Speedo Or Satnav.

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Which do you reckon is the most reliable indication of accurate driving speed?
 
I was told that speedos are built to show 2-3mph faster than you're actually going, so the satnav is actually correct.

However I still make sure that the speedo shows under the limit just in case :)
 
Sat nav, I think all cars are calibrated a bit faster than your really going. We used a sat nav in the works car, the sat nav read 75 and the car was just over 80.

Only one way to find out for sure, try going past a speed camera and see which one is right when you get the ticket:lol:
 
sat nav , checked mine against my speedo and my speedo underreads by around 3-4 mph all through the range , cross checked it on other cars and got the same result :)
 
CT said:
Which do you reckon is the most reliable indication of accurate driving speed?
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That would be the photograph they take from those little vans with the tinted windows Cedric :'(
 
Satnav, it's a completely digital system linked to high precision clocks, but I'm not sure how it handles speeds on slopes, I don't know if it takes the elevation change into the calculations.


However it will probably be pants at walking speed as it's only accurate to 10 metres :)
 
All I know is that they both show different... :shrug:


But as I have had both for a number of years and neither has been officially calibrated... :suspect: ...I may well be doing 80mph... :eek: ...when I actually think I'm doing 70... ish... :naughty:



Good thread Cedric... could have done with reading this before finishing THIS one off... :lol:


:p
 
I think it's generally accepted that speedos read over by about 10%. An indicated 34mph on my speedo is 30mph on the satnav. I do tend to go by the satnav reading.
 
I will have to go by my speedometer as I don't have (and will not have) a satnav.

From my experience, satnavs seem to turn people into idiots. I have seen drivers who know where they are going, follow a ridiculously more complex route just because the satnav told them too.

If I was driving with one it would be thrown out of the window after a few miles!


Steve.
 
CT said:
I think it's generally accepted that speedos read over by about 10%. An indicated 34mph on my speedo is 30mph on the satnav. I do tend to go by the satnav reading.
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Me too but the difference on mine is not so great... :shrug: ... 30 on the satnav is 32/33 on the speedo... ;)






:p
 
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IIRC the speedo can read up to 10% above your actual speed but not below it.

There's obviously a delay with a sat nav system however so I would put it out there that as long as you're on a straight road with good signal and driving a constant speed, you're sat nav will be the most accurate...
 
Most fords are within 10%, speedo at 30 will show about 27mph at the signal out from the vehicle speed sensor. sat nav usually confirms the VSS
 
Your speedo reads faster than you are actually going, manufacturers are obliged(sp?) to factor a up to +3mph greater reading.

I read that on PH forums, it may be an urban myth:shrug:, but it would make sense.
 
CT said:
Which do you reckon is the most reliable indication of accurate driving speed?
Click to expand...

Martyn... said:
That would be the photograph they take from those little vans with the tinted windows Cedric :'(
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Now there is a whole new thread in the making ;)

I was "reportedly" doing 44 MPH in a 40 some years back
Damned sure that I was doing 45 (speedo) though :cool:
 
Cobra said:
I was "reportedly" doing 44 MPH in a 40 some years back. Damned sure that I was doing 45 (speedo) though :cool:
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That would be back in the days before you were a Cobra, when you were a Black Mamba i suppose Chris? :coat:

I always believed the speedo had an accuracy tolerance of 10% of the actual speed. This seems to be borne out with my own tests involving different cars and 2 or more different (brand) satnavs.
 
hepburn said:
That would be back in the days before you were a Cobra, when you were a Black Mamba i suppose Chris? :coat:
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:D

Just a mere snakeling Heppers :cool:



 
Depends on lots of factors actually, ranging from tyre depth, to tyre pressures, tyre sizes and wheels fitted to the car. You'l find most models use the same speedo regardless to the wheels being 16- 17- 18's etc all have a slightly different circumfrance. Also speed, some are more accurate at slower speeds, then higher, and visa versa, but unless you have a proper calibrated speedo like HGV's or Police, then the speedo will be over reading, but by how much is the question.

Then on the satnav front some a re more accurate to within a shorter distance, then also it depends on how many sats its recieving.
 
Unless you have a calibrated speedo (as russdaz says,police cars and such have them) which are set to read the exact speed then your car speedo will "over-read" by 10%, iirc this is a legal requirement.
 
Steve Smith said:
I will have to go by my speedometer as I don't have (and will not have) a satnav.

From my experience, satnavs seem to turn people into idiots. I have seen drivers who know where they are going, follow a ridiculously more complex route just because the satnav told them too.

If I was driving with one it would be thrown out of the window after a few miles!


Steve.
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:agree:
 
I tend not to use the speedo anyway. I regulate my speed with the rev limiter..........
 
You can certainly use your satnav to calibrate your car speedo. Do this on a flat open road, giving enough time for the satnav to settle first. The satnav will be very accurate, ~0.1% error. Whereas the speedo can typically be 5% in error, due to the factors mentioned in other posts, tyre size/inflation/age, speedo wear, road surface.

However, the satnav reading on a steep slope will be slightly less than the actual road speed, as GPS measures the speed over the circumference of the earth, not the road distance the car is travelling.

Ed
 
I think I can answer this, I have a pickup truck with a speedo, calibrated tachograph and a satnav, the tacho and satnav show the same speed at 100kph but the speedo is about 7kph faster,at 30mph they're within 2kph of each other, so when you get a ticket that says you were doing 85mph chances are your speedo would be reading 92ish
 
MG TF 135 said:
I tend not to use the speedo anyway. I regulate my speed with the rev limiter..........
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Cool! Thats one way of doing it I guess :D
 
i work for a satnav company (begins with a G...) - ours are accurate to 0.1kt (knot).
cars can be calibrated to show you going up to 10% faster, but they're not allowed to show you going slower than you are.
 
When I first took my Tom Tom to the US I came across several of those radar speed indicators which tell you how fast you're going. Every time they displayed the exact speed that the TT was showing. Comparing my car's speed to the TT, at 70 TT speed then the car is showing about 72-73. Below that then it is only 1 to 2 mph above the TT speed.
 
SatNav - I use a TomTom and in my Cougar it reads about 9mph slower than the car's indicated speed at 75mph...so I drive at just under 80 in the UK if I'm staying legal on the motorways (but what are the chances of that?)...
In the Mustang, which has an electronic system, it reads exactly the same as indicated...
 
sat nav's are not accurate and neither is speedo's it is a hard one this

sat nav's are only realiable on the in built versions as there is more than one sensor built in to the car normal sat navs only have the one making them slower and hard to judge the correct distance i have noticed a few times that the sat nav was faster than the car

also the new digital speedos are meant to be accurate to within something close to the actual speed you re doing
 
wegotitugetit said:
sat nav's are not accurate and neither is speedo's it is a hard one this

sat nav's are only realiable on the in built versions as there is more than one sensor built in to the car normal sat navs only have the one making them slower and hard to judge the correct distance i have noticed a few times that the sat nav was faster than the car

also the new digital speedos are meant to be accurate to within something close to the actual speed you re doing
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err...I don't think so...the number of sensors the GPS has makes no difference - it's the number of satellite signals that determines accuracy...

Built-in GPS systems are notoriously innaccurate - compare a top-end TomTom to the GPS system in a BMW 335i and you'll see what I mean...the Beemer's system costs nearly £2,000 as an option and is utterly hopeless by comparison...

You'll have to trust me on this - I use Civilian and Military GPS systems at work and the hand-held Civilian ones are both more accurate and cheaper...
 
erm i think you will find majority are,
on the tom tom they only use one point of refrence the main unit not giving you a 100 % accurate reading where as in built gps uses several sensors which beam up to the satalite giving you more of a accurate reading to where you are

example vauxhall cavalier in built we went around bradford with this in aswell as the tom tom the tom tom was half a mile behind us as signal in bradford is useless but the inbuilt unit was accurate we took the tom tom along a it was the most up to date unit compared with in built unit

i have worked on several diffrent vehicles with these in and customers said they love them beacuse there more accurate then tom tom '

ok you have lets say one set of bad sat navs they do happen just sounds like the bmw's have them
 
OK - if you say so...I only have my own experiences to go by...
 
It's OK -I'm not having a go - my experiences with the Beemer satnav were less than happy - but I've never had any dramas with speed/location accuracy of the TomTom or Garmin e-Trex hand-held units...

That's not to say that the mapping on the TomTom is always 100% accurate - but that's nothing to do with anything other than the roads being built faster than the maps are updated...easier to deal with now that you can upload any map-errors yourself...Typically takes until the next quartely map-update to be rectified.
 
neither are accurate

sat nav take your speed over an average amount of time (albeit a short time), so the instant speed will always be wrong unless you are travelling at a perfect constant speed.
 
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