How accurate is the speedo

[Guest001]

Geez guys you certainly have this issue boiling away , laptops roadside signs wow . I also don't place any accuracy on those mounted signs as there were several of them on the Bruce Highway south of Gympie and one flashed me for Tailgating WTF there was not another car for bloody miles . The whole idea of this post was to see if it was an anomaly in our van because as I said the speedo and the OBD reading are the two that are different and I was curious why the two supposed readings from the on board computer would be different to each other no need to work out tyres and wheels dimensions as I usually drive with my main speed from the GPS as the speedo is always lower , the GPS nags me when I stray over by more than 5 % any way I can change it to be exact , but I like to stay real close to the allowable tolerance . I hate driving along highways and motorways and you come up to groups of cars all driving fractions below the limit and bunching up , it turns into a dodgem car drive as people change lanes . I just like to get past them and try to keep a clear run in front too many ratbags in what I call ratpacks .

The reason you come up on the "ratpacks" is probably because, you driving on your GPS reading are actually doing 100kph.

The ratpack is probably driving on their speedos and actually only doing in the mid to low 90's.

I get that all the time as I trundle along sitting on 106 on the dial (actual 100) and pass lots of them who look at me funny LOL

[Ben J]

Re: How accurate is the speedo

Jist drive about 10km/h faster than most, and you're probably on the limit.

Sent from my GT-I9305T using Tapatalk 2

[wai]

The whole idea of this post was to see if it was an anomaly in our van because as I said the speedo and the OBD reading are the two that are different and I was curious why the two supposed readings from the on board computer would be different to each other.

Ok, the digital readout and the analogue gauge readout will never ever be the same, simply because of the way the signals are displayed.

The digital readout direct from the vehicle computer will not necessarily be the same as the display on a digital speedometer because in most systems, this is where the manufacturer tweaks the display to be within the ADR.

The analogue readout will vary based on how the display is created. If it is a speedometer cable from the transmission, this normally has a magnetic coupling to the needle which is biased against a hair spring. These are notoriously inaccurate and the variation can be different throughout the range.

Other analogue displays are essentially an electrical meter where there is an electronic pickup on the transmission. The electrical signal is passed to the computer and this then outputs either a voltage or current which the gauge then displays as speed. These can be more accurate. If your gauges flick to maximum when you turn the ignition on, chances are it is this type of gauge.

While the second type of analogue gauge is more accurate, it will not necessarily match a digital speedometer.

So, there you have it. If you want to check the accuracy, then it is either a speed check gantry or a GPS unit, and here you need to check it at various speeds (40, 50, 60, 70, 80, 90, 100, and 110) as a variation may not necessarily be consistent across the range of speeds.

[Guest001]

Ok, the digital readout and the analogue gauge readout will never ever be the same, simply because of the way the signals are displayed.

The digital readout direct from the vehicle computer will not necessarily be the same as the display on a digital speedometer because in most systems, this is where the manufacturer tweaks the display to be within the ADR.

The analogue readout will vary based on how the display is created. If it is a speedometer cable from the transmission, this normally has a magnetic coupling to the needle which is biased against a hair spring. These are notoriously inaccurate and the variation can be different throughout the range.

Other analogue displays are essentially an electrical meter where there is an electronic pickup on the transmission. The electrical signal is passed to the computer and this then outputs either a voltage or current which the gauge then displays as speed. These can be more accurate. If your gauges flick to maximum when you turn the ignition on, chances are it is this type of gauge.

While the second type of analogue gauge is more accurate, it will not necessarily match a digital speedometer.

So, there you have it. If you want to check the accuracy, then it is either a speed check gantry or a GPS unit, and here you need to check it at various speeds (40, 50, 60, 70, 80, 90, 100, and 110) as a variation may not necessarily be consistent across the range of speeds.

Both mine are linear. That is the faster you go the greater the optimism

Eg Polo 4k fast at 70. 7K fast at 100 10 k fast at 120.

Passat is similar but 6k fast at 100

Checked the Passat against the Canbus reading which is identical to what my Tomtom reads as well

I wouldn't take prev posters advice and do 110 and not worry as the tolerance is less than that in QLD. I got a ticket for 4k over at 70.

[DonJuan]

How accurate is the speedo

My speedo reads 116/117 when my Polar FIS+ reads 110km/h. The 110km/h was also correct according to the highway overhead speed checker in Victoria. I now go by my Polar unit as that's reading the ECU.

[Transporter]

My speedo reads 116/117 when my Polar FIS+ reads 110km/h. The 110km/h was also correct according to the highway overhead speed checker in Victoria. I now go by my Polar unit as that's reading the ECU.

Nothing wrong with that and most people prefer it that way rather than paying the speeding fines.

[DonJuan]

How accurate is the speedo

Nothing wrong with that and most people prefer it that way rather than paying the speeding fines.

Agreed. But at least now I can have a discussion with any police officer that pulls me over and can advise him of the exact speed I was doing according to Victorian Government installed speed guides.