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Master Traveler
RJF
Posts: 4
Registered: ‎02-01-2012
Location: United Kingdom
Accepted Solution

Speed difference between my tomtom and my car

Hi, So I am new to this but I have noticed that there is normally about a 5 mile an hour difference between my speedo and the tomtom via 1000. Say I am doing 60 mph on the speedo....the tomtom says I am doing about 56 mph, why is indicating slower than actual?...any help appreciated!

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Neophyte Explorer
alan605
Posts: 12
Registered: ‎30-12-2011
Location: US

Re: Speed difference between my tomtom and my car

hi, i just got back from a road trip with my 1435tm...like your post..my tomtom and speedo was off..but not by much..anywhere from 1 to 3 mph off..not enough to bother me. if i were to guess why they are off ??..i would say it's because it takes some time for the satel light signal to bounce back and get read ?.this is just a guess.

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Lupus1
Posts: 5,478
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Registered: ‎21-09-2011
Location: Germany

Re: Speed difference between my tomtom and my car

Hi,

 

the difference is caused by your speedo. The speedos all have a certain allowance when being manufactured. As the speedo in every single car will show a slightly different speed there's always an allowance discount when being caught for speeding. The PND is by far more accurate via the GPS signal than your speedo which explains the difference.

 

Although when accelerating the GPS is a bit behind in getting the new speed but that is really only a bit as there are display latencies and transfer latencies. But you'll see when you've reached your speed that usually the speedo and the PND will show different speeds which is due to the mentioned above allowance and inaccuracy of your speedos.

 

 

Regards

Lars.

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TF
Apprentice Traveler
TF
Posts: 2
Registered: ‎02-01-2012
Location: Australia

Re: Speed difference between my tomtom and my car

It isnt the speedo at all. Its your tyres.

Basically when the radius (and circumference) of your tyres decreases - ie when you wear them out or buy low profile tyres - the car goes SLOWER than what your speedo indicates. When you buy new tyres it will be closer to the real speed. Your speedo can not be recalibrated either so I just trust my gps and go a little slower

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Sovereign Trailblazer
redcar
Posts: 74
Registered: ‎22-07-2011
Location: United Kingdom

Re: Speed difference between my tomtom and my car

The tyre wear argument is 80% myth.  Say the outer diameter of your tyre is 600mm, and you wear 16mm off of it, that is only going to reduce your actual speed by 2% for the same indicated speed.

 

BTW, on average your Tomtom  is MUCH more accurate than your car speedometer.

 

One side note is that older Tomtom devices e.g Go730 give a much more useful indication of the vehicle speed, having only a second or so lag.  The Go1000 series devices have about a 4 second lag before settling on the correct speed after a speed change.

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Lupus1
Posts: 5,478
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Registered: ‎21-09-2011
Location: Germany

Re: Speed difference between my tomtom and my car

Hi TF,

 

I doubt that it's the tyres. Due to the same reasons redcar provided as well as due to my own experience. In my family we have 5 different cars with no identical models but some of the same brand. With some of the cars the difference between speed displays / PNDs and the speedo is nearly nonexistant and with some of the cars we've got up to 10kmph more on our speedo than being displayed on the PND/speed display. This didn't change an iota after I changed from nearly run-down summer tyres (about 2mm profile) to new winter tyres (more than 7mm profile).

It's the same with all cars over quite some years now.

 

Also if it wasn't the speedo why have a speeding reduction due to speedo allowance when being caught at speeding?

 

 

Regards

Lars.

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Vanman
Posts: 351
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Registered: ‎24-03-2011

Re: Speed difference between my tomtom and my car

[ Edited ]

I don't know if its by law or British/EU standard but in the UK your car speedo can be from correct to 10% fast.

 

In this way, the Police should be able to assume that if you are speeding you must be indicating this on your speedo. Any 'allowance' the Police give is discretionary only.

 

Also I assume that car manufacturers will calibrate the speedo for the standard tyre in its most worn condition and err on the side of caution? In that case I would expect that almost all speedos read high.

 

EDIT: This is from a written answer in the house;

"Lord Whitty: The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulation 39. Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed."

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Founding Member
hope68
Posts: 368
Registered: ‎25-02-2011
Location: United Kingdom

Re: Speed difference between my tomtom and my car

see here under "Errors": http://en.wikipedia.org/wiki/Speedometer

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Sovereign Trailblazer
ghost_rider
Posts: 122
Registered: ‎03-01-2012

Re: Speed difference between my tomtom and my car

in most cases..in united states anyway..mostttttttt states will give you a 5 mph leed way ..depends if that cop had a good day or not i guess ??..lol. now..i do know for a fact..if you put some very large tires on a car or truck ( mostly trucks )..it WILL offset the speedo !!..my dad ( many years ago )..had been stopped for speeding..and yes..he did have very large street tires on his truck..went to court..showed the judge where he had installed large tires..was let off with a warning ( he did have to get a gear reduction to make up the difference though )..sooo..yes..tires matters..that is...IF YOU TIRES ARE VERY BIG ??!.  as to why the gps and speedo are not in sinch ??.i can't answer this...other than what i posted above ( alan605 )..the time difference it takes for the signal to bounce back and forth to the gps.

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Master Traveler
RJF
Posts: 4
Registered: ‎02-01-2012
Location: United Kingdom

Re: Speed difference between my tomtom and my car

Thanks Lars, this makes sense.....
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