What impact does wheel size have on speed

Currently have 19's, I have found a set of wheels that I like that are only 20's. What would be the impact on car performance? Is it just the weight of the bigger wheels on the corners?

That is a tricky question to answer. As it is a physics question, it is full of 'it depends' and corner case situations.

All things being equal, a car with a 20" wheel with similar aspect ratio tire will have a higher center of gravity than a 19". This may reduce lateral grip performance.
The 20" setup is also usually heavier than a similar 19" setup and as most of the weight is at the edge of the wheel, the moment of inertia for the 20" will require more force to overcome. This may reduce acceleration performance.
For the same reason but in reverse, it may reduce braking performance.

On the potentially positive side, top speed performance should be marginally increased providing aerodynamic drag is not considerably increased by raising the height of the car.
Gas milage may also increase.

Knowing what elements of the performance envelope you want to optimize would help.

20"s are pretty normal on sports cars these days because you can keep a small sidewall which is great for performance, and the contact patch increases.

The 991 GT3 RS for example, has 21" rear wheels on 325/30/R21 Michelin Cup 2 tires.

Had Porsche used 17" wheels they would have to have huge sidewall height (and then you can't get Cup tires) or just have smaller tires which would decrease contact patch.

So there's benefits. That's why you find dubs on top tier performance cars pretty often these days.

Any "benefits" to 20's involve appearance, unless you have 16" brakes where you'd generally need 20's to clear the outsides of the calipers.

There really isn't any need for wheel sizes to be any more than about 4" bigger than the bigger brake rotor size fitted to the car.

You may or may not get sharper turn-in response with 20's over 19's - that's more dependent on tire brand/model/size and wheel width. But if you use the 20's specifically for getting shorter sidewalls at the same tire outer diameter as 19's, you'll most likely end up with a combination that's more sensitive to camber (which might well demand stiffening the suspension and tweaking the alignment specs if you actually wanted to use the upper end of the available performance).

For a range of uses from street duty at the 'easy-driving' end up to semi-serious autocross or HPDE, the land of diminishing returns probably starts at sidewall heights a little taller than 4" (taken as tire OD minus wheel size, divided by two). Just to be clear, street duty in this sense does not consider appearance preferences.

FWIW and as counterpoint to the Porsche example above, the GT350, GT350R, and the recent Z/28 all use 19" wheels and tires. The upper-level European nameplates aren't blind to the idea of using appearance to sell their cars . . . featuring out-size wheels is the way concept cars have been displayed for decades.

Michelin's PSC2 is available in several 19" sizes that might be appropriate for S550 track use, and they'd make even more sizes if there was enough demand. Just saying . . .


Norm

Tire weights, when speaking of street tires are all usually with in a couple of pounds of each other, wheel mass on the other hand can vary greatly. Less mass = Greater Performance. Less total vehicle weight, less rotational mass to accelerate and decelerate, less mass for the suspension to control.

End of the day if you find a wheel that appeals to you even if it diminishes performance who cares? The driver mod will always overcome vehicle dynamics. Driving a heavy car well on street tires is more impressive than bolting on slicks and race wheels and having the car make you look good. Again, I have a long way to go driving wise, but laptime comparisons with other cars, including race set up cars poorly driven don't lie. I was looking at the average laptimes from last weekend at Palmer for the AER event. 1:52's. These are full blown race cars on slicks, while our cars have strong power I ran the 5th track day of my life there last year in a stock GT PP and turned 1:55's while leaving plenty of time on the table due to operator error.

The whole argument about wheel diameter seems silly when you consider F1 cars ride on 13" diameter wheels. The tire supplier to F1 wants them to go to 18" for marketing reasons not performance. 13" diameter tires are perceived by the public as bottom of the line tires.

redleg322 said:

20"s are pretty normal on sports cars these days because you can keep a small sidewall which is great for performance, and the contact patch increases.

The 991 GT3 RS for example, has 21" rear wheels on 325/30/R21 Michelin Cup 2 tires.

Had Porsche used 17" wheels they would have to have huge sidewall height (and then you can't get Cup tires) or just have smaller tires which would decrease contact patch.

So there's benefits. That's why you find dubs on top tier performance cars pretty often these days.

Click to expand...

F1 - 13"
NASCAR, IndyCar - 15"

20's aren't for performance. You can't get any of the good R compounds in 20" (and yes, the Cup 2 don't meet my definition of good R compounds, they win almost no races or autocrosses in the US).

They generally add weight, cost, and hurt the ride. If you are already at a 40 series or less, a shorter sidewall is not really going to help performance. And a short side wall has nothing to do with the contact patch.

They do open up the wheel selection and tire selection a bit. In particular, it is currently much easier to match the stock tire diameter with 20's than with 19's or 18's.

But really you are asking the wrong question. Ideally you want to fit wider, sticky tires, with light and strong wheels. You can certainly get 20" wheels (& tires) that will be an improvement over stock, or even some heavy and/or not particularly wide 18's or 19's. But you can potentially do better (get lighter & cheaper specifically) with smaller diameter wheels. Look very hard at what the wheel size does to your tire selection options.

Some interesting points here. So some of the best handling race cars in the world use 13" & 15" wheels. How did sport performance street cars end up with 18",19" and 20" wheels from the factory?

Built4Speed said:

Some interesting points here. So some of the best handling race cars in the world use 13" & 15" wheels. How did sport performance street cars end up with 18",19" and 20" wheels from the factory?

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That's easy! Looks.

And ride and handling, MPG etc. Big brakes on heavy cars require big wheels. When 10-11" brakes were the standard and cars were 3000 pounds a 15" wheel was acceptable.

dgc333 said:

The whole argument about wheel diameter seems silly when you consider F1 cars ride on 13" diameter wheels. The tire supplier to F1 wants them to go to 18" for marketing reasons not performance. 13" diameter tires are perceived by the public as bottom of the line tires.

Click to expand...

We won't know that unless the FIA stops restricting brake discs to <11 inches but saying "F1 only runs 13" wheels" is arguing that a $50,000 carbon brake disc on a $50,000 wheel/tire which supports a $10,000,000 car isn't proving that street wheels/tires over 13" on a street car are pointless.

There's a bit of a problem with that reasoning. To each their own, though. There's a reason you're seeing OEM wheels around the 20" size pretty commonly these days on street cars and hyper cars and its an engineering reason, not an aesthetic one.

I do like the look of 19/20 inch wheels, but if the non-restricted engineering behind a 13" wheel was indicating there are performance benefits, I'd be all for it.

Built4Speed said:

Some interesting points here. So some of the best handling race cars in the world use 13" & 15" wheels. How did sport performance street cars end up with 18",19" and 20" wheels from the factory?

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Sports cars have been getting bigger and generally a bit taller over the years, so the wheels have been getting bigger as a matter of proportion. Styling of the S550 and S197 Mustangs is much "thicker-bodied" than it was for the original mid/late sixties cars, even though the wheelbases are within an inch and overall lengths within four inches of the 1968 Mustang (oldest year I have dimensions & specs for).

They've also gotten a lot heavier, and in order to achieve tire load capacity at inflation pressures that won't put your fillings at risk, it's been more reasonable to get that load capacity partly through increased wheel diameter. It's probably possible to gain all of it via wider tires in the smaller diameters, but that might end up requiring tires too wide for other corporate standards or the wet-weather driving skill(?) levels of too many customers.

Tire load capacity is a function of the volume of air contained within it as well as the pressure that it's inflated to.


Norm

redleg322 said:

We won't know that unless the FIA stops restricting brake discs to <11 inches but saying "F1 only runs 13" wheels" is arguing that a $50,000 carbon brake disc on a $50,000 wheel/tire which supports a $10,000,000 car isn't proving that street wheels/tires over 13" on a street car are pointless.

There's a bit of a problem with that reasoning. To each their own, though. There's a reason you're seeing OEM wheels around the 20" size pretty commonly these days on street cars and hyper cars and its an engineering reason, not an aesthetic one.

I do like the look of 19/20 inch wheels, but if the non-restricted engineering behind a 13" wheel was indicating there are performance benefits, I'd be all for it.

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If you aren't running brakes bigger than 14", there isn't any engineering necessity for ponycar wheels to be bigger than 18's. Or 19" with 15" brakes.

Except for the matter of bragging rights, 14's with the right pads is as much or more brake than most ponycar drivers will use or ever need.


Norm

Norm Peterson said:

If you aren't running brakes bigger than 14", there isn't any engineering necessity for ponycar wheels to be bigger than 18's. Or 19" with 15" brakes.

Except for the matter of bragging rights, 14's with the right pads is as much or more brake than most ponycar drivers will use or ever need.


Norm

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I get what you're saying, Norm, but OP is asking if there is a performance benefit to 20's and a taller tire has a larger contact patch and in order to keep the sidewall stiff you use larger wheels on taller tires.

Now whether anyone in a pony car "needs" more than 18" wheels is another matter. You're probably right about that one. Its one thing to have a performance benefit, its another to need or be able to use it.

I wonder how much contact patch area you'd gain, going from let's say 285/35-18's at about 26" tall to 285/35-20's at 28". Assuming the same width wheels.


Norm