It's an easy rule of thumb only. If you want the definitive answer then check the service manual for the specific vehicle/engine combo.

simonjalsconz: Have you noticed that UK sites always talk about belt replacement at 100k miles and in NZ we talk about belt replacement at 100k. I am highly suspicious.

I'd say it's just a nice round number that pretty well guarantees the belt won't break if you replace it at those intervals. A lot of these sorts of recommendations are just arbitrary and for liability reasons have significant fat built into them.

100,000 miles or kms is for most owners a long period and it doesn't really matter whether it's miles or Km.

I'd also suggest that rather than spend time converting miles to kms when they edited the manual for a metric or imperial market it was quicker and easier to just change the unit of measure.

RunningMan:

It's an easy rule of thumb only. If you want the definitive answer then check the service manual for the specific vehicle/engine combo.

I've noticed this with servicing intervals too - quite often the NZ service interval is much shorter than the equivalent UK car.  Maybe it's in anticipation of kiwis keeping cars for longer - they seem to depreciate much more quickly in the UK.  The other interpretation is it's NZ importers just ripping us off in anticipated servicing income.  The UK used to be known as "treasure island" by the motor trade because of the significant and arbitrary price differentials between UK and europe, to the extent that it was often cheaper to special-order a UK spec car from a EU dealer and then import to UK, than just buy in the UK

Maybe NZ is the new treasure island/archipelago?

This is largely an academic question and the technical answer to your question is "marketing".

Timing belt change limits are normally paired with a calendar time limit which the majority of vehicles get to before the km's are reached, so you may well find that the timing belts are supposed to be done at the same "time" regardless of if it is the UK or NZ.

It is not only km's travelled that causes a belt to deteriorate, time causes the rubber and other materials to perish and degrade. Other things also cause wear on belt such as fluid leaks, water or dust getting up around the engine and driving style. KM's are a rough and unreliable measure because it isn't the rotation of the wheels that causes it to wear. For instance sitting stationary in traffic still causes wear on the belt but the odometer is not measuring this. Also driving in 2nd or 3rd gear also causes more wear than driving the same distance in top gear (or coasting).

In this vain, this is why some modern cars oil change periods are calculated on the fly by the computer and isn't a hard km limit any more - the computer monitors the work and driving cycles done by the engine and calls for an oil change sooner or later accordingly.

shk292: I've noticed this with servicing intervals too - quite often the NZ service interval is much shorter than the equivalent UK car. 

I know with diesels there was a difference because Marsden Pt produced poorer quality diesel than what was legally allowed in Europe, so diesels in N.Z. had higher emissions and fouled their oil quicker than the same car running on European fuel.