speedo changes with tyre size?

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.
http://www.exploroz.com/Vehicle/Tyres/SizeCalc.aspx

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.

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.

Jaycar have an electronic gadget for speedo correction. Ring them for details or look on their website. Michael

I use a Yellr Box speedo correction unit in my 100 series http://www.yellr.com/index.htm

Plug-n-play, works great.

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.

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 :)

wizzer

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.

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.
MG.