Bigger tyres , by how much does it effect speedo accuracy ?

Hi Guys

I got 33s on my car . I know the speedo isn't as accurate. What are average kmph it is out by ?

I'm in the middle of resetting my dawes and needle valve

thanks

Gday mate. With a GQ going to 33" inch tyres, you will be travelling at 100km/h when your Speedo says 95. But as all Speedos are slightly different you may want to double check with a GPS.
Also there are lots of websites with tyre size calculators, just Google away

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Originally Posted by taslucas

Gday mate. With a GQ going to 33" inch tyres, you will be travelling at 100km/h when your Speedo says 95. But as all Speedos are slightly different you may want to double check with a GPS.
Also there are lots of websites with tyre size calculators, just Google away

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It varies so much... my old patrol was 100% accurate with 33"s according to GPS. Same GPS on my new one and with the same 33"s on when my speedo says 100km/h I am actually travelling 110km/h

My Speedo is currently 8k above actual road speed, so 33's will bring mine up smack on..

As mentioned above speedo readings vary from car to car, as a rule (ADRs) a speedo on a standard vehicle should under read, i.e. when speedo says 100 kph you are actually doing say 97.

Use a GPS to check your actual to speedo reading. On my GU IV I replaced the speedo cable cog at the gearbox from a 16 tooth to a 15 tooth, the speedo now reads spot on with the three different GPS units I have checked it against.

This thread is worth a read

What I posted was the ratio change as stated on tyre size calculators.

But yeah if you do 100 on the Speedo in ten different standard vehicles you'll get ten different actual road speeds

would it be safe to just drive under 5kmph ?

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Originally Posted by Limmy

would it be safe to just drive under 5kmph ?

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Perfect. That's at a hundred k's.

thanks all for advice! just another thought.
how much off would it be with 35inch ?

I went from 31's to 33's on the GU and now the speedo is spot on against at least 2 gps'

used to read slower before....

My speedo is spot on with 33's on but with the 265's its out by 5 kmh at 60kmh and 10kmh at a 100kmh

i just downloaded an app for android mobile called SPEEDVIEW . uses GPS for speedometer . I will test it later

Larger tyres will also affect acceleration, braking and fuel usage

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Originally Posted by threedogs

Larger tyres will also affect acceleration, braking and fuel usage

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By how much?

Bigger rolling diameter will affect your take off and that depends on heaps of thing motor, diff ratios etc.
Heavier rolling diameter will affect your braking ability too,
You should know this cause if you put 31 back on, your GQ would fly

Yeah I know was just having a laugh..... huge difference between my 265s and the 33s.

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Originally Posted by Winnie

Yeah I know was just having a laugh..... huge difference between my 265s and the 33s.

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Must be a diesel thing coz I didn't notice the difference in my petty........ But then again that's probably due to me driving Miss Daisy :D

I don't know what it is really, the 33s that came on the car were bugger all different to my 265s but my Silverstones really suck the power from the car.

This should help

http://www.csgnetwork.com/speedocalibcalc.html

Can the speedos be calibrated?

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Originally Posted by Limmy

i just downloaded an app for android mobile called SPEEDVIEW . uses GPS for speedometer . I will test it later

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I tried this app and its the same as my GPS, so got confirmation that the speedo is out, even though I'm running standard size wheels and tires. Did notice that it has a 1/4 mile time as well. Interesting to see who has the slowest patrol.

Going from 31s to 33s. The speedo reads 94 when im doing 100. So 6%

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Originally Posted by Dales300exc

Going from 31s to 33s. The speedo reads 94 when im doing 100. So 6%

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What did it read with 31's

With 33s on my gq my speedo is spot on with 1km/h

It read pretty darn close to 100 at 100.

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Originally Posted by Dales300exc

It read pretty darn close to 100 at 100.

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I have found this is pretty much infinitely variable as every patrol seems to be different. A few examples- my gu was correct with 285/75/16 tyres, my sisters gq is out 5km with 31/10.5/15 tyres and her boyfriends gu was correct with 35/12.5/15s fitted.

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Originally Posted by nissannewby

I have found this is pretty much infinitely variable as every patrol seems to be different. A few examples- my gu was correct with 285/75/16 tyres, my sisters gq is out 5km with 31/10.5/15 tyres and her boyfriends gu was correct with 35/12.5/15s fitted.

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Same. Mine has come out spot on with 33's and 4.1diffs.. But others are way over.. Or under.
If I ever change to 4.3 diffs it'll revert back to close to 6k under like my 31's were

I went from 31's to 33's on my GQ and the speedo is actually on the money now .. checked with 3 or 4 different GPS's...just to be sure

All depends on how far and which way your speedo is out in the first place..

:)