Tyre sizes and engine numbers
A tyre marked 225/45R17 tells you its width in millimetres, its sidewall as a percentage of that width, and its wheel diameter in inches. Three different unit systems in six characters, and the middle one is a ratio rather than a measurement.
That is why fitting a wider tyre with the same aspect number makes it taller as well as wider, and why the speedometer stops being right.
Reading the size
In 225/45R17, the 225 is the section width in millimetres. The 45 is the aspect ratio — the sidewall height as a percentage of the width, so 45% of 225mm is 101.3mm. The R means radial construction, and the 17 is the wheel diameter in inches.
Overall diameter is the wheel plus two sidewalls: 17 inches is 431.8mm, plus 202.6mm of sidewall, giving 634.3mm — just under 25 inches.
The consequence of the aspect ratio being a percentage is that it is not a fixed height. Go from 225 to 235 while keeping the 45, and the sidewall grows too, because 45% of a larger number is larger. The tyre gets both wider and taller when you only meant to change one.
225/45R17
Sidewall 45% of 225 mm = 101.3 mm
Wheel 17 in = 431.8 mm
Diameter 431.8 + 202.6 = 634.3 mm (24.97 in)
What that does to the speedometer
A speedometer counts wheel revolutions and multiplies by an assumed rolling circumference. Change the tyre and that assumption becomes wrong — a larger tyre covers more ground per revolution, so the car goes further than the instrument thinks.
Going from 225/45R17 to 235/45R17 adds 9mm of diameter, which is 1.42%. An indicated 100 is really 101.4, and the odometer accumulates too slowly by the same proportion. Over 100,000 miles that is 1,400 miles the car never recorded.
Within about three per cent is the usual guidance for staying legal and keeping the ABS and stability systems working from sensible numbers. This change is comfortably inside that; a jump of two sizes might not be.
Size is not the only thing to match. Load and speed ratings matter as much, and a tyre that physically fits is not necessarily one the vehicle is rated for.
Work it out: Tyre Size Calculator →
Displacement is swept volume and nothing else
Engine displacement is the volume the pistons sweep: bore area times stroke times the number of cylinders. An 86mm bore with an 86mm stroke across four cylinders is 1,998cc — sold as a 2.0.
It is rarely exactly the round number. 1,998cc and 2,035cc are both badged 2.0, and the advertised figure is always a rounding of the real one.
The bore-to-stroke ratio is the more interesting number. Equal bore and stroke is called square, and it is the compromise most ordinary engines are built to. A long stroke favours low-down torque; a short one lets an engine rev harder, which is why performance engines tend to be oversquare.
What displacement does not tell you is power. Compression, breathing, valve timing and forced induction all matter more, which is why a 2.0 turbo routinely out-powers a naturally aspirated 3.0.
Work it out: Engine Displacement Calculator →
Towing: three limits, and the one that usually binds
Towing capacity is not one number. There is what the vehicle itself may weigh fully loaded, what the whole combination may weigh, and what the trailer itself is rated for — and the answer is whichever of them runs out first.
It is usually not the one people check. The combination limit and the vehicle's own loaded weight interact: adding passengers and luggage eats into what is left for the trailer, so a car that tows 2,500kg empty may not tow it with four people and a boot full of gear.
The other figure that matters is tongue or nose weight — the download on the towball, typically around ten per cent of the trailer. Too little and the trailer snakes; too much and the rear axle is overloaded. It also counts against the vehicle's own payload rather than the trailer's.
Work it out: Towing Capacity Calculator →
Questions
What does 225/45R17 mean?
225mm section width, a sidewall 45% of that width, radial construction, and a 17 inch wheel. Millimetres, a percentage and inches in the same six characters.
Does a wider tyre make the car taller?
If you keep the same aspect number, yes. The sidewall is a percentage of the width, so a wider tyre with the same 45 has a taller sidewall as well.
How much does a tyre change affect the speedometer?
In proportion to the diameter change. Going from 225/45R17 to 235/45R17 adds 1.42%, so an indicated 100 is really 101.4 and the odometer under-records by the same amount.
How far out can a tyre size be?
About three per cent of overall diameter is the usual guidance, for legality and to keep ABS and stability systems working from sensible figures. Check load and speed ratings as well as size.
Is a 2.0 engine exactly 2,000cc?
Almost never. 1,998cc and 2,035cc are both sold as 2.0 — the badge is a rounding of the swept volume.
Does a bigger engine mean more power?
Not reliably. Displacement is only the volume the pistons sweep; compression, breathing, valve timing and turbocharging matter more, which is why a 2.0 turbo can beat a naturally aspirated 3.0.
What limits how much I can tow?
Whichever runs out first of the trailer rating, the vehicle's own fully loaded weight, and the combined limit. Passengers and luggage reduce what is left for the trailer, so the empty-car figure is often not the real one.