Why does the tom tom 910 register a lower speed than I am actually driving on speedo?
Answers:
either one or the other is inaccurate,probably the speedo.Could be diff ratio or tyres but check it against mile posts to be sure.
Because your TomTom is accurate and your car speedo is extremely optimistic
Your Tom Tom is probably giving you a more accurate reading than your speedometer unless there is something wrong with the car, although I am surprised by size of the discrepancy.
The law states that your speedo cannot under read, so when your speedo states 70mph this is the fastest you can be travelling. Typical speedos will overread by around 5-10%.
car spedos are alowed to be something like 10% out, so that you think your goin faster than u are, and are therefore safer. my tomtom does the same. bit disapointing when u think ur doin 100.
your spedo properly need recalibrating
just a thought but your car isnt reading kilometers ?
this could be your cause, when a car is made and the wheels are fitted the speedo is calibrated with say 14"wheels on, if you go and put a different size wheel on the car (maybe for looks purposes) this will cause the speedo to give you a false reading,
Satellite navigation systems work using satellites (surprise surprise), and so sometimes they aren't always accurate if you don't have enough satellites that the Tom Tom can reach.
Your speedometer is allowed to over read by up to ten percent,UK law, but not under read.The TomTom,as with other satnavs, reads about every 50/60 yards so is highly accurate.Or so i have found.I have a garmin i3 streetpilot and i use it to check the accuracy of the various vehicles i drive.Can i make a suggestion?Drive to your speedo when passing speed cams and the plod.That way you will know for sure that you are legal.
Sorry Guys, but the Tom Tom is probably the one that is out. The GPS accuracy is going to be within 25-50 metres at best. Added to the update frequency rate of the TOM TOMs processor and possible screening effects of the Windscreen, Clouds Trees etc. that will come between you and the Satellites it all adds to the error.
I have an Origin B2 that consistently reads under the vehicle Speedo by about 5mph, regardless of which vehicle it's in. Which makes your TT's 14mph error look pretty bad
The speedometer gets its information from the speed the output of the gearbox turns at. It is then calibrated, taking into account the maximum diameter of the car's tyres when properly inflated and brand new. Then an extra bit is added, because by law it is not allowed to under-read but is allowed to over-read by 10%.
They are generally designed to over-read by about 5% to allow a small margin of error for tolerances while still meeting the legislation, but the error increases as the tyres wear, or if they are under-inflated.
The Tom-Tom unit registers the speed using satellite global positioning. Because it is not subject to the same legislation as the car's speedometer, it does not have to be designed to over-read to allow a margin of error. Also it will not be affected by changes in the effective rolling radius of the tyre caused by wear or incorrect inflation.
If you are getting the difference you suggest (14mph difference at a true 56mph is 25%), then there is either something wrong with your speedometer's calibration, or you do not have the correct size of tyres on the driven wheels, or both.
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