speedo changes with tyre size?

Submitted: Thursday, May 01, 2014 at 09:49
ThreadID: 107558 Views:2628 Replies:8 FollowUps:7
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Hi all, recently bought a Navara kitted out with 'sandgroper" upgrade ( big off road tyre/wheels, a 2" lift, B. bar + towbar, etc...). As the fancy wheels are expensive even to look at, I have taken off the 7" and put on 6" tyre/rim on steel rim.- speedo now over by about 12 kmh . Nissan say there's nothing they can do ( part of drive train) and its still within Aus standard accuracy!. this seems odd as I dont think the original Navara wheels ( new model D22) wouldn't have been 7".
any thoughts on this ?
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Reply By: FatGaz - Thursday, May 01, 2014 at 12:23

Thursday, May 01, 2014 at 12:23
Hi Mike,

Any change in the tyre circumference will alter the speedo accuracy.
If you post the details of the Original tyres Vs. the current tyres you will be able to calculate the difference.

Use this link to calculate the speedo difference.

There should be a tyre placard on the door that says what the acceptable tyre sizes for the vehicle are. Different states have different rules as to how much you can go over/under the recommended.

Sounds like you have the wrong sized tyre for your vehicle.
AnswerID: 531572

Follow Up By: John and Regina M - Thursday, May 01, 2014 at 12:42

Thursday, May 01, 2014 at 12:42
As above. Careful as you may now be illegal. And it is your choice if you stay that way of course.

However, there will be insurance and registration consequences, almost certainly insurance void if you are in an accident.

And the men in blue will be attracted to you as will inspectors.

And if you have active stability control, I am led to believe ANY mod to tyres, rims, suspension is NOT allowed, however a call to your local RTA may be advantageous.
FollowupID: 814606

Follow Up By: Member - mike g2 - Thursday, May 01, 2014 at 13:58

Thursday, May 01, 2014 at 13:58
thanks guys, will check for correct size, legality, placard etc..
original upgrade kit must have been legal otherwise Nissan couldn't sell it. removed the 7" off roaders because there worth quite a few $ea
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Follow Up By: Zippo - Thursday, May 01, 2014 at 19:04

Thursday, May 01, 2014 at 19:04
That calculator is ummmm - misleading at best. I punched in the size of my road tyres (205/60-16) and it gave a circumference of 2049.58mm. Crikey, what precision.

The actual rolling circumference (which is what you need for calculating speedo/odo figures) is NOT a "one-size-fits-all" for a particular tyre size as such calculators would have you believe. In my case (size above) the rolling circumference as quoted to me by Bridgestone:

"Rolling Circumference in 215/60R16 for these two patterns are as follows:
T001 – 1980mm
EL64 – 1988mm."

and Yokohama:

"The rolling circumference is 2022mm".

What I presume the calculator is giving is the physical circumference, not the rolling circumference - useful only if you are making tyre warmers.

As a footnote, the current Australian Standard allows speedo error at manufacture to be -0%/+10%+4kph.
FollowupID: 814632

Follow Up By: Member - johnat - Thursday, May 01, 2014 at 19:59

Thursday, May 01, 2014 at 19:59
And ... to add to the confusion, the rolling circumference is dependent on the wheel diameter, the profile of the tyre, and the tyre pressure as well as the load on the vehicle. If the OP sticks a stack of bales of hay in the tub, the rolling radius of the tyre (and hence the circumference) will be altered.

In addition, if the diameter of the tyre increases, either because the profile increases over the original (eg by fitting a 75% profile tyre when original was a 70%), or by overinflation (not as great), the indicated speed will be lower than actual - which is technically illegal.
FollowupID: 814642

Follow Up By: Zippo - Thursday, May 01, 2014 at 21:06

Thursday, May 01, 2014 at 21:06
Rolling circumference is dependent on the wheel diameter, but not in the simplistic way many believe. I have seen people measure the overall diameter, the axle height above ground and probably other ways to derive the RC. None are correct.

The RC is effectively the outer circumference of the tread belt which lies beneath the actual tread pattern. The simplest way to understand it is to consider the "caterpillar track" - that is what that belt does as it deforms to contact the road surface.

The notion that tread wear affects RC (in other than a second order way) is also misplaced. Picture a tracked vehicle with say 50mm blocks attached to each track segment. The RC would still be the same.

In the same way, tyre pressure only has a second order effect on RC. Unless it alters the geometry of the tyre dramatically the tread belt still has the same circumference.
FollowupID: 814653

Follow Up By: olcoolone - Friday, May 02, 2014 at 10:32

Friday, May 02, 2014 at 10:32
Jeeeez didn't know 4 wheel driving had so much calculus, or is it geometry due to the change of shape...... think I'll stick to my day job in quantum physics.
FollowupID: 814669

Follow Up By: The Bantam - Friday, May 02, 2014 at 18:23

Friday, May 02, 2014 at 18:23
If ya want to get picky rollong circumference also varies with speed...the faster the taller the tyre.
It also varies with torque and traction...the more torque applied th the tyre the shorter it will appear.

This is why neither the standards nor the law discuss rolling anything.
The only thing you can rely upon with any accuracy is measured with the tyre off the ground in free air.

All that said you do have to do something about the speedo accuracy.

The on line calculators are notoriously inaccurate at times....if you want reliable figures go to the manufacturers specs or the standards manual...every tyre shop should have a standards manual.

FollowupID: 814733

Reply By: Allan B (Member, SunCoast) - Thursday, May 01, 2014 at 13:51

Thursday, May 01, 2014 at 13:51
Hi Mike,
If you are running non-standard sized tyres and wish to have your speedo and odometer read correctly, devices are available to correct the speedo signal. Google "speedo corrector" to view some that run from $80 t0 $200.
Jaycar have a good kit for about $55 but you do need to assemble the electronic board yourself. See here.
There is a YouTube video on assembling, installing and calibrating the Jaycar kit here. A bit nerdy but informative.

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AnswerID: 531574

Reply By: Michael ( Moss Vale NSW) - Thursday, May 01, 2014 at 14:55

Thursday, May 01, 2014 at 14:55
Jaycar have an electronic gadget for speedo correction. Ring them for details or look on their website. Michael
Patrol 4.2TDi 2003

Retired 2016 and now Out and About!

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AnswerID: 531576

Reply By: Member - Andrew L (WA) - Thursday, May 01, 2014 at 15:55

Thursday, May 01, 2014 at 15:55
I use a Yellr Box speedo correction unit in my 100 series http://www.yellr.com/index.htm

Plug-n-play, works great.
AnswerID: 531580

Reply By: Alloy c/t - Thursday, May 01, 2014 at 16:42

Thursday, May 01, 2014 at 16:42
You don't state if the vehicle is brand new or 2nd hand ? Dealers have been known to 'dress up' and give even slow moving 'new' stock a going over , fancy wheels , chunky tires [instead of the factory originals] and give it a "butch" name and a sale soon follows , that said , have a look at Redbook with your vehicles build date / you should be able to find out the original wheel and tire size etc.& "if" the vehicle is original 'sandgroper' upgrade ….. nice photo.
AnswerID: 531584

Reply By: wizzer73 - Thursday, May 01, 2014 at 18:19

Thursday, May 01, 2014 at 18:19
Maybe just a cheap gps unit and perm mount it?

At least you wont get speeding fine if the speedo is reading over actual speed :)

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Reply By: Robin Miller - Thursday, May 01, 2014 at 18:47

Thursday, May 01, 2014 at 18:47
7" or 6 " is irrelevant MG.

Its the rolling diameter of the tyre that counts , you can get narrow tall tyres or fat low ones.

I guess what you are saying is that they fitted smaller diameter tyres, what size , and how do they compare to original.

If your speedo reads 100 and your GPS says less than 94 you can take it back to Nissan as long as it has the Nissan size fitted.

Had that on one Patrol , and they gave me another speedo which did the same thing and then I found a little internal calibration potentiometer and calibrated it myself.

The electronic correction gadjets mentioned above do work , although they have a little delay and don't generally work below about 20kph.

While GPS'es are your friend and some models in Garmin range at least have a Car trip computor feature which has a big speedo display compared to normal little one.

Sometimes easier to buy a heads up GPS based speedo as have a big easy to read display.

P.S. if you have much smaller tyres than standard then your odometer will clock up a higher mileage reading.

Robin Miller

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Reply By: Member - mike g2 - Saturday, May 03, 2014 at 10:19

Saturday, May 03, 2014 at 10:19
Thanks to all the tech gurus on tyres/rims and speed. I checked out the wheels I put on, they are legal and speedo being a little over actual is a likely result of wheel/tyre size change. also, my idea of 6" or 7" may be incorrect as its rim internal width I am told that this measurement applies to! I am also told going down a size is generally ok- but need to check aust standards and veh placard etc as you guys suggested.
vehicle sold as 'new', upgraded by dealer with "sandgroper package" . its only 2mths old.
I was mainly aiming at going to a road tyre and saving on fuel with a long road trip planned in Aug-possibly around the big block-at least 20K kms, not doing much off road. therefore not needing those offroad tyres.
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