….Okay, now with that education: “What are the constraints?”
Constraints could be as simple as the size of your current wheels and specific outer diameter needed to maintain the accuracy of the speedometer, odometer, traction control, and ABS functionality. Or it could be a little more in depth and limited by class specific rules (for competition) like maximum tire width, tread wear rating, or it can be very in depth with fitting the largest tire in the wheel well as possible.
1. Existing Wheels:
If you are not purchasing new wheels and are simply looking for new tires, look up the TREAD width of your current tires on TireRack.com – are they the same or 0.5” narrower than stock? If the tires are the OEM size, the tread width will probably be close to the rim width, and maybe even up to 0.5” wider.
- For example: Let’s say your car has an 18×10” wheel and a Michelin Pilot Super Sport (PSS) tire on it from the factory. It’s likely the PSS would be a 285/35-18 for this wheel which has a 10.2” tread width. Now following my generic rule of thumb, the best size tire would be a 275/35-18 that has a 9.6” tread width which will improve the tire’s response and grip, but this is relative to the same 275 tire on a narrower wheel.
In our example, the factory 285 is pretty good but isn’t optimal for our 10” wheel, and it could make even more grip and have better response on a 10.5” wheel. If you were to downsize the tire on the factory 10” wheel, you are optimizing the grip and response of a smaller tire, but that may be a slight net loss of grip for a slight improvement of response. Depending on the situation, downsizing the tire could have slightly less, the same or slightly more grip. You can go either way here, but depending on your needs, I would probably keep the factory 285 width UNLESS a better performing tire can be had in the 275 width (and tread width) range, then that would be a win-win over an inferior 285. Remember the “Top Tip” earlier in the article?
2. Tire label width (Nominal width):
If you compete in time trials or wheel to wheel racing, but your class has a tire width limit of say, a 275. By now you should know at least two things: 1) – The nominal sidewall width is useless and 2) – The tire model (compound/construction) is more important than width. Given a rule constraint like a 275, there is not much to do other than look at the true tread widths of the various tires and (more importantly) find out which compound is truly better.
3. Wheel well room:
This can get very complex when increasing the width and diameter of the tire beyond what the factory has intended; but there are 3 main things to consider: Width, Diameter, and Volume/Total Tread Surface Area (TTSA).
The width of a tire usually does not influence the ABS or Traction Control, and the constraints here tend to be limitations with steering lock and the tire rubbing the inner chassis at full lock. Optimizing the offsets and rim widths for the given amount of room is the goal here.
There are a few things to understand in regards to tire width. A WIDER TIRE:
- Does NOT change the contact patch SIZE when the vehicle weight and tire pressure is the SAME.
- DOES change the contact patch SHAPE when the vehicle weight and tire pressure is the SAME. The contact patch becomes wider and shorter (front to rear).
- (Typically) ALLOWS for LOWER PRESSURE which will then INCREASE the contact patch size (good).
- INCREASES the tires Total Tread Surface Area (TTSA), and makes it less prone to overheating and has a LONGER LIFE.
- Takes LONGER to WARM UP to its ideal operating range. Usually not a problem on heavy cars.
This has a huge influence on the ABS, Traction Control, speedometer, effective gear ratio, rotating mass, and much more. The easiest thing to do is to keep the diameter the same as the factory. I wouldn’t recommend going above or below this diameter until you know more about how it is going to affect the rest of the systems on your car.
Increasing the Diameter also changes the contact patch SHAPE, making it longer (front to rear) and narrower (width) for a given vehicle weight and tire pressure. You can typically also lower tire pressures slightly to increase the contact patch SIZE due to a larger diameter tire. Larger diameter tires generally help for longitudinal grip (drag racing) more than a wider tire. There’s a reason drag racing tires have small diameter wheels, large sidewalls and very large outer diameter tires.
Keep in mind that larger diameters also increases the tire’s Total Tread Surface Area (TTSA) which can be advantageous. As far as sizing goes, this Ferrari 458 Speciale is arguably one of the best performance cars ever built and is equipped with massive 245/35-20 and 305/30-20 Michelin Pilot Sport Cup 2 tires that have 26.8″ and 27.2″ diameters respectively. Interestingly, the 300/650-18 and 320/710-18 racing slicks commonly used on GTE racecars have very similar outer diameters.
While slightly smaller brakes and either 18″ or 19″ wheels probably would have sufficed, Ferrari felt this direction of smaller sidewalls and larger diameter wheels were worth it and it’s hard to argue against it especially when amazing supercars from the 918/P1/LaF trio to the Aventador SV, McLaren 675LT and Ford GT have all gone in this direction. Having driven the Speciale in bumpy pothole-riddled Toronto city streets, this package with its thin side wall heights isn’t as bad as you might think in the worst conditions and is just brilliant on the racetrack.
-TOTAL TREAD SURFACE AREA (TTSA):
A tire’s TTSA is an important and often overlooked aspect that directly affects a tire’s performance. Bigger isn’t always better and there’s a point of diminishing returns (which probably won’t be an issue for most readers) where a larger and wider tire will not be any faster. While a 245-width tire isn’t wide by today’s standards, Lotus Elise’s struggle to get heat into a 245 and tend to slip and slide on cold days due to their low weight and low center of gravity. On the other hand, a heavy 3,800lb car on 275s can lay down a fast lap or two but cannot last a 20 minute session without overheating the tires and being seconds off pace. All of this has to do with a tire’s TTSA, which is dictated by the tire’s overall diameter and width.
There are always exceptions to every rule. A car’s setup is like an inter-connected spider web where changing anything influences everything else. Often one change will have a secondary effect that overshadows the primary change.
Because of this, it’s possible that running a tire a lot wider than the wheel width would improve the overall performance if it were on a heavy car, with a less than ideal alignment that is very heavy for the wheel width. In this case, the increased VOLUME/TTSA of the tire, and tread width, would in fact improve the cornering ability of the car.
It’s important to understand the intended use of the car as well as the tire’s TTSA. You don’t want too much volume/TTSA (like the Elise example) if you are autocrossing, because you will never heat the tire up to its ideal operating range in time to make grip. Likewise if you have a heavier car, increasing the volume/TTSA is key to making tire last longer in a session.
Tire compound and tread design also plays a big role here and while swapping to a more aggressive tire is the easiest way to make a car faster and last longer before overheating the tires; increasing the TTSA will make the tires last even longer, and even allow for a tire choice that is less of a compromise for daily driving. Look at the following chart of various production cars and their weight-to-TTSA ratio:
This figure is calculated by adding the total front and rear Tread Surface Areas (tire width x circumference) then dividing the Total Tread Surface Area (TTSA) by the vehicle weight. The higher the mm per pound, the more the load is distributed across the tires and the more resilient the tire will be to overheating.