3 = 6.
Yes, really, at least according to Auto Union’s advertising agency.
The DKW of that unusual name had, like other Auto Union models up to the 1960s, a three cylinder two stroke engine. The somewhat implausible claim was that this was equivalent in terms of power to a six cylinder four stroke of twice the size on the grounds that each of those three pistons was producing power each time it descended rather than every other time.
In addition to three equalling six, it has been found that eleven equals fifteen.
Meet the Citroen Traction Avant 11.
It derived its name from its 11 CV, or horse power, engine.
Meet, on the other hand, the UK built (because of import tariffs) Citroen Fifteen, which was sold in Light or Big versions. The Big was, well, bigger, with a longer wheelbase and wider track.
So the 15 had a more powerful engine, with a hefty 15 bhp to shift a car weighing just over a tonne at a very leisurely pace? Mais non. Both had 1.9 engines with 55 bhp. So what’s going on, and how can 11, 15 and 55 all be correct?
It’s all down to tax. France at the time, and for many years after, taxed cars according to their power, except they didn’t. Cars were taxed according to an arcane formula which in the case of the 1.9 Traction Avant worked out at 11 fiscal hp or CV. The UK did the same but they used their own but equally arcane formula which produced what was called RAC hp. So the Citroen had 15 RAC hp and at the same time 11 CV and 55 bhp.
RAC hp, incidentally was derived from:
hp = ( D2 x n ) / 2.5 where D was the bore in inches and n the number of cylinders. Undersquare engines with few cylinders were therefore favoured.
French fiscal HP at the time of the Traction Avant (there were several changes over the years) was derived by:
CV = D2 x N x L x w x K, where D and N are as above, w is engine speed in revs per second and K a coefficient depending on the number of cylinders.
So there we have it. Three equals six, and 55 equals both 11 and 15.