Interesting. On a UK Honda I had once (back in the Eighties), the mph in top gear worked out at double the tach reading over 100. I later noticed that the same formula applied to two VWs, two Alfas, a Suzuki, a Volvo a Peugeot and a Citroen and to my wife's Ford today. Engines varied from normally aspirated petrol to turbo diesel, but all with manual transmission.