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Discussion Starter · #1 ·
No, not the painfully tight black swimwear from the 80's, but the speedometer in the MK5 !!

Heres the thing - I have a TomTom which shows the speed at which you are travelling, and it always shows about 10% slower than the car speedo reading; and I dont know which one is the accurate one.

I read something on this forum a while ago from someone else that basically suggested the same thing. This is a real ar5e as I dont know which to trust at the moment, and with the number of speed cameras round my way its a risk I dont want to take.

Anyone else had a similar thing ?
 

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Yes, same problem with the git and the roadangel2.

Basically I can't answer which is more accurate (I'm likely to go with
the RoadAngel as it's GPS based, even if it is normally abotu 2secs
behind speedwise), but I can say you will find a difference.

VW have to ensure that the speedo doesn't read wrongly in the sense
that it says you're doing 50mph and you'r actually doing 55mph, that's
kind of illegal.

So you tend to find it works the other way, the speedo will sometimes
be reading faster than you're doing in reality (so it says 55 and
you're actually doing 52).

I have also read that things like wheel size, tyre profile, and amount
of wheel in contact with the road surface will all skew your
speedo. Blackspot (RA manufacturers) also have this question in
their FAQ - I'll post a link in a sec here - which covers this TO A
DEGREE.

As such I find the following at each 10mph mark on the GTI:

Speedo ==> RA2

10mph = 10mph

20mph = 19mph

30mph = 28mph

40mph = 38mph

50mph = 47mph

60mph = 57mph

70mph = 66mph

80mph = 76mph

90mph = 85mph

100mph = 95mph

I'm guessing it'll get worse but I've only ever had 106 out of the car for licence reasons! [;)]

HTH (a bit)

C

PS - if you're really in doubt, trust the lowest one of course [y]
 

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I have Tomtom Bluetooth sat nav and in my previous car BMW318 it was bang on accurate with two differnt BM's I intend to try it in the Golf this Saturday.
 

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I would say the gps is accurate as long as you're doing a constant speed, i've found mine to be about 4-5% out ie 80 indicated equals 77 in the gps
 

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mine is crap ! my origin and sat nav both say 88mph and the car says 100mph , thought it might be something to do with running 18" rims
 

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I take this on the mk5 ? as it does seem way out for a modern car. I had similar on my old supra which deflated me a little to find out the speed I was REALLY doing

[:(]
 

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In my MkV the TomTom gives me a few mph less (circa 5 at high speeds)

When in my bros lotus, they were a dead match!

I'd say the Golf gives you a few in hand.
 

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Have found mine to be about 5mph slow at 60mphand gets considerably worse as speed increases. I think it also helps to explain the wide variation on MPG managed by a variety of people. I'll ignore the fact that it will also inflate the number of miles that the car has done!!

KeithM
 

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I've got Copilot on an IPAQ PDA with a Syson CF GPS unit and, at 70mph on the speedo in my Audi, the GPS speed is about 65mph.

In my wife's Focus 70mph is 68mph on the GPS.

I tend to trust the GPS when going through speed cameras and I haven't been flashed yet.

That's the death knell.

On a separate topic, I'm waiting for the dealer to call me back with a delivery date for my 4mo.

I'm excited, my wife says it's only a car.

No doubt she'll be stealing it at every available opportunity.
 

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The GPS is more accurate than what is on the speedo.

I was just out in a Golf testing our Road Angel Navigator & I was sitting bang on 40mph & the RAC was showing 37mph
 

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I went out in my mates 1.8T beetle and his speedo was well out to his Road Angel he sticks with the Road Angel readings.

Chris
 

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I too have used a GPS system to check speedo accuracy, but there may be some reders who either do not possess a GPS system or who may want to use the old-fashioned method. If so, please read on...

First find an empty bit of motorway near your home (easier said than done, sometimes). If it's level and straight as well so much the better. With a willing passenger armed with a stopwatch, pen and paper get up to and maintain a steady speed (say, 50 60 or 70). Ask your passenger to time how long it takes to pass 5 marker posts at the side of the motorway. Do this three times for each speed and (later with a calculator) calculate the average time. Your true speed is found from : 894.77 divided by the time (to two decimal places) eg If at an indicated 70 mph your average of three timed runs is 13.14 seconds then your true speed is 68.1 mph (Why use 894.77 ?. This is derived from the fact that at a speed of 1 mph it takes 894.77 seconds to cover 400 meteres-the distance between five posts).

I hope that this advice may be useful to readers.

Regards, Beddoe
 

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The true speed is a little different on my car, and I find out like this:

Get to a speed, say, 70mph. Put cruise on.

Then press and hold (to reset) the trip computer.

Set it to avarage speed, and that's how fast the computer thinks you're going.

Since you just reset the trip computer, your average speed will only have been read from the last few seconds.

hope you understand what I'm on about!

EDIT: Guess who's just posted in the wrong forum? Although what I said might still stand!!
 

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What is the smallest tyre/ wheel size on the mk5? I am guessing that VW will ensure that the speedo is accurate for this size/model, and anything larger will merely under-read, therefore not challenging the law.
 

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What is the smallest tyre/ wheel size on the mk5? I am guessing that VW will ensure that the speedo is accurate for this size/model, and anything larger will merely under-read, therefore not challenging the law.
Surely overall tyre diameters on Golf 5's are all identical? The wheel sizes vary, of course, but the tyre wall depth reduces as the wheel gets larger in order to achieve the same 'rolling diameter'. Thats why cars with 18" wheels have thin 'low-profile' tyres.

Anyway - as regards speedos - my results are pretty much the same as everyone elses. If Tomtom sat nav says I am doing 70 mph, the speedo will read 73 to 74mph.
 
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