March 19, 2026
Briefly
- RNZ passes on a claim by Auckland Transport that Cost of driving 15km in Auckland nearly double that of public transport. Specifically, they say we have “a 15-kilometre single person commute now costing roughly 80 cents per kilometre, which is equal to about $12 for the total trip.” That seems a lot? I won’t claim to be an expert on driving costs, but the nearest petrol station for which I can see data (Mobil on K Rd) is charging $3.099/litre. To get 80c/km you would need to use a bit over a 1/4 litre per km (more precisely, 0.258l/km). This is 25.8 L/100km (for our US readers, about 9 mpg).
- The US celebrated Pi Day this week, and there was a niche popular video describing how to estimate π by coin tossing. Toss a coin until there are more heads than tails. The expected value of the proportion at this point (which is obviously more than 1/2 and at most 1) is π/4, so by repeating this procedure lots and lots of times and averaging the results you can estimate π/4, and thus π. This is not a good way to compute π, but it’s surprising that it works at all (sadly, the explanation for why it works is not very illuminating)
- From newsroom, “Mazda NZ’s Driving Good promotion promises the company will donate five trees to environmental charity Trees That Count and that “over each vehicle’s five-year warranty term, these five trees will not only mitigate any environmental impact from CO2 emissions, but they will significantly contribute to the ecosystems in which they are planted”.” Whether this is true turns out to be sensitive to your interpretation of “mitigate”. There is a sense in which planting any trees provides some mitigation of the environmental impact from CO2 emissions, but you might think Mazda was saying something more like “will absorb as much CO2 as the car emits”. If that’s what you think, you agree with Lawyers for Climate Action, who say it would take 41,000 trees to absorb a comparable amount of CO2.
- StatsNZ said “food prices were up 2.5 percent in January 2026 compared with December 2025. The correct increase was 2.1 percent.” They’ve fixed it, and it didn’t impact the CPI.
Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »
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At first I suspected Auckland Transport did its test trip using a Hummer… but that typically gets 13.5 litres per 100 km.
Are there any stretch Hummer limos operating in Auckland?
2 weeks ago
I wondered if someone had perhaps mixed up 25 km/L and 25 L/100km
2 weeks ago
The per km cost will be based on a total cost of ownership for an average car. The IRD reckons it costs $1.17/km to drive a petrol car a year ago taking everything into account.
Lots of that will be depreciation and you pay a decent chunk of that whether you drive the car or leave it parked up.
2 weeks ago
Thanks for the link, that’s useful. I still think that’s the wrong comparison because a lot of it is depreciation (also, it’s now too high rather than too low).
The same IRD page gives 37c/km as the marginal cost (Tier 2), implying the increase in fuel costs since then is about 40c/km, which isn’t plausible.
2 weeks ago