Speedo vs SatNav - which is more accurate?

[WaldoMcFroog]

If I look at my speedometer and compare it to my satnav, there is a discrepancy in the displayed speeds.

If I'm doing 70 on the speedo, the satnav will be showing somewhere around 63 MPH; at lower speeds the discrepancy is less pronounced - it's not out by a constant MPH, but seemingly by a ratio.

This has been the case in three different cars, all different makes, so either it's a massive coincidence that all my cars have had defective speedos or there is something real at work here.

My instinct is to say that since a Satnav uses relativity to determine position, it's likely to be more accurate than an electronic speedo that relies on vehicle speed sensor picking up magnetic fluctuations from a toothed disk on the transmission.

Should I drive to my speedo or my satnav?

Normally, of course, this isn't a significant issue; I pooter along at 20 MPH regardless of what sort of road I'm on...

[jake-the-peg]

Satnav is very much more accurate.

Speedometers have a permitted tolereance of reading high by 10% or so I think but 0% reading low.

So your spedometer may not tell you you're travelling slower than you are in practice that means most are engineered to over read

[Ankou]

i think it is because of the laws for car manufacturers in that the speedo within a car must never understate the the actual speed, so they effectively build in a slight buffer that over states it slightly by around 10% because of the potential for the mechanics/electrics of the speedo in your car to lag over time.

"The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h. The standards specify both the limits on accuracy and many of the details of how it should be measured during the approvals process, for example that the test measurements should be made (for most vehicles) at 40, 80 and 120 km/h, and at a particular ambient temperature. There are slight differences between the different standards, for example in the minimum accuracy of the equipment measuring the true speed of the vehicle."

[WaldoMcFroog]

Thanks both, but you appear to be saying the opposite to what I've observed. You both seem to be saying that speedos may not report a speed that is slower than a car is going, yet my speedo reports *slower* than my satnav which Jake confirmed should be more accurate.

Did I misinterpret what you said?

[Ankou]

is your satnav built in (i.e. from the car manufacturer) ?

[jake-the-peg]

You said that if you were doing 70 speedo that your sat nav said 63.

So your speedo is reading high and over reading as I said.

It's telling you you're going faster than you are for safety

[Jemisa]

"I pooter along at 20 MPH regardless"

We were behind you for miles on the M4 the other dat. GgRrrr!!
jem

[Ankou]

i was a bit narked when he did 70mph through my village the other day, but its ok because really it was only 63mph.

[mibn2cweus]

Waldo, Pass some of that over this way. ;o)

[WaldoMcFroog]

Sorry, JtP, I did indeed say that.

I retract my previous brainfart.

[rov1200]

I would be more inclined to use the speedo. The reason is because this registers in real time. With a Satnav there is a slight delay between your satnav receiving a signal. This is evident when you do not follow an instruction and it takes several seconds to correct itself on the new route.

[NazNomad]

Ignore the SatNav ... it'll be the speedo they test when you deny speeding (if caught). ;-)

[heathfield]

I once owned a 3.4ltr Jag. This model was credited with having the most accurate speedo of any production car. With my speedo showing 70mph, I'd be overtaking those motorists whose speedos were also showing 70, but whose actual speed was around 67mph. I mentioned this recently to a friend who is an Audi mechanic. He said that many Audi speedos are calibrated to show 70 at 68mph. Otherwise, tyre pressure, tyre wear, and tyre size will all affect the accuracy of speedo readings, whereas with GPS these won't matter.

[wildwood]

Also, the difference between new tyres and 'borderline' tyres can easily be 3% of your speed.

[Andyvon]

All speedos are accurate to within 1 or 2 mph between 30 - 60mph. Below and above that they become progressively inaccurate due to the mechanical limitations of the speedometer drive on the gearbox. When a speedo says you are travelling at 100mph you are probably doing an actual 86-90mph.

If your speedo says 70mph Waldo then you are probably travelling at an actual 63 -65 mph which your satnav bears out.

[rov1200]

I would have thought that worn tyres show a higher mph reading than new ones? Because the diameter of the wheel is smaller 1 revolution of the wheel means less distance travelled.

[ChuckFickens]

Errr, rov...

"With a Satnav there is a slight delay between your satnav receiving a signal"

Yes, of course there is, that's how satnavs work, they know the time of the delay and it's used to calculate your position, it's what make them accurate, not what makes them less accurate.

The sat nav will be more accurate than the speedo at a steady speed, it won't respond to changes in speed as fast as a speedo though.

[mibn2cweus]

OP, you have a PM in your alternative.

[Castaway]

My situation is similar. When I do 33 mph (speedo reading) my satnav reads 30mph. When speedo reads 70, satnav reads 63. All that started when I changed the wheels on my car from 17" to 15". When the 17"were on there were no discrepancies at all between speedo and satnav, readings were exactly the same. Truth be told I don`t really get it why the change of wheels can be a cause of such difference in speed readings.

[Castaway]

Hi, My situation is similar. But it has not always been. It all started when I changed my wheels from 17" to 15". When I had 17" ones on my Rover both speedo and satnav read exactly the same speed, all the time, no difference.
However when I put smaller 15" wheels on I started noticing discrepancies between readings: 30mph on satnav when speedo read 33mph. 63 on satnav when 70 on speedo. I cant understand how change of wheels to smaller could`ve caused such difference in readings.