Reading A European Listing: WLTP Figures Without The Headache

The Spreadsheet With Three Truths

A client of my agency was speccing a small delivery fleet and sent me the spreadsheet he had built for it. One German-market diesel he liked was listed at 6.3 L/100km. A local petrol model sat beside it at 5.1. A third column, added from an American review, showed 46.12 MPG, and by the time the sheet reached me the diesel looked like the thirsty mistake and the petrol car looked like the obvious buy. The sheet was tidy, the arithmetic in it was wrong in three separate ways, and every error pointed the same direction: against the diesel.

Here is the cleanup I ran on it, written as rules because I have run the same cleanup often enough that it has become one. The short version: the 5.1 and the 6.3 were produced by two different laboratory tests a generation apart, and the 46.12 MPG was the 5.1 wearing a US gallon it had never met. Name the test, name the unit, and the phantom gap that was about to steer a fleet purchase mostly evaporates.

Rule One: Name The Test Before The Number

Every European fuel figure was born in a laboratory, and the laboratory has had two regimes. The old one, NEDC, measured cars on a gentle cycle designed decades ago. The current one, WLTP, replaced it: WLTP applied to new models from September 2017 and became mandatory for all new cars in September 2018. Any European listing from before that change carries NEDC figures or transitional values calculated back from WLTP, and any listing after it carries WLTP. The figure alone does not tell you which regime produced it. The car's approval date does, and that date is the first thing to write next to any number you copy out of a listing.

This is the same trap, wearing a European coat, as The UK Car That Nearly Lost To A Wrong Gallon: a correct number attached to the wrong label, steering a real decision. A gallon mix-up changes a figure by about 20 percent. A test mix-up, as the next section shows, can change it by just as much, and it hides better, because both numbers arrive wearing the same unit and the same confident decimal point.

Two Tests, Two Different Exams

The reason the same car scores differently is that the exams differ in almost every dimension that matters.

A longer test at higher speeds with sharper acceleration burns more fuel, so the WLTP figure for a given car lands higher than its NEDC figure did, and the European Commission's own consumer guidance says so plainly: the car did not get thirstier, the measurement got stricter. This is where the question every reader asks arrives on cue: fine, then what is the conversion factor? There is not one. The gap depends on the individual car, its gearing, its weight, how much of its work the old gentle cycle let it dodge. Manufacturers published both values side by side during the transition years precisely because no honest multiplier existed. A site that offers you a single NEDC-to-WLTP factor is selling you a guess in a lab coat.

The rule I gave the client that day, and the one that has survived every fleet sheet I have touched since, is this: one test cycle per column, never one column per car. A figure that travels without its test named next to it is not data. It is a rumor with a decimal point, and I now refuse to let one into any comparison I sign my name to.

Rule Two: One Unit, One Gallon, One Column

The second error in that spreadsheet was quieter. The American review's 46.12 MPG had been computed on the US gallon, while the European figures sat in L/100km, and a helper column had converted the diesel's 6.3 L/100km into 37.34 MPG, also on the US gallon, then compared the two MPG numbers as if they shared a test as well as a unit. They shared neither. The discipline that prevents this is dull and total: pick one unit for the whole comparison, and on this site that unit is L/100km, because European labels already speak it and because it needs no gallon at all. When a figure arrives in MPG, it goes through the converter on this page with its gallon named out loud, US or UK, before it is allowed into the table. A number whose gallon nobody can name does not get a row.

A fuel figure is a witness statement. Read the name of the test and the name of the gallon before you believe a word of the testimony.

The Cleanup, Worked On One Car

Here is the client's diesel-purchase question rebuilt honestly. The German diesel: 6.3 L/100km, WLTP, which is 44.84 MPG on the UK gallon if MPG is truly needed. The local petrol model: 5.1 L/100km, but NEDC, from the model's older brochure, which is 55.39 MPG on the UK gallon under a test that no longer exists. Laid side by side with their tests named, the comparison stops claiming the petrol car wins by 1.2 L/100km. What it actually says is narrower and more useful: the diesel was measured under the stricter modern exam, the petrol car under the retired gentle one, and nobody on earth can say from these two brochures which car drinks less on a Multan delivery route. For that, the fleet needed the petrol car's current WLTP figure, which the maker's newer listing provided, and one fill-to-fill week in each candidate, which no brochure can replace.

The use case under this whole article is that buyer, and there are thousands like him: a small fleet owner, a family importing one car, a dealer writing an advertisement. The stakes are not academic. A phantom 1.2 L/100km gap, believed, prices out to real money across a fleet's year, and it can push a buyer off the car that would have served him better onto the one whose paperwork was merely older.

Reading Any European Listing In One Minute

The habit I now run on any European listing takes about a minute. First, find the approval era: before September 2018, suspect NEDC or a transitional value; after it, the figure should be WLTP, and the listing usually says so in the small print near the emissions line. Second, refuse any comparison in which two figures carry different test names, and go find the same-test figure for both cars or measure both yourself. Third, convert everything into L/100km once, with gallons named, and let no second unit into the sheet. Fourth, remember that even a clean WLTP figure is a laboratory result, closer to the street than NEDC ever was and still not the street.

One last link in the chain matters for that fleet buyer in particular: once the figures finally stand on one test and one unit, the question stops being about economy labels and becomes about money per kilometer, which is where fuel type enters. The Petrol Or Diesel Break-Even takes the comparison from there, with the same discipline applied to prices: named, dated, and never mixed mid-table. Read listings this way and the headache the title promises to remove mostly never forms. What remains is the part that was always real, two cars and your roads, and that part a spreadsheet was never going to settle anyway.