Calculate with the following numbers:

1. One litre of petrol costs NZ$ 2.00 at present, and your car will most likely require about 8 litres per 100km if it's an average rental car, like a Toyota Corolla 1.8 for example.

2. Your average travel speed in NZ is usually 60km/h – meaning, that's as far as you can get in one hour without breaking any speed records. If you reduce your driving to about 5 hours a day, that means you should allow for an overnight stop every 300km, and it would be just enough to tour NZ's North island extensively within a two-week timeframe.

3. A rental car will normally cost you around NZ$ 60 to 120 per day, depending on choice.

4. A motel room runs between NZ$ 70 and NZ$ 160 a night, but they often have kitchens, so you can save on eating out.

5. Eating a proper three-course meal with alcoholic drinks in a nice restaurant will run you as high as NZ$ 50 to 80 per person. If that's too much for you, there are ways of eating cheaper, especially the corner stores, called "Dairies" here, are a very cheap source of pretty yum food which is more filling and less expensive than fast food chain food. We're talking pies and sausage rolls, sandwiches now.

6. You can find accommodation for as little as NZ$ 25 a night per person in backpackers' hostels, of which there are heaps down here.

I hope these numbers helped you a little.
Have fun on your trip!
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