How accurate is the speedo

[Sunny43.5]

I recently purchased a new gadget which is a HUD for our vans , strange thing is I get three different speed readings . The unit connects direct to the OBD connector and gives out speed / tacho/ fuel consumption and will warn if you holding the revs too long in each gear plus you can select a temperature warning as well . But the GPS tells me I am doing 100kmh the speedo is showing 104 kmh and the HUD shows 98kmh . I wonder why the ODB is different from the speedo . The photo does not do it justice it is much clearer to the eye than the camera is

[tigger73]

It will all depend on what tyre size you are running and how much tread you have on your tyres (i.e. rolling diameter). Typically the ECU has an uncorrected speed (which is what you're reading over the OBD port). This then has a "correction factor" applied to it to account for you fitting wheels/tyres that are 1 x oversize for your vehicle. This "corrected" value is then displayed on the speedo/MFD.

This means that the vehicle manufacturer is "safe" with the speed that is shown. You shouldn't be travelling faster than the indicated speed - it's a legal requirement that you speedo is not under speed.

[Guest001]

The legal settings for speedos in Australia if I remember correctly is up to +10% +- 4% or very similar It may not read less than the actual speed.

My Polo reads 107 at 100 The Passat 106 The reading from the computer in the Secret menu confirms this.

In the Polo was checked against a TOM TOM and VCDS software. We were unable to correct it better.

My previous ride a 100 ser Landcruiser was bang on at 100kph according to the GPS

[vdubmotorworx]

All cars ive tested at 100kph on dash usually read around 95-98 on VCDS ....quite normal now...as above its the 10% +4 law...different wheel options make a big difference.....I tested a jetta with factory 16" rims and was 95 ...tested mine with factory option 18" and was 99..issues happen when people fit bigger wheels than factory size like 20" and then the speedo can read the wrong amount over...VDUB...

[Guest001]

Here is the ADR rule


The current Australian Design Rules require speedos to be calibrated when the vehicle is unladen, fitted with normal tyres inflated to full pressure with an allowance for tyre heating:
"Unladen vehicle" means the vehicle in running order, complete with fuel, coolant, lubricant, tools and a spare wheel (if provided as standard equipment by the vehicle manufacturer), carrying a driver weighing 75 kg, but no driver's mate, optional accessories or load."Tyres normally fitted" means the type or types of tyre provided by the manufacturer on the vehicle type in question; snow tyres shall not be regarded as tyres normally fitted;"Normal running pressure" means the cold inflation pressure specified by the vehicle manufacturer increased by 0.2 bar.
The current ADR prohibits any under-reading:
"5.3. The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified in paragraph 5.2.5. above, there shall be the following relationship between the speed displayed (V1 ) and the true speed (V2).0 ≤ (V1 - V2) ≤ 0.1 V2 + 4 km/h."
This formula means that the vehicle's actual (true) speed must not be greater than the displayed speed. (Displayed speed minus true speed must be greater than or equal to 0, and less than or equal to 4kmh plus 10% of true speed). This means that if your vehicle's actual speed is 100kmh, the displayed speed is permitted to be anywhere between 100kmh and 114kmh.

Prior to July 2006 the ADR allowed the speedo read ±10% of true speed. This means that cars sold new prior to 1 July 2006 could comply with the ADR even if the speedo under-read by 10%. Despite this being theoretically possible, due to the testing procedures and the reasons stated below it was unlikely to occur in practice.

[Rebuild]

I,ve gone through those trailer mounted read speed devices, and for around 60km/h speedo reading, the roadside device reads around 3-5 km/h less.
Standard 17" rims and tyres on 2004

[Sunny43.5]

My main puzzle was the difference between the ODB reading and the cars reading I would have thought they should be the same , I know GPS is pretty accurate and I expect the car to be lower which it is but can't figure out why the two onboard readings are different . Oh and by the way I don't think the rozzers in NSW make any allowances even though ADR,s tell a different story .{not that I have been booked } in enemy territory .

[Guest001]

I,ve gone through those trailer mounted read speed devices, and for around 60km/h speedo reading, the roadside device reads around 3-5 km/h less.
Standard 17" rims and tyres on 2004

I wouldn't bet the house on the accuracy of one of these They are slow to change and are only there to wake you up a bit so don't need to be absolutely spot on I guess

[Guest001]

My main puzzle was the difference between the ODB reading and the cars reading I would have thought they should be the same , I know GPS is pretty accurate and I expect the car to be lower which it is but can't figure out why the two onboard readings are different . Oh and by the way I don't think the rozzers in NSW make any allowances even though ADR,s tell a different story .{not that I have been booked } in enemy territory .

It depends where the OBD is getting its reading from I guess.

You can see it when you use a VCDS cable and can get an accurate reading in the secret menu which correlates fairly closely with a GPS (Not the RNS510)

On reading your OP the answer is that the speedo is as shown in my prev post is designed to be optimistic by law. Depending on the manufacturer it will vary but must conform within the posted limits A GPS will always be a few seconds late as it digests the data as you are moving.
The OBD is probably getting it from the ECU or wherever the Secret menu gets it from as well.

Someone may explain it better than me

[Sunny43.5]

Thanks for all your answers guys but "secret menu ' in the words of Pauline Hanson Please explain ??

[Guest001]

Thanks for all your answers guys but "secret menu ' in the words of Pauline Hanson Please explain ??

If you have a RNS510 Navigation unit you can access this and it gives a whole lot of readings direct from the Canbus.
If you don't have one close your ears LOL

[wai]

Find a long straight level stretch of road and use a GPS unit to verify the accuracy of the speedometer.

Any device that measures the speed by referencing the rate at which the wheels rotate will not give you an accurate reading simply because you will have to get to the ideal settings on a whole lot of inputs and by the time you account for the tolerances of each input, you result is not going to be that accurate. If you like use two GPS units to verify the accuracy of the speedometer.

On my Caddy, the speedometer (both digital and analogue) indicate a speed that is 5% higher than true. This is using two GPS units for reference (ANS-610 and AlerteGPS G200). This is also confirmed when I have driven under the speed check gantry just south of Albury on the Hume Hwy. I have also been tailed by VIC and NSW highway patrol vehicles for many km and have had no issues using cruise control. Victoria has a tolerance of 3 km/h on speed.

On roadside trailer display units, I would not trust them. I was once in a traffic jamb with a police vehicle alongside in the adjacent lane when there was a roar of laughter from the police vehicle. We were all at a standstill with no vehicles allowed in the opposite direction when the roadside unit flashed a reading of 90 km/h! We were at a standstill in a 60 km/h roadwork zone!

[The_Hawk]

Our Multi reads about 105/106 km/h when you're really going 100 km/h.

I don't have my laptop with me today at work but I spent a whole bunch of time calculating the differences that would theoretically be possible.

I grant that for a system measuring rotations that in order to not read over they have to assume the largest possible wheel and tyre combo to make sure it *never* reads over. At the time I dug up every stock wheel/tyre option from the factory for the Golf and calculated the differences. From memory there was ~2% difference.

What that means is that the speedo *could* be set to read dead accurate for that largest option then in the worst case you would be 2% under with the smallest wheel/tyre combo.

Tyre wear will also make a difference, but by my calculations from new to 10mm wear there is ~3.5% difference.

IMHO the speedo should not be out by more than that 2% differential in wheel options, yes it will get worse over time, but when we are starting with ~5-6% error and wearing through another 3.5% that can make a huge difference.


On a side note, the odometer appears to be dead accurate. The average speed and I assume fuel consumption rely on this accuracy too. To test this theory, find a long stretch of flat road, set your cruise control to 106 km/h then reset your MFD and watch the newly calculated average speed come up... it's 100km/h. So it seems the car knows how fast it's really going. Double check this against your GPS and let me know how you go.

[Guest001]

Our Multi reads about 105/106 km/h when you're really going 100 km/h.

I don't have my laptop with me today at work but I spent a whole bunch of time calculating the differences that would theoretically be possible.

I grant that for a system measuring rotations that in order to not read over they have to assume the largest possible wheel and tyre combo to make sure it *never* reads over. At the time I dug up every stock wheel/tyre option from the factory for the Golf and calculated the differences. From memory there was ~2% difference.

What that means is that the speedo *could* be set to read dead accurate for that largest option then in the worst case you would be 2% under with the smallest wheel/tyre combo.

Tyre wear will also make a difference, but by my calculations from new to 10mm wear there is ~3.5% difference.

IMHO the speedo should not be out by more than that 2% differential in wheel options, yes it will get worse over time, but when we are starting with ~5-6% error and wearing through another 3.5% that can make a huge difference.


On a side note, the odometer appears to be dead accurate. The average speed and I assume fuel consumption rely on this accuracy too. To test this theory, find a long stretch of flat road, set your cruise control to 106 km/h then reset your MFD and watch the newly calculated average speed come up... it's 100km/h. So it seems the car knows how fast it's really going. Double check this against your GPS and let me know how you go.

Save your laptop look here Custom rims, wheel tire packages for your ride - RIMSnTIRES.com

Put your stock tyre size in one side and work out the difference on the other.

For instance to bring the Polo back to dead correct would take a tyre 40mm larger diameter which is illegal.

[Sunny43.5]

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 .