Inaccurate speedometer

[Graham Dicker]

When the speedo and digital readout tells me I'm doing 70 on cruise control, my satnav tells me I'm only doing 65. Similarly when the speedo says 30, the roadside radar display units that warn if you are not going fast enough ( ) say 27 or 28. Anyone else observed this? I wonder if I should ask the dealer to recalibrate it.

[guest5251]

speedometers by law are not allowed to under read as this could lead to the driver speeding
without knowing,but for the speedo to overread is ok i think they can be 10% out.
the other governing fact would be as your tyres near the legal limit your wheels will be
turning quicker so the difference between your actual speed and your dislplayed speed
will be reduced.

[John Ratsey]

According to my satnav my Mk 3 (SE CVT) is indicating 1 to 2mph high at 60 mph. my previous two Jazzes have also had similar slight over-optimism. The Kia I had before that was about 10% high.

I'm surprised that your Jazz is behaving different to mine in this respect. The speedo error will increase slightly as the tyres wear so that the wheels have to go faster for a given road speed. What tyres do you have? Mine are the standard 15".

[guest1372]

All cars I've driven recently have faster speedos than the GPS indicated speed from the phone's satnav.
For me, about 74 on the dash is 70 gps.

As others have said, the margin of error must be on the positive side, and tread wear will have an impact.

A 185/55R16 tyre is 610mm diameter full tread and will reduce to 600mm over it's life; so 1916mm per revolution will reduce to 1885mm that's about 3%

Mind you this also means a car's odo that shows 74k miles has only really done 70k.  Reports state it's accurate.

The OBDII port outputs speed, think I might compare that as well to see if the car is really more accurate than the display.
--
TG

[trebor1652]

I have the EX (16inch ) when approaching 30mph road side warning signs, if I have the cruise control set to 30mph the sign stays blank, if set to 31mph it flashes 'to fast'.


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[Graham Dicker]

I have Michelin 185 60 R15 tyres. I am comparing the Jazz with my previous car, an Accord. The over-read at 70 was barely 2 mph. I will probably not raise the issue with my dealer just yet, just bear it in mind and see how things develop as the tyres wear down.

[John Ratsey]

I have Michelin 185 60 R15 tyres. I am comparing the Jazz with my previous car, an Accord. The over-read at 70 was barely 2 mph. I will probably not raise the issue with my dealer just yet, just bear it in mind and see how things develop as the tyres wear down.

Those are the same tyres as I have. The only factor I can think of is that my Jazz is one of the first batch. Might Honda have changed some setting, in this case for the worse.

My complaint on miles-related matters is that the computer's mpg is about 9% higher than the real values. That is even more optimistic than my previous Jazzes.

[ColinS]

I looked into this years ago when I first got Sat-Nav so not sure if these facts are still current.

A speedometer is never allowed to show less than the actual speed of the car and not more than 110% of actual speed + 6.25mph.  So at 70mph  (real speed), it could be showing 83.25mph.  Clearly car manufactures will set them high in order to remain within these parameters and how high they show depends on the amount they choose to over clock them and how well they are manufactured within the tolerance.  So it will vary from batch to batch.  And of course as has been pointed out, the measurement is taken from the drive shaft so it will also be effected by the diameter of your tyres.

As people have pointed out this will also reflect on odometer, mpg, cruise control, speed limiters and anything else that relies on time over distance measurement.  For example I never trust the mpg it tells me, as this is always way over what I measure from filling the tank and looking at the odometer difference (yeah I know the odometer is also inaccurate but I just can't get my head around that).

Sat-Nav technology is much more accurate, setting aside that it doesn't allow for changes in altitude.  So unless you are going down a 1 in 1 hill is is probably showing your actual speed.  As it is distance over time, the faster you go, the more accurate it is.

[John Ratsey]

I think manufacturers are tempted to keep the speedometers reading high so that drivers think how nicely their cars are running at, say, 70 mph, when in reality it is nearer 60 mph. The same applies to the mpg reported by the computer.

I've never noticed significant over-estimating by the odometer. The journey length reported by the car will be close to that indicated by Google Maps or other route planning software. Tjhere's no technical reason why it can't be accurate when the tyres are new.

[ColinB]

When I have the cruise control set to (say) 70 and the car's travelling at a constant speed, I find the speedo indicates slightly under that (around 68 or 69). That suggests the CC (and presumably the speed limiter) is working on actual speed rather than indicated speed. That suggests the car "knows" the actual speed and is working to it, and it's just the speedo indication that's adjusted to read slightly high. That might also explain why the speedo over-reading isn't reflected in the odometer reading. Pure speculation of course.