Tyres affect on speedo

Hey guys,

Hope this is in the right place! I've just driven my new car back from QLD and have found that my speedo is out a bit, through trial and error and following other car's speeds I've got a good idea, just wondering if anyone can shed some light on different size tyres affecting the speedo as a constant or percentage... let me make that a bit clearer.

When I drive at 100kph my speedo reads 90, so 10kph difference, or 10%
When I drive at 60kph will my speedo read 50 (10kph) or 54 (10%)

I'm assuming 10% is the way to go but I have nothing to base that on.

Thanks all.

The tyre effect in % will be linear IE 10 at 100, 5 at 50, 1.7 at 17 etc
BUT
that doesn't mean your Speedo indication error IE the speedo itself is linear as it may have differing errors % wise at differing speeds unrelated to tyres or diff ratios

Cheers mate. Appreciate it.

You can install one of these speedo correctors.

https://www.youtube.com/watch?v=bMs7w72QTVM

For hwy driving I would rely on a GPS of some description, The Aust Gov gives car companies a 10% error in speedo readings
The Victorian Police allow you only 5% if that, do the math lol
I run 285s and find the speedo near enough to correct,
when I fit my 315s its out same as when I had 265s on it
For hwy driving @ 110kph Ill set the CC to 105kph Im in no rush
and my ETA in marginal over 2.5 hrs

Quote:

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Originally Posted by threedogs

For hwy driving I would rely on a GPS of some description, The Aust Gov gives car companies a 10% error in speedo readings
The Victorian Police allow you only 5% if that, do the math lol
I run 285s and find the speedo near enough to correct,
when I fit my 315s its out same as when I had 265s on it
For hwy driving @ 110kph Ill set the CC to 105kph Im in no rush
and my ETA in marginal over 2.5 hrs

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The ADRs allow the speedo to over read up to 10% error, but the speedo must never under read, so the speedo will nearly always indicate a higher speed that actual, so if you sit on the speed limit according to the speedo you will never by speeding (assuming standard size tyres fitted).
GPS is the best way to check, but there is no calibration traceability so not really suitable for evidence in court.
To answer the original question, tyre size changes the ratio, so it tyre size changes indication as a percentage.
Graham

Quote:

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Originally Posted by GeeYou8

snip...
The ADRs allow the speedo to over read up to 10% error, but the speedo must never under read, so the speedo will nearly always indicate a higher speed that actual, so if you sit on the speed limit according to the speedo you will never by speeding (assuming standard size tyres fitted).

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Yes and No...
The ADR (ADR 18) was amended in 2006 effective from Jul of that year
The requirement you have mentioned is only for vehicles manufactured after 2006.
Vehicles manuf prior to 2006 are allowed a Speedo error of +/- 10% of the actual speed when travelling above 40KPH

For our beloved Patrols that means...
All Y60's and Y61's up to GU series III and the earlier series IV can have Speedo errors up to +/- 10%
IE at 100 KPH actual, the indication can be 90 to 110 KPH

Late series IV Y61 and up and Y62 that are post ADR 18 amendment and must only have an error faster than actual
IE at 100 KPH actual, the indication can be 100 to 114 KPH

Thanks guys, loving the assistance on here!

Does anyone know someone that's fitted the corrector or similar ?

Jay77

i fitted a Marks Adapter speedo correction unit not long after i bought the car (previous owner gave it to me on test drive when we got to post office when picking up his mail) :)

was simple to install, hardest part was hooking up the power feed. Also had to drill a hole in trans tunnel, grommet came with kit & i sealed up with sixaflex. The plugs plug straight into existing nissan harness.Has 2 settings for different size tyres.

Calibrated against a Hema HN7, which was a simple process.

I have a 2005 GU IV with 33 tyres on it.