NEW ALLOY WHEELS ADVICE

[fixit man]

Hi,

I am thinking off getting some new alloy wheels to go with bigger tyres , so here's the plan if anyone can tell me if I'm missing anything before I order.

Front 18" x 8.5J ET40 with 235/40 18 tyres ,
Rear 18" x 9.5J ET40 with 265/ 40 18 tyres

The original wheels have ET47

according to a wheel offset calculator , I will have 1mm more clearance on the inside of the front wheel and 13mm extended on the outside.

The rear will be 12mm less on the inside and 26mm extended on the outside !

Any tips advice welcome..thanks !

 

[Dav the wheel nut]

In simplistic terms as you are changing your tyre sizes you are also changing the wheel and tyre circumference.
It is advisable to keep the circumference of the fronts and rears as similar as possible to each other as well as to the originals. This avoids inaccuracies in the speedometer reading.
In order to do this I would fit 225/40/18s on the front and 265/35/18s on the rears. This will keep the difference in the circumference of the wheels to under 1% as opposed to nearly 4% using the tyre sizes you suggested.
From experience the slightly thinner tyre on the front will also reduce the possibility of rubbing.
That is my advice, I'm sure others will have their own. :wink:

 

[fixit man]

In simplistic terms as you are changing your tyre sizes you are also changing the wheel and tyre circumference.
It is advisable to keep the circumference of the fronts and rears as similar as possible to each other as well as to the originals. This avoids inaccuracies in the speedometer reading.
In order to do this I would fit 225/40/18s on the front and 265/35/18s on the rears. This will keep the difference in the circumference of the wheels to under 1% as opposed to nearly 4% using the tyre sizes you suggested.
From experience the slightly thinner tyre on the front will also reduce the possibility of rubbing.
That is my advice, I'm sure others will have their own. :wink:

OK that makes sense, I knew about the bigger rear Tyre affecting the speedo.... I didn't know it's relation to the front wheel/Tyre affects it too.

Did you use some calculator to figure out the ratio's or just mathematics and knowledge ?

Thanks for the info

 

[BRC]

Try this calculator to get a feel for the differences you can expect along with % variation in speedo reading: http://www.rimsntires.com/specs.jsp

 

[fixit man]

Thanks ...

 

[Leesfarm07]

Try this calculator to get a feel for the differences you can expect along with % variation in speedo reading: http://www.rimsntires.com/specs.jsp

What a fantastic link

 

[Dav the wheel nut]

In simplistic terms as you are changing your tyre sizes you are also changing the wheel and tyre circumference.
It is advisable to keep the circumference of the fronts and rears as similar as possible to each other as well as to the originals. This avoids inaccuracies in the speedometer reading.
In order to do this I would fit 225/40/18s on the front and 265/35/18s on the rears. This will keep the difference in the circumference of the wheels to under 1% as opposed to nearly 4% using the tyre sizes you suggested.
From experience the slightly thinner tyre on the front will also reduce the possibility of rubbing.
That is my advice, I'm sure others will have their own. :wink:

OK that makes sense, I knew about the bigger rear Tyre affecting the speedo.... I didn't know it's relation to the front wheel/Tyre affects it too.

Did you use some calculator to figure out the ratio's or just mathematics and knowledge ?

Thanks for the info

To calculate the rolling circumference of the wheel and tyre is simple maths really. Take your 225/40/18 for example.
Firstly one inch =25.4mm so the diameter of the wheel in mm is 18x25.4mm which is 457.2mm
The sectional height of a 225/40 tyre is 40% of 225 which is 90mm
The outer diameter of the wheel therefore is the diameter plus the sectional height top and bottom ie. 457.2mm+90+90=637.2mm
So to calculate the rolling circumference the formula is 0.96x637.2mmxPI(3.14159)=1921.7mm which is the unladen circumference of that wheel and tyre. Simples

 

[andysat]

In simplistic terms as you are changing your tyre sizes you are also changing the wheel and tyre circumference.
It is advisable to keep the circumference of the fronts and rears as similar as possible to each other as well as to the originals. This avoids inaccuracies in the speedometer reading.
In order to do this I would fit 225/40/18s on the front and 265/35/18s on the rears. This will keep the difference in the circumference of the wheels to under 1% as opposed to nearly 4% using the tyre sizes you suggested.
From experience the slightly thinner tyre on the front will also reduce the possibility of rubbing.
That is my advice, I'm sure others will have their own. :wink:

OK that makes sense, I knew about the bigger rear Tyre affecting the speedo.... I didn't know it's relation to the front wheel/Tyre affects it too.

Did you use some calculator to figure out the ratio's or just mathematics and knowledge ?

Thanks for the info

To calculate the rolling circumference of the wheel and tyre is simple maths really. Take your 225/40/18 for example.
Firstly one inch =25.4mm so the diameter of the wheel in mm is 18x25.4mm which is 457.2mm
The sectional height of a 225/40 tyre is 40% of 225 which is 90mm
The outer diameter of the wheel therefore is the diameter plus the sectional height top and bottom ie. 457.2mm+90+90=637.2mm
So to calculate the rolling circumference the formula is 0.96x637.2mmxPI(3.14159)=1921.7mm which is the unladen circumference of that wheel and tyre. Simples

Simples???-that looks heavy math for this time in the morn!-well done!

 

[Leesfarm07]

Funny.. Sitting in that maths lesson all those years ago staring out of the window thinking what is the point I'll never use this pie thingy in my life......wrong. Wish I'd paid more attention now!

 

[fixit man]

Funny.. Sitting in that maths lesson all those years ago staring out of the window thinking what is the point I'll never use this pie thingy in my life......wrong. Wish I'd paid more attention now!

Ha me too !, I could do it but I'd have to google it, I'll never forget PI 3.142, but how to apply it, maths is not my strong point !

 

[fixit man]

In simplistic terms as you are changing your tyre sizes you are also changing the wheel and tyre circumference.
It is advisable to keep the circumference of the fronts and rears as similar as possible to each other as well as to the originals. This avoids inaccuracies in the speedometer reading.
In order to do this I would fit 225/40/18s on the front and 265/35/18s on the rears. This will keep the difference in the circumference of the wheels to under 1% as opposed to nearly 4% using the tyre sizes you suggested.
From experience the slightly thinner tyre on the front will also reduce the possibility of rubbing.
That is my advice, I'm sure others will have their own. :wink:

OK that makes sense, I knew about the bigger rear Tyre affecting the speedo.... I didn't know it's relation to the front wheel/Tyre affects it too.

Did you use some calculator to figure out the ratio's or just mathematics and knowledge ?

Thanks for the info

To calculate the rolling circumference of the wheel and tyre is simple maths really. Take your 225/40/18 for example.
Firstly one inch =25.4mm so the diameter of the wheel in mm is 18x25.4mm which is 457.2mm
The sectional height of a 225/40 tyre is 40% of 225 which is 90mm
The outer diameter of the wheel therefore is the diameter plus the sectional height top and bottom ie. 457.2mm+90+90=637.2mm
So to calculate the rolling circumference the formula is 0.96x637.2mmxPI(3.14159)=1921.7mm which is the unladen circumference of that wheel and tyre. Simples

Very good, makes sense ....I see how you got everything there except 0.96 whats that ? C= D x pi ? maths lessons on the forum now