….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).
-WIDTH:
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.
-DIAMETER:
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.
EXCEPTIONS:
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.
25 comments
Thank you, very informative article!
Loved this article. But how do you calculate or optimize vehicle weight/hp to tire size?
Amazing article. Changes my whole perception of what i am looking for. 2017 mustang gt. Drop 1 inch, tiger grip wider. Staggered or not too. Like 275 on all 4 and do I look for height and 45s height compared to 50s height for stance
Glad to hear it helped.
Nice, but what’s the optimal weight to mm for tire width? I want a good compromise for street/weekend warrior use and looks on my 09 daily driven Scion tC. Don’t wanna take too much of a hit to mpg, but I want more room to soak up hard cornering and, as I said a more aggressive look.
Stock sizes are 215/45r17 and 225/40r18. I know a 235 will fit, and I’ve seen a couple of 245 and 255 fitments online, but it’s mostly 245 or below. Currently running BFG sport comp 2 a/s tires and like them. Live in Florida and was recommended those for my application. The car is FWD with a manual and 160hp, I intend to run a square setup.
Thanks for pointing out that choosing the right tires will be able to dictate the success of the race and also keep you safe from accidents. I will share this information with a friend of mine since he will be joining races once he gets his car customized. It has really been his goal to do so after being obsessed with a series of movies that focuses on this.
This is probably the best ‘layman’ article on tires I’ve read! Here’s why I’m here. 2018 Mustang GT, stock except exhaust, air box and multiple tunes including E85. Daily driver with track weekends. Stock rims are 18 x 7.5 rolling on Pirelli P Zero Nero P235/50ZR18. I want more grip in the rear. Don’t care to change wheel diameter bc Ford Engineers are smarter than I. Contemplating going to 10″ in rear with MT ET Street SS 285/40R18. Not married to them and am open to any suggestions you may offer. I’m interested in grip and of course a better ET.
Thank you for the kind words, it’s great to hear that feedback (which was the goal). To be honest, I’m not the best source for tire sizing when it comes to drag tires since I do not have firsthand experience testing how wheel widths affect longitudinal grip and launches of drag-specific tires.
What did you end up doing? Mine is a 2017 and trying get a better stance look with a drop 1 inch with a little wider tire,but my big concern is 40 45 or 50 height tires
Thanks for listing all the details which are of importance for proper tyre selection.
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Yet, I did miss the part explaining the influence of “aspect-ratio”.
I actually trashed a set of all-weather tyres and damaged my rim, due to driving a bit to fast on a very poor rural road. Consequently, I searched for information regarding rally tyres on the web. Interestingly, the majority of rally tires of Michelin have aspect ratios of 60-65. Yet, these tyres have very stiff side-walls. What is your opinion regarding the influence of aspect ratio?
In addition, you mentioned that you consider load-index not to be be very useful. Yet, does load index influence the strength and stiffness of the side walls? Consequently, the higher the aspect ratio, the more important load index, especially using poor roads? Appreciate your view/
Mr. Johnson, your article is the most clearly explained writing I’ve found on the matter. Still it is not very clear for me what would happen to my 2019 Toyota Camry, with original 17″ x7.5 original wheels if I change original tire size of 215/55/17 to 235/55/17. My intention is to gain half an inch in underbody clearance, and of course any gain in performance would be wellcome. I appreciate any light you can give me on this matter.
Going from a 215/55 to a 235/55 would give you a tiny bit of ground clearance but would also change the circumference of the tyre, which may throw off your odometer and other systems. I suspect it’s not a good idea, if you want more ground clearance the proper way to do it is to install adjustable coil-over suspension.
The wider tire will lose steering response and feel more vague and ‘mushy’ as the sidewall deflects more before loading up. Peak grip will likely increase. I’m not sure you’ll gain 0.5″ in ride height from making that switch. Check out a tire size calculator to be sure.
Than you for writing this article! It is very informative! What I did miss was information on what would be a balanced mm/Lb for track use. You present a table with multiple vehicles, but do not comment on whether you find those values too high or too low, with the exception of the Elise, that isn’t even on the table. What would be a good range for mm/Lb?
It depends on the tire. Whatever it takes to keep a tire in its ideal operating temperature window is what should be targeted.
First of all, excellent article, i appreciate the time and effort placed into explaining all the nuances of selecting a damn tire. Ive already screen shotted some portions for my digital “Car” notes.
I just have one critique from a data driven, engineering minded geek.
In the TTSA chart, TTSA seems to be calculated only using half of the tires….1xFront+ 1xRear… thats not total…I realize that may not be your chart, but when i come across calculations like these that aren’t completely logical, it makes me ask…why? In the end, i know it will just change those numbers by a factor by 2, but to me it makes more sense to use the TTSA that uses all four tires. Of course, if there is an industry standard that uses 1xFront and 1x Rear, then so be it; i am ignorant to it (alibi: i am new to this game)
Once again, thanks for making your experience available for the entire world to learn.
Cheers!
Haha, thanks. As far as TTSA goes, feel free to multiply all the numbers by 2. For comparisons sake, only looking at half the tires does not affect the results.
That was very helpful, thanks. On my Lotus Elise S1 (1800lbs), I’ve always found that I could not get heat into my tires (205F/245R). For many years I was following the ‘wider is better’ gang. But this year I’ve decided to try a smaller size (195 & 225). I’ll be saving weight and hopefully heat them faster too! Over the time I’ve also came up with the thinking of choosing a tire first then get the wheels. Thanks for the great article !
Glad to hear it. The #1 thing is the compound of the tire and keeping it in it’s operating temperature range.
Good article but the information I’m looking for doesn’t get addressed.
How do I choose tyre sizes for my car based on power and weight?
My car weights 550kg and has rear wheel 60bhp
I currently have 13×7″ rims wearing 195 section tyres.
I’m sure that’s too much rubber.
Should I get narrower rims and save weight or would this cause a lack of traction?
It depends on what your goals and objectives are, if you’re limited by a certain UTQG or ‘class’ of tire, and once you determine what tire you’re going to run, then you would need to look at your tire temps. If you’re overheating your 13×7″ 195 tires, then you will likely improve grip and laptimes by going with wider tires. If you are not getting your tire up to peak operating temperature, then going down on tire size will put more energy into the tire, and get the operating temps higher where the tire will make more grip. So it depends on your current setup and data.
Great article! But just want some clarification when you refer to wheel width are you referring to the width of bead seat to bead seat or the overall wheel width?
I’m so glad I found your article somewhat randomly while trying to understand which “load index” to select. You gave me a set of completely new dimensions to look for and especially to match wheel width and tire width. Spot checking shows that most 275/35R18 track focused tires i was tentatively loooking at are too wide for 9.5″ rims. Thank you!
I think your TTSA and weight ratios comparison chart is wrong. All of the numbers in the mm/lb column are half what they should be. The TTSA used is only for one tire per axle, meaning the TTSA is doubled, halving the mm/lb column.
Take the third example of the Dodge Viper, a 295/25R19 front. That’s a tire circumference of 1,980 mm x the 295 width, I get 584,100 but I don’t know the actual tread width, so close enough for this to your 587,451. That however is one tire, for both front we must double it to 1,174,902 for the front total.
Do the same with the rear and we get a rear total of 1,550,924, add front and rear together for 2,725,826. This should be the number in the TTSA column, not 1,362,913. This means the mm/LB goes from 413 to 826.
Hello, planning a time attack build. Been looking at multiple sources and fitment industries was saying to add an inch to listed wheel widths as typically the bead width and not the full width is listed. I know you’re saying to mark a half inch up for better handling. Just wondering your thoughts on what wheel width to go off of to hit this mark, thanks.