Sat Nav Says 69 Mph, Car Speedo Says 71 Mph. Which One Is Right?

[amaltman47]

sat nav says 69 mph, car speedo says 71 mph. which one is right? could i be done for speeding?

[tonyav]

Sat nav is more likely to be correct out of the two.

[ToraToraTora]

cars mostly over read

[Hopkirk]

Legally cars are not permitted to under read, so I believe, so they tend to over read.

[bhg481]

Sat Nav is correct. Speedos must not under-read, so are usually set to over-read by a small percent.
The diameter of a worn tyre is about 6mm less than that of a new tyre. The speedo only knows how many RPM the wheels are doing; it has no knowledge of how far the car travels with one revolution of the wheels - it calculates it from an assumed wheel diameter and the RPM. Therefore, as the tyres wear the accuracy of the speedo will vary.

[paul1763]

i use a garmin sat nav and it always reads 5mph slower than speedo,i allways assumed it was a bit behind due to time delay between car and satelite as i've quite often just passed a junction before the sat nav gets there (if that makes any sense)

[Hopkirk]

Paul, that's because you drive so fast that it can't keep up.

[paddywak]

I've a Tomtom and when the speedo reads 60mph the satnav reads 56mph.

[sunny-dave]

SatNav will be right - albeit with a slight lag when accelerating/decelerating quickly.

All car speedos over-read - some very badly - I've had one that is over 10% out - a real pain until you can calibrate it with a SatNav or similar.

[douglas9401]

The satnay's right. My speedo reads 33 @ 30mph as per one of those smiley signs and the nav agrees.

[Baldric]

Depends where you are, if you're in a 30mph limit then Yes!

[THECORBYLOON]

Speedometers are permitted by legislation to indicate a speed 10% higher than the actual speed but must not indicate a speed lower than the actual speed.

[jomifl]

If you go by you speedo then you stand less chance of being nicked. Before the advent of satnavs I used to calibrate the speedos of cars I was driving by timing the tenth of a mile markers on motorways. It didn't make me drive any slower though :-)

[Old_Geezer]

I believe that the Sat Nav is more accurate but due to the satellite link, delayed. Maybe you were accelerating ?

[1ozzy]

I agree with nearly everyone who has answered.
Speedo will always err on the side of caution, ie: read a bit higher for the reasons already posted..
Satnav will be more accurate.

[1ozzy]

As OG says, a rapid acceleration may not immediately register on your satnav.

[joggerjayne]

I agre. The Sat Nav MUST be more accurate.

A Sat Nav measures your movement across the ground.

A speedo makes a bit of a pi calculation based on a presumed radius and circumference of your wheel + tyre.

Think of it this way ...

If you skid at 70mph, with your brakes on, your speedo will think that you have stopped because the wheels are not turning.

Your Sat Nav will know that you are still moving.

[VHG]

You are never going to get done for speeding if you are doing 71 in a 70 limit, or 31 in a 30 limit.

The police know that most speedos in cars are inaccurate and a good lawyer could easily prove that the person doing 71 was probably doing under 70.

So the police only book someone if they are doing well over the figure, say 75/76 in a 70 limit, or 35/36 in a 30 limit.

[Old_Geezer]

Oh, not 71 in a 30 mph limit zone then ?

[bhg481]

Sat Navs only calculate your position on the earth about every 2 seconds and work out your speed from the distance moved since the last calculation, hence an average speed as opposed to the instantaneous speed of a speedo.
It gives you the interesting situation where, if you could drive VERY quickly in a circle, so that you were back where you started from 2 seconds ago, the sat nav would give your speed as zero. (back-of envelope calculation - about 100mph on a 100ft diameter circle). Anyone going to give it a try?