Speedo or Sat Nav?

[PepnKook]

I have a Tomtom sat nav that we use in both a car and our van. Whilst in the car, the sat nav and speedo read the same, but in the van, the sat nav reads around 5-6 mph lower than the speedo. What one do I believe??!! I get edgy setting the cruise control to over the speed limit on a dual carriageway, even though the sat nav says I'm under! Also, setting the cruise at 73 mph in a 110 kph (68 mph) zone, my co-driver tends to get a little twitchy.....

 

[gus-lopez]

sat nav. speedo's have to overread rather than under read to prevent claims against the manufacturer's

 

[maison]

Definitely the SatNav. Satspeed calculation is more accurate than the speedometer which is usually set to “over read” to help comply with speed limits.

We use it as our speedo, especially when in Europe, as we can set it to km/h.

 

[Vic]

As above, Sat Nav.

 

[Kingham]

Op - It depends on what you want !

Most of the time your sat nav will give the most accurate speed of travel, but on occasion it can be more inaccurate than your speedo.

Therefore most of the time, relying on your sat nav will have you travelling accurately at whatever speed you choose.

Relying on your speedometer will have you travelling just below whatever speed you choose, therefore has a built in safety margin, should your attention be elsewhere.

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[teddybear]

Have to use sat nav as Speedo is not visible

 

[68c]

Have to use sat nav as Speedo is not visible

Why is it not visible?

 

[Coolcats]

Have to use sat nav as Speedo is not visible

Now then if you can’t see the speedo are you sure you are not a back seat driver? If not what else cant you see....

 

[Rapido925M]

When I meet devices in towns which display my speed they usually agree with the satnav BUT sometimes they read lower, agreeing with the speedo.

Given that the difference is typically 5mph, does it matter very much if you drive according to the slower one? I'm usually not rushing in MH so I drive that little bit more sedately and pull in when possible to let those behind me get past.

Alternatively, hope that Police Forces grant you 10%+2mph so at 70mph you can be doing 70+7+2 = 79mph.
[BUT if the law says 70mph, why should you get away with anything? I understand that in France 30kph means 30kph!]

Your decision: you choose but don't complain if you get punished for speeding.

Gordon

 

[34127]

Alternatively, hope that Police Forces grant you 10%+2mph so at 70mph you can be doing 70+7+2 = 79mph.
[BUT if the law says 70mph, why should you get away with anything? I understand that in France 30kph means 30kph!]

Your decision: you choose but don't complain if you get punished for speeding.

Gordon

One of the reasons for the 10% + 2 mph tolerance is to take into account the inaccuracy of the speed camera so I wouldn't assume that if you are doing 79mph in a 70 limit that you will not get done.

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[gus-lopez]

Why is it not visible?

It is probably a Fiat with the unviewable speedo/dash.
Having said that when driving here with my sunglasses on I can only vaguely see the dash.

 

[icantremember]

I prefer the satnav, although as I found out to my cost, they can have a time lag when accelerating.

Accelerating away from a roundabout on a tree lined city street I was caught just above the 30mph limit, although I'm sure my sat nav showed under the limit.

 

[AJM2]

I remember HonestJohn in the Telegraph changing his mind on this. Initially he said satnav but it was pointed out to him that the margin of error within its tracking system meant it was far better to rely on the speedo.
If there's a guy hanging out of a police van with a camera (and I spot him first) I'd choose whichever said the higher speed!

 

[tonyidle]

When I meet devices in towns which display my speed they usually agree with the satnav BUT sometimes they read lower, agreeing with the speedo.

Given that the difference is typically 5mph, does it matter very much if you drive according to the slower one? I'm usually not rushing in MH so I drive that little bit more sedately and pull in when possible to let those behind me get past.

Alternatively, hope that Police Forces grant you 10%+2mph so at 70mph you can be doing 70+7+2 = 79mph.
[BUT if the law says 70mph, why should you get away with anything? I understand that in France 30kph means 30kph!]

Your decision: you choose but don't complain if you get punished for speeding.

Gordon

Using the satnav doesn't mean you have to drive anywhere near the limit. I use my satnav to set cruise control to maintain 65mph on the motorway. If I used the speedo to do the same I'd be travelling at 60mph.

 

[teddybear]

With my height and the angle of the Speedo the top arc of the gauge is Hidden behind the steering wheel

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[AG 2602]

I'd choose Speedo's every time my current pair are circa 1998 so a little bit tight and slightly laddered on the left buttock!

 

[Rapido925M]

Using the satnav doesn't mean you have to drive anywhere near the limit.

I totally agree but the OP said

I get edgy setting the cruise control to over the speed limit on a dual carriageway, even though the sat nav says I'm under! Also, setting the cruise at 73 mph in a 110 kph (68 mph) zone, my co-driver tends to get a little twitchy.....

I can only offer advice on what I understand - Gordon

 

[kevenh]

I prefer Sat Nav indicated speeds to a speedo that is allowed to be inaccurate.

My last two cars have had speedos that have shown similar speeds to the Sat Nav.

At the end of the day the last paragraph summary from Car Clinic trumps all our {contrary } feedback:

On a straight and level road, at constant speed, the GPS-derived speed reading will be accurate.

However, there is usually some lag in the reading, and accuracy can be affected by GPS signal quality, bends in the road, inclines and speed changes. Fortunately, modern factory-fitted systems also use data from the car itself, integrating it with the GPS information so as to enhance the accuracy.

So, in general, your sat nav is likely to be more accurate than the car’s speedometer, but for safe driving, rely on the latter because it will never understate your speed.

EDIT: added a link to the source: https://www.driving.co.uk/car-clinic/car-clinic-which-should-i-trust-my-speedometer-or-my-sat-nav/

 

[TheAleMan]

Sat nav is accurate. My work truck gets set on cruise at 74, and that is 70 on the sat nav.

 

[DerryW]

As far as a Ducato is concerned it has to be the Sat Nav. It's near on impossible to read the speedometer particularly at night. I cant make out how Fiat are allowed to get away with it.

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[Kannon Fodda]

Given that the difference is typically 5mph, does it matter very much if you drive according to the slower one? I'm usually not rushing in MH so I drive that little bit more sedately and pull in when possible to let those behind me get past.

Differences are more likely to be a percentage value, than a fixed speed tolerance. so perhaps only 1 or 2 mph at 30, but 6 or even 7 mph at 70.

Alternatively, hope that Police Forces grant you 10%+2mph so at 70mph you can be doing 70+7+2 = 79mph.
[BUT if the law says 70mph, why should you get away with anything? I understand that in France 30kph means 30kph!]

As far as I know any 10% plus a bit is an unwritten rule, not guaranteed to apply. I recall some articles a while ago where some police forces / safety partnerships were threatening to remove any tolerance "improve" safety and enforcement (so raising more money).

So problem is really what is the accurate reading, speedo, sat nav? By law the speedo can't give an over-reading so most vehicles will show a higher speed than you are actually doing. Many police traffic vehicles will have a specially calibrated speedo (often denoted with a home office logo) so those officers can rely on their vehicle. Us mortals have to keep guessing but if our speedos were accurate, perhaps we wouldn't all be guessing and be less tempted to do our different interpretations of how fast.

 

[Rapido925M]

Differences are more likely to be a percentage value, than a fixed speed tolerance. so perhaps only 1 or 2 mph at 30, but 6 or even 7 mph at 70.

So typically about 5mph, as I said!

As far as I know any 10% plus a bit is an unwritten rule, not guaranteed to apply. I recall some articles a while ago where some police forces / safety partnerships were threatening to remove any tolerance "improve" safety and enforcement (so raising more money).

Hence my comment "hope that the Police force will grant you 10%+2"

So problem is really what is the accurate reading, speedo, sat nav? By law the speedo can't give an over-reading so most vehicles will show a higher speed than you are actually doing. Many police traffic vehicles will have a specially calibrated speedo (often denoted with a home office logo) so those officers can rely on their vehicle. Us mortals have to keep guessing but if our speedos were accurate, perhaps we wouldn't all be guessing and be less tempted to do our different interpretations of how fast.

So if you do not exceed the limit as denoted by your speedo you should be OK.

How often do we need to be travelling at the highest permitted speed? I came back from Dorset last week maintaining 70mph (using cruise control as measured by my speedo) on the motorways and close to national speed limits on other roads but that was under special circumstances: most times I would be 10mph lower.
My greatest concern isn't exceeding the speed limit per se so much as the speed limit changing and me failing to spot it.

I totally approve of average speed restrictions because they allow you to exceed the instantaneous limit when overtaking whereas a spot camera will catch you in one place but not all the others.

Gordon

 

[matt34]

sat nav for me

 

[autorouter]

A speedo is precise and consistent, but not accurate. That means you can tell the difference between 54 and 55 mph readings, even if the true speed is 59 and 60 mph. A satnav will be very accurate when travelling at a constant speed in a straight line, but might show stupid speeds when crawling along in traffic.

For the best accuracy, you can look at both, and remember how much error the speedo has at each speed. You'll soon learn, for example, that 60mph on the satnav is 65mph on the speedo.

 

[34127]

There a few comments on this topic about speed limit tolerances.
The National Police Chiefs Council give guidance on these tolerances but it is up to individual police forces as to whether or not they use them.

https://www.npcc.police.uk/2018%20FOI/Operations/030%2018%20Speed%20Enforcement%20Guidance.pdf

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[PepnKook]

Totally agree that it's best to err on the side of caution when travelling at lower speeds, but what I'm looking at specifically is when I'm tanking across Europe on a autoroute or autobahn overnight and I've got miles and miles of clear road in front of me at 130kph and I'm itching to get to where I'm going, I want to be able to set my cruise control to the speed limit so I don't break it and get nicked, but setting it to 85-86mph on the dash and getting pulled over, don't think the fuzz would pay much attention if I said "But the satnav said...."

 

[kevenh]

<snip>..I've got miles and miles of clear road in front of me at 130kph and I'm itching to get to where I'm going, I want to be able to set my cruise control to the speed limit so I don't break it and get nicked, but setting it to 85-86mph on the dash and getting pulled over, don't think the fuzz would pay much attention if I said "But the satnav said...."

When you're away from built up areas is when a Sat Nav will definitely trump a speedo's built in inaccuracy.
And imo the transient bad locks a GPSr gets from bad reception that affect speed readings are rare enough to make it reliable compared to a speedo. No need to adjust for tyre wear, etc.

 

[two]

Neither is 100% accurate.
Sum them and divide by two.