how accurate are sat-nav speed ratings?

[daviebroon]

got a navman n60i which i love to bits. been trying to figure out just how close to the truth the mph reading is. when i'm doing indicated 40 on the speedo, it says 37. when i do about 70 it says 65. when i touched 150 on the clock (down hill)... it said 140.

i know the speedo is more and more inaccurate the faster you go. the navman updates every second and is accurate to a couple of feet iirc.

anyone know?

 

[rory182]

I think they're maybe more accurate than the speedo. Cruise control gives a different figure again.

 

[biojo]

well my tom tom 6 at

speedo tom tom
30 28
70 67
80 76

if i remember correctly

 

[daviebroon]

Cruise control gives a different figure again.

wouldn't know. i've got the poor-man's sport

 

[Mells]

well my tom tom 6 at

speedo tom tom
30 28
70 67
80 76

if i remember correctly

Thats exactly what I get with my tomtom in the clio. When there are cameras about I always use the speedo on the car because it over reads. Stick to 30 on that and it will be well away from the 35 mph needed to trigger the camera.

 

[Graham Gooch]

tut tut at a peak of 140mph !

 

[Harry182]

Every car speedo is approx 3% out.

Consumer being ripped off by Car Manufacturer

Just think! You lease a car, with 12,000 miles a year allowance. Then realise that your speedo is actually 10% out, This means that you are now only allowed to do 10,800 miles and will be Charged at least 35 pence per mile = £420.00
Belive it, IT HAPPENS.
Even Worse. You own a car that is showing 100,000 on the clock and you go to trade it in, In reallity your car has only done 90,000 miles but you daeler looks in his little book and gives you the price that you car is worth. This can be up to £2500.00 less because of the diffrence shown on the clock.
Is This Fair. NO
Speeding, You drive down the motorway @ 70mph, You are actually doing 63mph.

Goverment guidelines for manufactures set by VOSA
SPEEDOMETER READINGS

• THE SPEEDOMETER MUST NOT SHOW A GREATER SPEED

AT

Speedo Reading / Actual Speed

• 30 23
• 35 27
• 40 33
• 50 39
• 60 44
• 70 53

As you can see the guidelines set are so far out how could anyone know what speed they are doing.
Simple Test

Use a GPS/ Navigation system, Even these are not 100% accurate but there not far out. You will be able to calulate just how much your Odometer could be wrong.

 

[daviebroon]

so 140mph on the sat nav would nearly be actual speed? surely quite good then?

was quite gutted as the speedo read about 152, and looked at sat nav reading just 140. i was only at that speed for about a second before coming off throttle... so maybe hadn't updated quick enough?

good thing is then... it warns me when avg speed cameras start and finish... so can use this and fly past everyone who are going by speedo lol

 

[Graham Gooch]

so 140mph on the sat nav would nearly be actual speed? surely quite good then?

was quite gutted as the speedo read about 152, and looked at sat nav reading just 140. i was only at that speed for about a second before coming off throttle... so maybe hadn't updated quick enough?

good thing is then... it warns me when avg speed cameras start and finish... so can use this and fly past everyone who are going by speedo lol

GPS is 99.9% accurate, well at the end of the day it has to be lol.

I've only got a 128mph peak on my garmin lol. Bad thing is it stores the max speed forever lol. Yeah its very handy for speed cameras.

 

[daviebroon]

yeah mine stores it on about 4 different sub-menus lol... i use it to show off my rather fast cross-country darts lol.

wasn't sure about the accuracy of navman as i know they're not the market leader. but it was 350quid and was better spec than most of the others i looked at. i love it!

 

[Graham Gooch]

My gps is spot bollock to those signs that flash up your speed in local villages lol.

 

[daviebroon]

My gps is spot bollock to those signs that flash up your speed in local villages lol.

there's one by my work that used to be national limits. it's a dual carriage way and have reduced to 40 for roadworks. always trying to get it showing 3 figures lol

 

[Chris1.4RT]

My needle reads 40mph and when I flick cruise on it'll be about 37. Gps will say about 35.

 

[KDF]

As long as your GPS device is locked on to a good few sattelites eg. 3 or more, then its pretty damn accurate.

 

[Tom]

Speedo readings are not directly related to odometer readings anymore.

You'll find that most cars use a uncalculated reading to supply the odometer. So nobody is being conned by speedos that read 10% higher etc etc

 

[Revels]

I wish TomTom gave you those kind of details.

Id like an average speed etc.

 

[daviebroon]

the navman can lock onto 12 sats... even indoors i pick up 10 of em! has a signal for like ever.

also good as it's got a half decent camera on it... and full gps logging to show where you've been. love this satnav

 

[M1TCH]

well my tom tom 6 at

speedo tom tom
30 28
70 67
80 76

if i remember correctly

same here on my go500.

 

[daviebroon]

 

[derv]

i have had my tomtom go, my Origin B2 and gone past one of those speed signs and they all showed the same speed, the speedo on my van however is out by 9mph at anything over 40mph.

 

[Ex-User (19928)]

Well GPS will give you an EXACT reading, IF you're doing a constant speed.

Simply the way it works, it won't give an EXACT speed when accelerating.

It logs your position, then logs your position again (about a second later) and gives you the average speed over that short distance, the faster you're going, the more accurate it will be.

GPS will give you a reading to the nearest 5 or 10 metres depending on conditions.

I have often been tempted by the thought of taking some GPS gear from work for the weekend (including a basestation which will give you a reading to the nearest 10mm quite a few times a second), and taking it to a track, just for a laugh to map it later.

Still, all car speedos are out by a bit unless it came with 14s and then you put 19s on it (it will then over read).

 

[UncleGrumpy]

My gps is spot bollock to those signs that flash up your speed in local villages lol.

there's one by my work that used to be national limits. it's a dual carriage way and have reduced to 40 for roadworks. always trying to get it showing 3 figures lol

legend.

Get a picture if you do.

 

[C.slicker]

My 1.2 is too quick. It dodges the satellite's signal.