Why are wheels getting bigger and tyres getting smaller?

QUESTION

Hi John,

Thanks for your continued output of videos and articles and your unique blend of intelligence and entertainment! They help keep me sane in here (rehab).

What are your thoughts about large diameter wheels and very low profile tyres? I note that most manufacturers offer larger diameter wheels on higher spec models. Is there a functional advantage or is this simply a perception thing?

Also what do you think about after-market large diameter wheels and ultra low profile tyres fitted to vehicles which were not designed for them?

Regards,

Greg

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ANSWER

Hey Greg,

Thanks for the kind words and your interest in my reports mate. Sorry you’re in rehab - doesn’t sound like fun, in the midst of a pandemic.

The advantage of the lower profile tyres is handling performance, so on cars where the suspension is expertly tuned to maximise the performance of tyres such as those, you get better handling.

Ride quality generally suffers, all other things being equal, but not by so much as people make out. Dampers and bushes can compensate somewhat.

On cars tuned to (say) 55-series tyres and where 40-series (or 45) have been added, the results are usually imperfect unless dampers, bushes and springs are re-tuned accordingly.

Lower profile tyres are also more susceptible to causing wheel damage if you hit a pothole (because there’s less sidewall to absorb the shock).

Arbitrarily changing the tyres can also change the overall gearing, resulting in speedo error and also worse fuel economy, especially at cruising speeds.

Generally, I advise people to buy a car that delivers the performance they require (engine performance, handling performance, tow performance, off-road performance, refinement, etc.) without modification, because modifying it generally makes things worse. A lot of people ruin the handling of performance cars by installing wheels and tyres of a different size…

All the best mate.

MALS - or death!

JC

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