Hang on a minute...

[Scuba_Steve]

well heres some info I found that might help

What about the odometer?

Inaccurate speedos especially concern drivers of diesel vehicles. They assume an inaccurate speedo also means an inaccurate odometer, which would result in them paying too much in road-user charges. Fortunately, most odometers are accurate and calibrated differently from the speedo. The international rules say an odometer must read within four percent of the distance travelled.

If you're concerned about your odometer's accuracy, don't test it using the speed readout of your GPS. Instead, do several point-to-point tests using the GPS distance readout over straight roads of 5km or more.

but heres something I really was not aware of

Hubodometers are notoriously unreliable and inaccurate. they go on to say It is unconscionable for government to collect some $800m in revenue, relying on mandated distance measurement devices that are notoriously inaccurate and are not required to meet an established technical standard for accuracy. The NZTA has continued to resist any opportunity to test hubodometers for reliability and durability and have instead required owners of vehicles to rely on hubodometer manufacturer warrantees.

[Berries]

They are now doing GPS based electronic RUC as well which seems to be the future.

[Pseudonym]

Just change the back sprocket on ya car…
Do I have to think of everything?