Tyres & engines
Tyre sizes, speedometer error, engine displacement, power and towing limits.
Tyre notation is three unit systems in five characters: 225/45R17 is 225 millimetres wide, a sidewall 45 per cent of that width, on a 17 inch wheel. Nobody would design it that way now, and it is why changing size without checking the rolling diameter quietly puts the speedometer out.
Engine numbers are similarly full of inherited convention. Displacement comes from bore and stroke and says nothing about power on its own; power and torque are the same measurement at different rotational speeds, which is why their curves always cross at 5,252 rpm on an imperial chart.
- Engine displacement Capacity from bore, stroke and cylinder count — plus what the bore-to-stroke ratio says about it.
- Horsepower Convert between hp, kW and PS, and work out the torque behind the power.
- Towing & payload Whether an outfit is inside both weight limits, not just the obvious one.
- Tyre size What a tyre size actually measures, and what changing it does to the speedometer.