Are Many Speedometers Faulty?


Consumer this month has an interesting test, revealing that many speedometers in our cars are actually showing a lower than actual speed.

The magazine itself has been told this by readers as well, explaining why you do 100k down the motorway but cars pass you as if you are doing 85k.

The truth apparently is, that you probably are doing 85.

I always assumed speedos were one part of a modern car’s electronic that was painstakingly accurate, in view of the small margins you have these days when passing a speed camera.

Consumer says car makers argue that setting the speedo this way, is a deliberate way to compensate for any low tire pressures or modified wheels.

It doesn’t sound right to me.

So how do you know when you're over the limit?

The magazine points out that in-car GPS units they tested on a straight stretch of road gives another way to measure your speed and, their tests,  confirmed the finding.

The magazine quotes NZTA saying any inaccuracies would result in motorists driving slower, not faster.

Well, that’s not a helpful excuse when you get pulled over.

And if 100k is really 85k, should the speed limit be changed to 110 or 115 to be more accurate!

The article is in the April edition, along with a test of GPS units.

Anyone noticed the problem?




  1. Gus says:

    This is fairly common from what I’ve heard.

    My car is about 7km out when doing 100km/h - I know this from the roadside “Your Speed” signs (more than one). Most people who I’ve talked to about it have had similar results, it seems.

  2. Richard says:

    I remember in the sixties changing from cross ply tyres to radials and with the smaller radius that resulted my speedo was way out. I had to have it re-calibrated. I understand 5kph at 100kph error would be very common. However, 100kph@ 85kph is far too much!

    It just goes to show if you change tyres or wheels from the original equipment you should have your speedo checked

  3. Paul Quinlan says:

    From years back, I’ve always heard that for a WOF the standard allowance is 10% variance - not that it’s ever checked! And this is the standard police speeding allowance as well, from what I’ve heard.

    On my car tested both using a GPS and roadside radar, at 53km/h I am really doing 50 km/h. We have tested three cars using GPS, and they are all overstating the speed by 6 to 8%.

    With people inflating tyres to different pressures, it’s not unexpected that the odometers will be out.

  4. Jeremy Harris says:

    If we are all driving slower than we think we are, how high would the road toll be with accurate speedos..?!

  5. Mike Maguire says:

    When renting cars in the UK I installed a GPS. I noticed also that the GPS speed was often 5 (or more) mph out (7 or 8 kph). I researched it and found that in Europe it is illegal to sell a car with a speedo that might read slow. So to avoid a lawsuit, the manufacturers set their speeod’s to read faster than actual. However the odometer seemed to be calibrated correctly (at least according to my GPS).

    This is what I learnt from Wikipeadia:

    United Kingdom

    The amended Road Vehicles (Construction and Use) Regulations 1986 permits the use of speedometers that meet either the requirements of EC Council Directive 75/443 (as amended by Directive 97/39) or UNECE Regulation 39. [12]

    The Motor Vehicles (Approval) Regulations 2001[13] permits single vehicles to be approved. As with the UNECE regulation and the EC Directives, the speedometer must never show an indicated speed less than the actual speed. However it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles’ maximum speed if it is lower that this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.

    For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph. There is also the added problem of cars not complying with the United Nations standards, being imported and allowed to be registered, making the situation even more complicated. This needs further investigation.

  6. Neil says:

    My understanding was that speedos were allowed to show a variation of +10%/-0% ie they can show you as going faster but not slower than the actual speed you are travelling at. I have seen this in my car when comparing to my GPS unit which seems to be more accurtae if the roadside things are anything to go by.

  7. rtc says:

    Germany allow’s a 5% tolerance on speed cameras i.e. travelling more than 105km/hr on a roads signposted at that speed will result in a ticket. Seems Germany either has better cars or people calibrate their speedometers. Like people say, if you own a car you need to be aware what happens when you change tires etc.

  8. Adrian says:

    Yes, I’ve frequently passed the roadside ‘Your speed’ signs showing me going at a lower speed than I think I am. Like another post - my speed according to the ‘Your speed’ signs is about 7km/h slower than my speedo is showing.

  9. Joshua says:

    rtc - don’t think that is a problem as the tolerance is +10% -0%, therefore when you think your doing 100km, you should really be doing between 90km - 100km. Anything outside that and you speedo is not within tolerance. It is also your responsibility to maintain your car, so if you change anything you are at fault unless it has been certified. Apparantly some countries have 0 tolerance for going over the speed limit.

  10. Phil Hanson says:

    I don’t buy Consumer’s pitch. For years, NZ Autocar magazine has electronically checked the speedo of each car it reviews and most indicated speeds are close to the actual speed, certainly well within the 10% “tolerance”. I also drive a variety of cars and find their readings quite close to, or the same as, the speed being recorded by my TomTom satnav.


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