18" 17"? Does car wheel size matter?

[treet]

Whats the deal with car wheel size? We are currently looking for a car secondhand and the ads seem to emphasise that it has 18" wheels which is apparently better than the standard 17". The man at the garage didnt really seem to know beyond saying that larger ones look better. As these are only family car SUV/MPVs I am a little sceptical that it matters so what benefits would I get by going for the bigger wheels? Is there any difference in tyre cost etc?

need less expensive

Again, this is not necessarily the case.

Bigger rims to fit bigger brakes and lower profile tyres.
Low profile tyres for less sideways movement cornering under load.

The above applies to a racing car. On a Vauxhall Astra or an SUV it's just about the looks and pretending you're Max Verstappen. And smashing them up on kerbs and speed bumps as you try to find the 'line'.

Well no, that’s clearly not right. If just increasing the final drive ratio decreased fuel usage then every car would have a ludicrously high final ratio.

Every choice of final ratio is a compromise between several factors such as noise, response, economy and wear.

Well yes that’s obvious, but at a given speed shortening the FD will increase fuel consumption. Unless we’re talking absolute extremes?

Where on Earth are you getting that idea from? If you are below the torque peak then decreasing diameter will improve fuel economy.

That shows a worrying lack of understanding of mechanics and physics…. As the car wheel size goes up, the sidewall depth tends to go down. The resulting rolling radius is broadly the same whatever the size of wheel specified for a given model of car. But, larger wheels tend to have wider tyres, which create more resistance as they have a larger surface area (caused by the width) and so use more fuel. Which is one reason why the same model of car can have different CO2 emissions for the different wheel options, the other being drag caused by air resistance. It’s the gearbox and gear ratios that turn engine output into forward motion, and the wheel size is just one part of that equation..

That’s not the case either. Picture a petrol engined car with a peak torque at 7k RPM. Say motorway cruise in 5th is around 4k RPM, now cruise at that same speed in 4th - you’re trying to tell me that’s more economical? It definitely isn’t!