How accurate is your speedo ?

[Wats_in_a_name]

Mine reads about 3 - 4 km/h higher than my actual speed, so when it says 100km/h I am doing 96 - 97. This suits me as this gives me a buffer if there are any speed cameras (ie if my speedo says I am doing the speed limit I have about 3 - 4 km/h to spare and shouldn't get pinned). With GPS I have heard they are only accurate to within 50m, so there could be a variance between a speed reading and actual speed.

 

[PIR4TE]

As I understand it, speedo inaccuracy (in a VE anyway) stems from a basic assumption that tyre circumference never changes much. Which is true for the most part except the actual way the calculation of circumference is performed i.e. Rolling Diameter used by the ECU in its speed calculation (also called the radius method) which is twice the distance from the ground to the centre of the axle.

However the ECU's method doesn't take tyre deflection / ballooning caused by change to vehicle weight, tyre inflation, acceleration / deceleration and overall speed into account. Anyone with a high powered RC car, off-road / sand tyres can appreciate the ride height and ballooning due to centrifugal forces and weight (mainly) first-hand.

This difference in the calculation of circumference using radius (distance to ground) or diameter as opposed to measuring actual rollout - actual linear distance travelled in one tyre revolution. This difference (error) is less with a low profile tyre with less deflection, and between tyre brands, but it presents an unknown variable value affecting the ECU's calculation (calc.vss) of vehicle speed and that reported (SAE.VSS) by the wheel sensors. Hence the in-built margin for error in {factor}.

vss = rpm x tire_diameter / (trans_ratio x axle_ratio x factor)

CALC.VSS uses:
vss is in MPH or km/h,
rpm is SAE.RPM (engine speed) or GM.TIS (trans input speed),
tire_diameter is in inches or mm,
trans_ratio is GM.TOS (trans output speed for A6),
axle_ratio is hard coded (e.g. 3.29),
factor is 336.135 (imperial) or 5305.16 (metric).

This calculated vss can be compared against SAE.VSS to see if the speedo is calibrated and adjusted correctly:

# ==============================================================================
# File details
# ------------
#
# This section defines various details about the file format.

*FILE

#Parameter  Value            Description
#---------- ---------------- ---------------------------------------------------
VERSION     7.5.5            File version
DECSEP      .                Decimal separator used in this file



# ==============================================================================
# Units
# -------------------
# See sae_generic.txt for more information on the *UNITS section

*UNITS

#Code     System     Abbr     Description
#-------- ---------- -------- -------------------------------------------------------------

None      None       ""       "No units"


# ==============================================================================
# Add slot definitions here
# --------------------------------
# See sae_generic.txt for more information on "SLOT" formats
#
#Units             Low          High      Fmt  Expression
#------------ ------------- ------------- ---- --------------------------------------------------------------
*CLC-00-060
mph                0.0          175.0      .0  "{GM.TOS} * 27.01 / (3.29 * 336.135)"
kph                0.0          280.0      .0  "{GM.TOS} * 686 / (3.29 * 5305.16)"


# ==============================================================================
*PRN - Parameter Reference Numbers
# --------------------------------
# See sae_generic.txt for more information on the *PRN section
#
#Code                      PRN  SLOT         Units            System           Description
#------------------------- ---- ------------ ---------------- ---------------- ------------------------------------------
CALC.VSS                   F060 CLC-00-060   "mph,kph"        Calculated       "Calculated VSS"

 

[rory]

My patrol with 33's (305/65/17) is about 7% over according to GPS. An online calculator states that it should only be 3% over... It always seemed to go faster than other cars on the highway but I have gone past speed cameras on the limit with the 31's on and not been flashed. I normally sit at 75kmh on 80kmh roads with the 33's.

 

[Benzito]

With GPS I have heard they are only accurate to within 50m, so there could be a variance between a speed reading and actual speed.

Domestic GPS units in optimal conditions(perfect satellite coverage, no nearby buildings or cliffs to reflect signals, etc) are accurate to 3m. GPS can actually be accurate to within a centimetre or so, but for basic civilian usage they actually restrict the accuracy to prevent the units being used for home-made weapon systems. I guess they are worried some terrorists will buy a navman and build a cruise missile?

edit: I am going totally off topic, but I remember seeing a doco on the construction of some massive building in Dubai. I don't know which one, cause they seem to keep throwing up a new 'worlds tallest building' every year. But anyway, they actually used super accurate GPS positioning to ensure the building was staying perfectly straight as they went upwards. Obviously being out by 1 degree makes a big difference at the top when a building is hundreds of metres tall. Anyways, that gives you an idea of how accurate GPS can be.

 

[Hangman]

The VS is out by 3 k's at 100 according to the digital speedo on the speed alert you can bring up. Dunno how accurate it is though.

The VT I can sit on the limit as indicated through speed cameras and not get done so it's accurate enough.

The LJ does 70mph standing still, according to the speedo:

 

[maginoodle]

at 113 on the speedo my crewy is doing 110 according to my tomtom.

 

[Group C]

My VZ is 100% accurate according to my GPS, after market 18's

 

[Garth]

acording to my gps, when doing 110 on the gps my speedo says im doing 120,
doing 60km gps says 68, so basicaly i dont trust either, ill do the speed as per my speedo, may be slower then other cars, but hey, at-lest i wont get a ticket

my patrol would depend on what day of the week it was, one day it was acurate, next day it wasnt, i swear it had a mind of it's own.