A litre of diesel on an Italian motorway now costs more than €2.70. About half of that is tax — and it is two different taxes that behave in opposite ways. Fuel duty is a fixed amount per litre (67 cents in Italy) and does not move when oil moves. Sales tax is 22% — and a percentage of a bigger number is a bigger number.

So when a war pushes up the price of crude, the state's share grows at the same time, entirely on its own. Nobody votes on it; nobody signs anything. Italy built a mechanism 18 years ago to hand that extra money back. This year it was switched on three times and off three times; the shortest run lasted twelve days. And one more detail shows how the whole thing is assembled: that 22% is charged on the fuel duty too — about 15 cents a litre of sales tax, levied on money that was already tax.

Prices at Italian pumps have been climbing since 3 July — the day the last discount on fuel duty expired. Since 8 July, Brent crude has risen by roughly a quarter, and this week it passed $100 a barrel. At an ordinary pump the national average is now about €2.00 a litre for petrol and €2.20 for diesel; on the motorway, diesel has passed the €2.70 we started with.

Italy's small-business research office estimates households and companies will spend around €13.5 billion more on petrol and diesel this year than last — an increase of about a fifth, in one year. The government says it will step in using something called the mobile excise duty.

At the bottom of the litre is the fuel itself — what the oil cost, refined and delivered. On top sits fuel duty: a fixed number of cents on every litre whatever the fuel underneath costs. In Italy it has been 67 cents since 1 January, now identical for petrol and diesel. This is also the money that pays for roads.

Then comes sales tax at 22% — and the detail that does the work is that it is calculated on the fuel and the duty together, not the fuel alone. European rules require the duty to sit inside the base, in every member state. That has two separate consequences worth keeping apart. First: 22% of 67 cents is about 15 cents a litre of sales tax charged on the state's own tax — and that never moves. Second, the one that matters today: the sales tax on the fuel part is a percentage, so when crude rises the state's total take per litre rises with it, automatically, without a single decision being taken.

Italy has known about this for a long time. In 2008 it built a mechanism aimed at exactly this problem: the mobile excise duty — mobile because the rate is meant to move, down when oil goes up and back again when it comes down. When crude rises the state collects extra sales tax it never budgeted for, and that surplus is used to cut the duty by a matching amount. It was created under a centre-left government and reformed by a right-wing one, which tells you the problem with it is not political.

The problem is that it is not really a mechanism. The finance ministry has to calculate how much extra sales tax actually arrived last month — a figure not ready until after the first week of the following month. Then two ministries sign a decree. And the decree carries an expiry date. In March an emergency decree cut the duty by 25 cents (more than the surplus alone would have covered) and ran 20 days; a May cut funded from April's surplus ran 12 days; a June decree expired on 3 July. Not one lasted a full month. The collecting happens by itself. The giving back requires a signature.

On 1 January the duty on diesel went up by 4 cents a litre — on top of the 67 — and petrol came down by the same 4 cents. The reason had nothing to do with oil prices: for years diesel had been taxed more lightly, and the EU classified that gap as an environmentally harmful subsidy, because diesel engines emit more of the fine particles and nitrogen oxides that damage lungs. The plan was drawn up in 2025 and meant to arrive gradually over five years. Then it was pulled forward into this year's budget, and five years' worth landed on a single day.

Judged on its own terms that is a defensible decision, taken a year in advance for stated environmental reasons. And it arrived in the same year the fighting with Iran moved the price of oil. Nobody made a mistake here. The environment people were doing environmental policy; the tax people were doing tax policy. What nobody had, anywhere in the process, was a room where the two decisions were added together and someone looked at the total.

A rule instead of a decision. Write the formula into the law: revenue above budget, and the duty falls automatically — no signature, no expiry. The cost: the treasury loses money it has usually already promised elsewhere; a budget written in October would depend on where oil goes in March; and high prices are the one thing that reliably makes people drive less, so this design softens them exactly when they would have worked — in the same country that raised diesel duty this year to clean the air.

Go after the base. Charge the 22% on the fuel alone and the 15 cents disappears. The cost: 15 cents on every litre sold, permanently, is a hole another tax must fill — and Italy cannot decide this anyway, because the base is written into European law.

Chile. Since 2014 the finance ministry has added a variable amount to the duty — plus, minus or zero, with nobody signing and no expiry. This is design A, finished. The cost is visible in this same oil shock: about $220M spent holding pump prices still, $140M of it in one week in March; then an emergency lever in the same law sent petrol up 32% and diesel 62% overnight, the president's approval fell six points in a week, and the truckers began organising. Automation doesn't remove the cost of a shock — it moves it into the budget and stores it there, until the budget says no.

About half of that litre is tax, and a slice of it is sales tax charged on tax. None of this is hidden and none of it is an accident — every piece was built deliberately by people with reasons they could defend: the duty pays for roads, the shape of the base is written into European law, and the change to diesel was made to clean the air.

What is lopsided isn't the money. It is the speed. One side runs entirely by itself: oil goes up, the state's share goes up with it, in real time, attended by nobody. The other needs a ministry to finish its arithmetic, two signatures, and a fresh decision every month — and then it expires. So the question worth carrying out of this isn't whether fuel should be taxed. It is who decides, and how quickly, when a tax grows on its own because a war moved a price somewhere else. A minister with a decree each month, or a formula that nobody has to sign. One of those answers is late. The other one is expensive. There is no third answer that is neither.