Tyre profiles, what do the numbers mean?
30 December 2010 | 03:23 PM
#1
Tyre profiles, what do the numbers mean?
Hi chaps, I presently run 235/40/18s and I am considering running 245/35/18s instead? I was wandering what the difference will feel like dropping from a 40 profile down to a 35 profile?
I have heard that the profile number is percentage of the tyre width > ie a 235/40 should be 235 divided by 100 x 40 = 94mm. I have just measured the width of my tyres using a ruler and yes sure enough it is 235mm wide. I have measured the side wall as best as I can and measured 45mm to the bit of rubber sitting proud (rim protector?) and 50mm to the edge of the wheel rim? So I am guessing the 40% / 94mm is the tyre profile at the top & bottom added together?
So if the above is correct the 245/35 tyre is 245 divided by 100 x 35 = 85.75mm? so there is only a 8.25mm difference in side wall from these two different sized tyres, and then if you work out how much side wall difference there is from the rim to the tarmac it is only half of 8.25mm = 4.125mm.
If my working out above is correct then I am guessing that the 4.125mm less side wall will hardly be noticable?
Is my working out correct? has anyone tried these two different sizes that can advise the driving difference?
NOTE > this is only for track day tyres so I am not bothered about pot holes and the likes
Cheers,
Lee.
I have heard that the profile number is percentage of the tyre width > ie a 235/40 should be 235 divided by 100 x 40 = 94mm. I have just measured the width of my tyres using a ruler and yes sure enough it is 235mm wide. I have measured the side wall as best as I can and measured 45mm to the bit of rubber sitting proud (rim protector?) and 50mm to the edge of the wheel rim? So I am guessing the 40% / 94mm is the tyre profile at the top & bottom added together?
So if the above is correct the 245/35 tyre is 245 divided by 100 x 35 = 85.75mm? so there is only a 8.25mm difference in side wall from these two different sized tyres, and then if you work out how much side wall difference there is from the rim to the tarmac it is only half of 8.25mm = 4.125mm.
If my working out above is correct then I am guessing that the 4.125mm less side wall will hardly be noticable?
Is my working out correct? has anyone tried these two different sizes that can advise the driving difference?
NOTE > this is only for track day tyres so I am not bothered about pot holes and the likes
Cheers,
Lee.
30 December 2010 | 03:29 PM
#2
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From: Slowly rebuilding the kit of bits into a car...
In a perfect world you'd fit a wider tyre to a wider rim... but there is room for manoeuvre !
I run 235/35 's on mine and the shorter stiffer sidewall helps with the turn-in, but as you say this also reduces the ride quality a touch ( for road use ).
dunx
P.S. In your example the car will accelerate quicker, and the speedo will over-read due to the reduced gearing...
I run 235/35 's on mine and the shorter stiffer sidewall helps with the turn-in, but as you say this also reduces the ride quality a touch ( for road use ).
dunx
P.S. In your example the car will accelerate quicker, and the speedo will over-read due to the reduced gearing...
30 December 2010 | 03:50 PM
#3
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From: Working in Belfast and living in Bangor, N'orn I'ron
http://www.tyresave.co.uk/tyreinfo.html
Google knows everything 
windyboy
TYRE SIZES.
Modern tyre sizes consist of 3 numbers: the width of the tyre, the profile (aspect ratio) and the wheel size: so a tyre size 155.70.13 is 155 mm wide, has a sidewall which is 70% of the width and fits a 13 inch wheel. The tyre size is decided by the vehicle manufacturer. More powerful cars require wider tyres to give more grip (to handle the power output and for better cornering ability). Larger cars also need bigger wheels and tyres to handle the weight of the car.
Tyre profiles range from 85% down to 35%. The lower the profile of the tyre the better the road holding when cornering. This is because the narrower sidewall of the tyre has less room to flex. Low profile tyres tend to give a bumpier ride because the tyre has much smaller sidewalls to cushion minor surface irregularities.
Do not fit a lower profile tyre than originally specified unless you also increase the wheel size to compensate for it: so that the rolling radius of the tyre remains the same! Failure to observe this precaution means the vehicle speedometer will be incorrect; the vehicle will use more fuel; the ride will be uncomfortable and the vehicle may catch the ground over "speed bumps". Tyres which are too small may also overheat and your insurance could be invalid.
After the 3 numbers of the tyre size there is usually a number and letter combination which indicate the maximum load per tyre at a certain maximum speed. The load and speed are determined by manufactuer's design and testing based on the tyre size and tread pattern. A coarse pattern will heat up more quickly therefore a lower speed rating will usually apply at the stated load when compared to a normal pattern.
Modern tyre sizes consist of 3 numbers: the width of the tyre, the profile (aspect ratio) and the wheel size: so a tyre size 155.70.13 is 155 mm wide, has a sidewall which is 70% of the width and fits a 13 inch wheel. The tyre size is decided by the vehicle manufacturer. More powerful cars require wider tyres to give more grip (to handle the power output and for better cornering ability). Larger cars also need bigger wheels and tyres to handle the weight of the car.
Tyre profiles range from 85% down to 35%. The lower the profile of the tyre the better the road holding when cornering. This is because the narrower sidewall of the tyre has less room to flex. Low profile tyres tend to give a bumpier ride because the tyre has much smaller sidewalls to cushion minor surface irregularities.
Do not fit a lower profile tyre than originally specified unless you also increase the wheel size to compensate for it: so that the rolling radius of the tyre remains the same! Failure to observe this precaution means the vehicle speedometer will be incorrect; the vehicle will use more fuel; the ride will be uncomfortable and the vehicle may catch the ground over "speed bumps". Tyres which are too small may also overheat and your insurance could be invalid.
After the 3 numbers of the tyre size there is usually a number and letter combination which indicate the maximum load per tyre at a certain maximum speed. The load and speed are determined by manufactuer's design and testing based on the tyre size and tread pattern. A coarse pattern will heat up more quickly therefore a lower speed rating will usually apply at the stated load when compared to a normal pattern.
windyboy
30 December 2010 | 06:46 PM
#5
i have just done exactly that.the only difference though is the 235/40/18 tyres i have are toyo r888's on my track wheels and the 245/35/18 tyres are khumo ku31's on my road wheels.to look at them both there is a very slight height difference on the 40 profile but whether that is down to it being a different make of tyre to the 35 profile i couldnt say.so far with the 245/35/18 tyres i havnt noticed any difference at all with them, and are actually quite good on the crappy roads too.i think the only real way to compare the 2 sizes would be to have 2 sets of the same tyre,but tbh i dont think there is any difference in them really.
30 December 2010 | 06:53 PM
#6
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Tyres which are too small may also overheat and your insurance could be invalid.
Also Im sure that as you went larger it slowed the acceleration down but improved top speed but something posted earlier made me think that is not the case.
30 December 2010 | 07:49 PM
#7
The point is you keep the rolling radius the same while reducing the sidewall The purpose of which is about stiffness, handling putting down power etc etc etc Zzzzzz. 
Rolling radius of OEM 225/45/17 is the same near as damn it as 245/35/18
However, you have reduced the sidewall from 4" to 3.4" in favour of more rim and greater width
http://www.miata.net/garage/tirecalc.html
The purpose of which is about stiffness, handling putting down power etc etc etc Zzzzzz. Rolling radius of OEM 225/45/17 is the same near as damn it as 245/35/18
However, you have reduced the sidewall from 4" to 3.4" in favour of more rim and greater width
http://www.miata.net/garage/tirecalc.html
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30 December 2010 | 08:26 PM
#10
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From: North Yorkshire
Availability, looks (subjective), comfier ride, speedo being closer to displaying its true speed. Are there any serious downsides? I know its not supposed to grip as well but isnt the handling meant to improve i.e. more feel?
31 December 2010 | 01:28 AM
#11
thanks for the feedback fellas, greatly appreciated. I did forget to state that the reason I am considering the 35 profile is because I am assuming that the 245/40/18s will not fit in my rolled arches or they may catch the arms on full lock?As a few know I have an 06 Spec C, I am looking at purchasing a spare set of wheels which are 18x8.5 ET35, I want to fit the fattest tyre possible to the tyres to increase grip, so I did not want to fit a to low profile tyre that reduced grip?
Bigarf > how come you run the 35 profile tyres on the road and the 40 profile tyres on the track, I would have thought (hence the questions above) that the wider tyre with the lower profile would provide the most grip
Cheers,
Lee
31 December 2010 | 09:43 AM
#12
Just wandering, if lower profile tyres are so good at cornering, why do Formula 1 cars have such a big side wall on there tyres. They appear to run smaller wheels with huge side walls
31 December 2010 | 10:11 AM
#13
before i went to the 235/40's i was on the 7.5in wide rims so i stuck to the 225/40's.but then when i bought my 8.5in wide rotas i wanted a wider look and more grip on track so i went for the 235/40 r888s.which i have to say did both jobs just fine.but tbh i dont know whether the r888's just naturally have a high side wall or whether its just becasuse the tyre was a little wider but they did look to have a high sidewall look to them.when i bought the road wheels i just wanted to see how the 245 tyre fit basically but i thought that as i was going with the wider tyre again that the sidewall would look even bigger again.as i didnt want the big bouncy tyre look, i decided to drop the profile slightly which i think has worked out pretty well,as it seems to have given me the profile look of the 225/40 but i still have the extra width and grip from the wider tyre.
Last edited by bigarf; 31 December 2010 at 11:39 AM.
31 December 2010 | 11:09 AM
#14
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From: Slowly rebuilding the kit of bits into a car...
F1 isn't anything like a normal car, it's probably a heat related issue, a larger surface area can dissipate more heat, and the sidewalls provide most of the suspension...
dunx
dunx
31 December 2010 | 11:13 AM
#15
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From: Slowly rebuilding the kit of bits into a car...
The overall change in gearing was almost undetectable.
HTH
dunx
31 December 2010 | 11:50 AM
#16
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What about the insurance side of things? I started a thread in the insurance section but no replies. I was looking to get some Vredesteins today to beat the VAT rise but they come in 225 45 17, will the insurers care?
31 December 2010 | 01:32 PM
#19
I would guess each insurance company may have a different idea on what is acceptable, however I would have thought that as long as you are not making your car more dangerous/prone to having accidents they wouldnt be bothered? I have never given it a second thought tbh, I have informed them though of the increase to 18" wheels.
31 December 2010 | 01:36 PM
#20
before i went to the 235/40's i was on the 7.5in wide rims so i stuck to the 225/40's.but then when i bought my 8.5in wide rotas i wanted a wider look and more grip on track so i went for the 235/40 r888s.which i have to say did both jobs just fine.but tbh i dont know whether the r888's just naturally have a high side wall or whether its just becasuse the tyre was a little wider but they did look to have a high sidewall look to them.when i bought the road wheels i just wanted to see how the 245 tyre fit basically but i thought that as i was going with the wider tyre again that the sidewall would look even bigger again.as i didnt want the big bouncy tyre look, i decided to drop the profile slightly which i think has worked out pretty well,as it seems to have given me the profile look of the 225/40 but i still have the extra width and grip from the wider tyre.
I am assuming the 255's will definitely not fit I noticed when looking at a set of R888's the corner of the tyre from tyre surface to side wall was a lot more angular than normal road tyres. I assume that this is to increase contact patch with the same width tyre. This would make the side wall look bigger I reckon
31 December 2010 | 06:02 PM
#21
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From: Slowly rebuilding the kit of bits into a car...
For a given tyre size there is a preferred spec. of rim, i.e. OEM STI is 7.5" with a 225/45/17
But that tyre is safe on any rim from 7" to 8", if not more...
So unless you go for that wonky Euro-look of a narrow low-profile tyre stretched onto a ludicrously wide rim, then you should be o.k.
If you do change wheels & tyres the weight and speed ratings are pertinent to an insurer, remember these are the people who don't know a rollcage from an ARB.
HTH
dunx
P.S. If you are paranoid try and find the suggested pressure to run a non-std tyre at...
But that tyre is safe on any rim from 7" to 8", if not more...
So unless you go for that wonky Euro-look of a narrow low-profile tyre stretched onto a ludicrously wide rim, then you should be o.k.
If you do change wheels & tyres the weight and speed ratings are pertinent to an insurer, remember these are the people who don't know a rollcage from an ARB.
HTH
dunx
P.S. If you are paranoid try and find the suggested pressure to run a non-std tyre at...
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