I've read that snd it makes no sense because it doesn't show me how the tread width affects the rotation of the wheel
face palm!
- Thread starter adam1942
- Start date
So answer me this I have 245 40 18 on the rear of my car and 225 40 18 on the front but then when I measure the outside "profile" of the tyre it measures 40mm on them both? Is this a coincidence?
Ok, to quote the 13" example. With a 155/70R13 tyre, the diameter of the tyre is 547mm.
You want to fit a wider tyre, say a 185.
With a 185/70R13 the diameter is 589mm - quite a lot more.
The 13" hasn't altered, but the 70, instead of being 70% of 155mm is now 70% of 185mm.
The solution is to reduce the aspect ratio.
With a 185/60R13 the diameter is 552mm, which is within 1% of our original 547mm.
You want to fit a wider tyre, say a 185.
With a 185/70R13 the diameter is 589mm - quite a lot more.
The 13" hasn't altered, but the 70, instead of being 70% of 155mm is now 70% of 185mm.
The solution is to reduce the aspect ratio.
With a 185/60R13 the diameter is 552mm, which is within 1% of our original 547mm.
Because the width effects the profile which in turn effects the diameter.
185/50/15........Smaller diameter.
225/50/15........Bigger diameter.
The 50 bit.....in the middle..... it is does not Equate to 50mm, it is NOT a measurement.
It is a percentage of the width.
185/50/15........Smaller diameter.
225/50/15........Bigger diameter.
The 50 bit.....in the middle..... it is does not Equate to 50mm, it is NOT a measurement.
It is a percentage of the width.
No, it is because you aren't very good with a tape measure.
The total diameter of a 225/40R18 is 637mm / 25.09"
The total diameter of a 245/40R18 is 653mm / 25.72"
If we take away the 18" / 450mm of the wheel, we are comparing 7.09" with 7.72" or say 187mm Vs 203mm. This refers to the total height of the tyre, i.e. the sidewalls on both sides, so we divide these by two to get the height of just one side, and are then comparing 93.5mm with 101.5mm. Given the curved edges of the tyres, this is now quite difficult to measure accurately. Plus you have to take account of the amount of tyre inside the rim of the wheel; my 17" wheels measure 18.5" from rim to rim, so we need to deduct a further .75" / 20mm. This gives a figure of 73.5mm Vs 81.5mm.
I am assuming you are measuring at the side, rather than at the bottom of the wheel, as the pressure and compression of the tyre will alter things further.
If you want to confirm the difference a more accurate way, remove one of each and measure the circumference.
Ralphy..... You ever been to Glossop? Things are...... a bit different there....... Time, Space, Physics..... It all goes out of the window 
right heated discussion commence:
ure telling me about all these measurements of the rim width etc etc but what on earth has that got to do with the width of tread on the tyre?
more to the point. if its got a 40 or whatever "aspect ratio" what is this a percentage of? the width? in that case is a 305 35 19 that come off a porsche have a tre side wall width of 106mm? is it a bus tyre? lmfao
if you get a tape measure and measure from the nurling to the edge of the print on a tyre it will show 40. 45 or whatever the tyre might be regardless of the width.
point 2 on my toyota i have 165 70 14 if 70 is a percentage of the width please show me the equation to work this out.
ure telling me about all these measurements of the rim width etc etc but what on earth has that got to do with the width of tread on the tyre?
more to the point. if its got a 40 or whatever "aspect ratio" what is this a percentage of? the width? in that case is a 305 35 19 that come off a porsche have a tre side wall width of 106mm? is it a bus tyre? lmfao
if you get a tape measure and measure from the nurling to the edge of the print on a tyre it will show 40. 45 or whatever the tyre might be regardless of the width.
point 2 on my toyota i have 165 70 14 if 70 is a percentage of the width please show me the equation to work this out.
The aspect ratio is a percentage of the total tyre width, as has been said several times already in this thread.
If a tyre is 215/45/R17 its 215mm wide and the tyre wall is 45% of the total width so 215x(45/100)=96.75mm
Why is it hard to believe a Porsche tyre is 106mm? When the maths add up perfectly! Dont forget there is also the asthetic variable, where huge wheels make the tyre visually look smaller too.
Also a truck/bus tyre may be something along the lines of 315/80/R22.5 so thats 252mm or just short of 10inches in old money.
Because your doing it wrong... the AR is measured from the bottom of the bead to the outer rolling edge. Not from the wheel rim to the edge of the print. (As show in PPT's image earlier on)
Remember some of the tyre (and more than you think) is inside the wheel itself.
165x(70/100)=115.5mm
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The ratio counts for the side wall on each side of the tyre, and includes the beading behind the rim (your 18" rims are probably 19.5" in terms of the exposed wheel), so if you think about a Porsche with a 305/35 tyre, the height of the tyre wall is as follows;
Height of tyre side wall, on both sides of wheel, from bead = 305mm x 35% = 106.75mm
For one side of the tyre = 106.75mm / 2 = 53.5mm
Then deduct the part behind the rim of the wheel, and you are at around 35mm.
The side wall on a 305/35R19 is about the same as a 175/60R19, as follows;
Tyre side wall, both sides, from bead = 175mm x 60% = 105mm (already this proves the argument)
For one side of the tyre = 105mm / 2 = 52.5mm
Deduct the part behind the rim of the wheel, and you are back to around 35mm
On your Toyota, with a 165/70R14 tyre, the height of the tyre on both sides is 165mm x 70% = 115.5mm
For one side, divide by 2, = 63mm, then knock off the part behind the rim.
Height of tyre side wall, on both sides of wheel, from bead = 305mm x 35% = 106.75mm
For one side of the tyre = 106.75mm / 2 = 53.5mm
Then deduct the part behind the rim of the wheel, and you are at around 35mm.
The side wall on a 305/35R19 is about the same as a 175/60R19, as follows;
Tyre side wall, both sides, from bead = 175mm x 60% = 105mm (already this proves the argument)
For one side of the tyre = 105mm / 2 = 52.5mm
Deduct the part behind the rim of the wheel, and you are back to around 35mm
On your Toyota, with a 165/70R14 tyre, the height of the tyre on both sides is 165mm x 70% = 115.5mm
For one side, divide by 2, = 63mm, then knock off the part behind the rim.
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