mph discrepancy as tolerance for the accuracy of the GPS on app/satnav and possibly wavering when I look at the Speedo and then the app/satnav.
 

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Given Kia like to change wheel sizes on different spec so, 2 3 or 4 could have different wheel sizes. So you could have 2 being 16", 3 being 17" & 4 being 18" with resulting different profile sizes.

As #4 what does the sticker on the door pillar say or the handbook for tyre size?

Any error should show as being slower. so in mine 70 on speedo is 67 on waze or google.
 

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If you are thinking of doing what Birdman suggested it could still bring the difference down to something nearer the real speed. If you do it would be interesting to know your results.
 

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the speedo is a stepper motor with a clock spring to bring the pointer down to the zero stop, the motor is told where to go to depending on pulses from the sender unit, the pointer is the indication, the pointer is removable and could be refitted in a more accurate position, they are a bit tight to get off but they do come off
 

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Discussion Starter · #17 ·
So are we thinking it could be a faulty sender unit ?
Any idea on location / ease of swapping ?
Or another option is to remove the Speedo and reseat the pointer.
 

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it is possible that it is the sender unit, it creates pulses as it rotates, never heard of one giving out more pulses than it is supposed to but with electronics anything is possible, as previously mentioned the prop flange nut is a known issue so It is always best to start with the cheap stuff, tighten the flange nut then at least that is out of the way,
 
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