Accuracy of Speed

[JayW]

A friend of mine was caught speeding recently at 32mph in a 30mph zone.

I note, consistently, that the speed on my TOMTOM (Live 1000) is lower than the speedometer on the Car itself. When the speedo or digital Cruise Control shows 30mph . TomTom shows 28mph ( e.g Tom Tom says 30mph when speedo & cruisecontrol shows 33mph). At speedo of 70mph, TomTom shows 67mph.

Which is likely to be correct.

 

[dhn]

The GPS will be accurate. All speedometers are purposely adjusted to be up to 10% higher than the accurate value.

 

[Andy_P]

Confirmed....

It's apparently illegal for car manufacturers to produce a car where the speedo under-reads the true speed AT ALL.
So, as it's really hard to get perfect accuracy in a mass-produced, un-calibrated instrument, they always err on the side of over-reading so as to be within the law and it's usually chosen to be somewhere between 5-10%.

It's a bit like the old Baker's dozen.

 

[JayW]

Thank You :thumb:

 

[canderson]

The GPS will be accurate. All speedometers are purposely adjusted to be up to 10% higher than the accurate value.

There are exceptions to that, and of course, a small error in replacement tire (tyre?) selection can throw everything off.

My Hyundai Santa Fe with new rubber continues to show exactly the same speed as my TomTom units up to 85mph (not sure what happens after that). Was true with the OEM tires, was true with my current Nitto Crosstek tires, was true with my old 720, my 740, and my beta unit. When the speedo is on 85, all units read 85.

My 300M reads only 1mph slow on the GPS with new rubber at 80.

So some of them are pretty tight, but understanding that tires wear, there will always be a drift towards a faster-than-reality indicated speed on the speedo.

But to the OP, I'd always trust the GPS speed, just as has already been recommended to you.

 

[canderson]

Confirmed....

It's apparently illegal for car manufacturers to produce a car where the speedo under-reads the true speed AT ALL.

... with the tires that are supplied with the vehicle from the factory. Of course, after that, all bets are off, and it is surprising how many people don't appreciate that picking up an off-size tire can foul things up royally. Many people still think wrongly about profile, and assume that a 255/60-16 is just a wider version of a 215/60-16 and don't realize that the diameter is changing. The speedo would be off by over 7% (the wrong way!) in that example.

 

[mikealder]

The speedo in my Volvo is within 1MPH of the GPS speed up to the legal limit, its out by around 3MPH once you get to 100MPH (on a race track of course). Its fitted with non standard size tyres though which actually made the vehicle speedo more accurate!

Fitting odd ball size tyres can cause other issues, I did hear of a Mercedes sports car that had a new tyre fitted due to a blow out, after this the anti lock brakes and traction control threw a hissy fit, Merc changed the gearbox under warranty as that was their diagnosis, all the car really needed were two tyres of the same diameter on the same axle, the combination of a worn tyre (but still legal) and a new one caused the cars systems to indicate a fault that wasn't really present - Mike

 

[dhn]

What I'd like to know is if, in Britain, drivers do get tickets for just going 2 mph over the speed limit!!

If so, no wonder the fixation Brits have with reliable Speed Camera Warnings on their sat navs .....

....... and why so many rely upon TT's data set instead of subscriptions to sites such as PGPSW.

 

[mikealder]

What I'd like to know is if, in Britain, drivers do get tickets for just going 2 mph over the speed limit!!

You would have to be having a really bad day to get caught at only 2MPH over the limit, to be honest I doubt it would end up in court.

If so, no wonder the fixation Brits have with reliable Speed Camera Warnings on their sat navs ......

It is handy having a decent warning system with frequent accurate updates due to the average speed camera installations that seem to follow the roadworks around on the UK's Motorway network, these are the ones that can catch you out, as are the mobile traps.

....... and why so many rely upon TT's data set instead of subscriptions to sites such as PGPSW.

The distinct lack of mobile camera locations when using the TT system is its main drawback, the inaccuracy of the average speed camera locations on the Motorway is nothing short of pathetic with some instances of locations being out of date by three years!
I guess those that use the TT service are ignorant of the fact that better data is available for use on their hardware from third party sources - Mike

 

[JayW]

Yes I can assure you that the person concerned did get fined in Court. The formal documents show that he was exceeding the 30mph limit by 2mph. He was fined and given points on his licence.

I certainly am one of the people referred to - I was and am not aware that there are 3rd party sources.

 

[dhn]

Well, you are now:

See here:
Pocket GPS World - SatNavs | GPS | Speed Cameras Forums-viewtopic-Why choose the PGPSW database?