Is your Speedo accurate?

[Buckshot]

I've always wondered how accurate my speedo was so I tried to work it out.

An article in one of the Landy mags a while ago inspired me.

I travelled along a dual carriageway at a set speed and timed how long it took me to travel a set distance.
Dual carraigeways are good because they have the little distance markers every 100 meters. The green, white and red posts with numbers at the top.

I travelled 9 Km in 5 minutes, 21.32 secs.

With a little school boy maths 'A car takes 5 mins 21.32 secs to travel 9 Km, how fast was it going in MPH?' and using the conversion factor below I worked out that I was going 62.66MPH

My speedo showed 70MPH the whole time. This is over a 10% difference.
I expected the actual speed to be less - but not that low.
However my Landy is a 1986 vintage so that may account for it.

My other car, a 1997 Rover took 5 mins 11.23 secs to do the same distance.
Can anyone work out the actual speed of this car?

Try it on your car, you may be surprised...

Usual disclaimer though. Be careful, it's best to have someone using the stop watch and counting the markers and then someone else driving :shock:

Coversion rates
1 KM = 0.621371192 miles
1 mile = 1.609344 KM

Cheers

Mark

 

[Squidders]

I have tried this a few times over the years...

My old ZZR1100 I had calibrated for 100% accurate at 150mph when I had it dynoed and at 30mph my actual speed was around 32 and at 180mph my speed was 179 using the same method (distance/time = speed)

Using 2 GPS recievers, a Garmin StreetPilot III and an Etrex my car does an actual 68mph with a dashboard indicated 70mph which isn't bad really.

I am buying a new motorbike in a few weeks so I’ll test that too.

 

[Rob]

We have to take tyre diameter into account when we change between the road and off-road tyres on the Landy.

The larger diameter of the off-road wheels makes me drive slower (on the speedo) past the speed cameras.

I had an old MK2 Escort once, I always wondered why nothing was overtaking me when I was doing 70 on the motorway. Turns out it was really doing 90 I was quite impressed really.

 

[Andy]

The only speedo I have goes on a pedal cycle. I worked out it was accurate to 0.04%

Speedos have to be accurate to 10% at 30mph. I think they tend to get worse as you speed up. What I find funny is a speedo on a merc and a spedo on a cheap car have a very different level of accuracy, yet they are made in the same place. How much would it really cost to sort them out so they are all accurate?

 

[Moonraker]

And there was me thinking this was a thread about swimming trunks and how well they indicated the contents :?:

 

[Buckshot]

:rolmao: :rolmao: :rolmao:

 

[Adi007]

One thing to remember about using GPS as a speedo is that most units work out your speed by averaging the speed over two readings taken a second apart (normally) so it's great if you aren't making any drastic changes in speed.

 

[Richie]

Speedos are normally out. Unless you have an IRS Speedo which means that it is guaranteed to be with in 5% of the actual speed that you are doing.

Which is why ACPO guidelines for speed cameras is legal limit plus 10% plus 3mph. (70mph + 7mph + 3mph = 80mph) However, watch it in the 30's and 40 limits the county forces can be set nearer the mark. If you are in Met land you will be safer.

 

[Edi_M]

Some GPS units measure the straight-line distance between two points in three dimensional space, others take lat/long and then perform a haversine based calculation (in laymans terms, the distance between two points relative to the centre of the earth taking into account the average curvature of the earth - I won't quote the maths because I can't remember & seem to remember it's a fairly wordy formula) to calculate the speed in knots. That is then converted into either mph or kph. Given the required accuracy of the numbers, there is likely to be rounding being performed. When I tried to take the formula from a VB function & put it into a sql procedure (for the programmers, don't ask me why, I was instructed to do it, should have gone into a C module), the result of calculating current position relative to current position using 32-bit floating point numbers had me a couple of cm away from myself. The further apart the two points, the worse the accuracy. If the GPS isn't reporting in perfect conditions, this will be worsened.

 

[Adi007]

For quite some time now most GPS receivers have used haversine based calculations to improve on accuracy of the information displayed. I seem to remember that older units (you can spot these in the UK by their inability to display 10 digit OSGB grid reference, displaying instead 8 digits) did 3D point-to-point calcs instead that were less accurate. I know that all current Garmin and Magellan receivers as well as any receiver that claims to use "SiRF" technology should now use haversine calcs.
Parallel 12 channel receivers (units that can listen to and receive information from up to 12 satellites simultaneously) also improved accuracy because individual errors were averaged out better. New 14-channel receivers will improve this accuracy even more.
Remember too that the biggest nemesis of GPS receivers is the good ole pine tree ... those needles filled with moisture are ideal microwave antennas and absorb the signal really well.

 

[Brendan]

According so my satnav system whilst on the M25 I was travelling at 63mph while my speedo was showing 70mph (2002 VWGolf)
I trust the GPS more than the car but dodgy speedo has probably saved a few speeding tickets!

 

[Rob]

Good when the malfunction operates in your favour

 

[havingagiraffe]

I once read of a Land Rover which had an infinitely adjustable bicycle speedometer fitted to enable quick adjustment between small dia road tyres and large dia off road tyres. It had the sensor mounted on the brake backplate taking a reading from a magnet mounted on the brake drum sending the info to a digital display in the cab. Cost less than a tenner and weighs less than a packet of fags. Very clever. I think old terratrip rally navigation equipment worked on a similar principle.

My Lightweight has Letraset stickers over the original numbers on the speedo to reflect the true speed - Rover 95 diffs and large tyres up the gearing by about 20%. Overtakes lots on the flat, gets overtaken lots on the slopes

 

[Colin]

For what it is worth, I was wondering this when returning from Scotland a few weeks ago along the M74. I have a Nissan X-trail with sat-nav which was standard when I bought it. While holding a constant 70mph I reset the trip computer which is linked to the sat nav. The average speed was initially zero but this very quickly increased until it appeared to hold steady at 67mph after some 10 mins or so. Now because it had started at zero, I thought that it would never reach 70 but would 'top out' at about 69 point something. So after about 15 mins of steady 70mph, I slowed down and held 65mph (v little traffic) and lo and behold, the average speed dropped to about 63mph. So bearing in mind that the sat nav seems quite accurate in terms of position and the clock in it is very accurate I would tend to believe the sat nav rather than the speedo and reason that the speedo overreads by between 2 and 3 mph at motorway speeds which of course errs on the good side in terms of speeding and speed limits.

 

[demographic]

I found that the indicated top speed of my bike changed when I used a front tyre (the speedo drive is taken off the front hub) that had a different aspect ratio.

It used to have a 120/60 by 17" and had an indicated top speed of 150 but then I changed to a 120/70 by 17" which has slightly higher tyres withthe indicated top speed went down to about 145.

This was done on a private test track and in no way did this private test track resemble the M6 on a quiet morning

Aparently when Performance Bikes magazine tested the Sigma Sport bicycle speedo that has the wheel circumferance input into it by the user they found it to be half a MPH out at 180 MPH.
Not too shabby I thought.