{UAH} Ever wondered why today's auto manufacturers putting 18", 19", 20" even 22" tires on their cars?

There are four reasons why most late model vehicles use larger rims and lower profile tires.

1. On the road performance: Vehicles so equiped with large rims - 18″ and larger will usually have very low profile tires, in the order of 45 series to 30 series. Low profile tire means the radial cords are much shorter compared with 70 series and that translated to much more precise corning control as the short sidewall radial cords will deflect less to make turns. All the later models of high performance cars are equiped with those tires. On tracks they can corner with over 1.10 g forces compared to .78g for the older 70 and even 60 series tires. Since tire body deflection is much less, less heat is generated so the top useable speed is higher as well. In 2022, The F1 cars will change from 13″ rims to 18″ for that reason. A lot of those high-end cars are capable of greater than 180 mph as some people considered them as factory authorized legal race cars. The sidewall of Bugatti Chiron is made from carbon fiber material and heavily strengthened and custom made by Michelin for about $10,000 per tire. A far cry from the average polyester cord radial tires.
2. Fuel saving: The reduction in the radial cord length due to lower aspect ratio tires means the air volume is also lower. To carry the same weight inflation must be increased. The old high profiles have inflation pressures around 30 to 35 psi, but the new low profile tires must be inflated to 47 to 50 psi, especially on EVs due to their battery weight. That translated to better fuel mileage, usually no more than 2% or so.
3. Larger rims mean they can install larger rotors for better braking performance. As more cars are turbo-charged with horsepower over 700 and as much as 1500, the braking power must increase as well. Larger rotors and special carbon-fiber ceramic rotors - at $8000 option are appearing more frequently. You will find16.5″ rotors on Porsches that weight less than 4000 lbs. Even Camrys and Corollas are coming up with larger rotors for cooling and improved brake performance.
4. Uniformity: Since rims that comes with those tires are either forged - high end units or casted aluminum alloy. They are machined so the out-of-round specification is very much better than stamped steel rims. Since the radial cords are much shorter, it is much easier to have tires being more uniformly round. That translated to better NVH numbers - Noise, vibration and harshness. Those number are more important as speed goes up. I am not too crazy on these larger rims and extreme low profile tires as I value the quality of my ride and our roads are not condusive for performance driving, especially the number of pot holes keep growing faster than the government can repair them. Part of the change may be progress as even 2019 Honda Civic type R used 245/30R20 as original equipment. For others It may be just visual appeal. For me 0 to 60 mph under 6 seconds is more than sporting unless you enjoying paying traffic fines. My take away is choose wisely, any low profile tire with aspect ratio smaller than 40% , you are choosing high speed handling and performance over utility and durability. Your gain may never be realized as few cars are driven to that extreme on a daily basis. May be I am getting too old for ultra high performance stuff.

1. Sidewall distortion on turns
2. Shorter radial cords by using lower aspect ratio reduce heat generation thus improve "G" forces.
3. 25 aspect ratio tires mounted on car and damages from poorly maintained roads or hitting curbs.

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 · Answer requested by Gene Blodgett

262 comments from Bernard Ching and more

Promoted by FIXD Automotive

Isaac Wittenstein

·

Updated March 23

Engineer at FIXD (2019–present)

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