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Discussion Starter #1
Hi All,

I have an FN2 and noticed when cruising at 50mph it felt slower than normal so I did a few tests

Speedo 80mph - Sat Nav 75mph
Speedo 70mph - SaT Nav 65mph
Speedo 40mph - Sat Nav 35/36 Mph
Speedo 33mph - Sat Nav 30mph
Speedo 30mph - Sat Nav 26mph

As you can see the higher the speed, the variance is about 5mph and could be up to 6mph, where as lower speed it's around 4mph.

I'm sure when I first got the car, the inaccuracies were less, is there something faulty?

Granted my Sat Nav is quite old, but doesn't mean its inaccurate at a constant speed.

Thanks
 

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Premium Member
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664 Posts
Hi All,

I have an FN2 and noticed when cruising at 50mph it felt slower than normal so I did a few tests

Speedo 80mph - Sat Nav 75mph
Speedo 70mph - SaT Nav 65mph
Speedo 40mph - Sat Nav 35/36 Mph
Speedo 33mph - Sat Nav 30mph
Speedo 30mph - Sat Nav 26mph

As you can see the higher the speed, the variance is about 5mph and could be up to 6mph, where as lower speed it's around 4mph.

I'm sure when I first got the car, the inaccuracies were less, is there something faulty?

Granted my Sat Nav is quite old, but doesn't mean its inaccurate at a constant speed.

Thanks
better off finding one of those 30mph village signs the kill you speed signs,, then compare your speedo and sat nav at 30mph with it,,
 

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Premium Member
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That's right, I'm sure the speedo shows more mph than satnav, exactly the same in my citroen grand picasso has well has the R, wouldn't worry about it.
 

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#MrGrumpy
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13,117 Posts
This is pretty similar across all cars tbh as manufacturers err on the side of caution (as having a speedo that under-reads rather than over-reads could land you in trouble in terms of speeding fines)
 

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This is the same on all cars. It is illegal for a speedo to understate the sped therefore all overstate by a few percent.

This also goes some way to explaining why computer mpg's are always optemistic.
 

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Discussion Starter #8
I know that there are inaccuracies, but at 30mph i'd expect it to be about 2mph below, not as much as 4 or 5mph!
 

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A/C fixer-upper'er
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856 Posts
I usually experienced about 4mph across the legal speed range, initially didn't realise it and thought the 'Your Speed Is' sign in our village was always broken - until I borrowed a satnav unit for a long journey as the inbuilt Honda one wasn't working. Satnavs never lie so you can always find your actual speed from that and work out how much 'extra' you have to play with ;)
 

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Stage 3 Hybrid
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6,223 Posts
Standard across all the 8 gen.
Fitting 225/45/18 tyres on 18" rims brings it down to 2 ish difference.
Fitting correct tyres on correct rims brings it down ? :confused:



This is pretty similar across all cars tbh as manufacturers err on the side of caution (as having a speedo that under-reads rather than over-reads could land you in trouble in terms of speeding fines)
Mr. Sambo is right, again....




ALL car speedos over read by 10%.

True 30 is 33...

True 50 is 55...

...and ALL HGV speedos are bang on.

So, in a 50 contraflow, and your doing 50, and a truck is right up your posterior, but can't overtake 'cos the outside lane has a width restriction...it's 'cos you're really doing 45 ! [smilie=tongue.gif]
 

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A/C fixer-upper'er
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So when you blast through a speed camera over the limit, going by the police who state that the cameras allow for +10%+2mph over the posted limit, is that before or after the speedos 10% inaccuracy is factored in?

Ultimately and purely hypothetically, if I were at 79mph through a camera as shown on my speedo, would I get a stern letter or a penalty notice through the door? :police:

Hypothetically speaking of course, I'm a safe and lawful driver at all times when on the road, and would never exceed the limits.....
 

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Premium Member
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So when you blast through a speed camera over the limit, going by the police who state that the cameras allow for +10%+2mph over the posted limit, is that before or after the speedos 10% inaccuracy is factored in?

Ultimately and purely hypothetically, if I were at 79mph through a camera as shown on my speedo, would I get a stern letter or a penalty notice through the door? :police:

Hypothetically speaking of course, I'm a safe and lawful driver at all times when on the road, and would never exceed the limits.....
I imagine the reason why the 10% is factored in is due to speedometer inaccuracy.

Sent from my GT-I9300 using Tapatalk
 

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Type EX,A guy called Mark
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3,435 Posts
Fitting correct tyres on correct rims brings it down ? :confused:
Correct size is 225/40/18 not 225/45/18

standard size 5/40/18

Diameter 25.1 in
Width 8.86 in
Sidewall 3.54 in
Circumference 78.8 in
Revs/Mile 804.3

Alternative size 25/45/18

Diameter 26 in
Width 8.86 in
Sidewall 3.99 in
Circumference 81.6 in
Revs/Mile 776.9

info from Tyre Size Calculator

Having had 25/45/18 tyres I think they are a better fit and slightly a better ride, but the cost is higher as they are not such a standard size.
 
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