Does Tyre width reduction "badly" effect Speedo accuracy ?

[davidsl500]

Has anyone lost a dummy?.. I seem to have found one on the floor.........

 

[rorywquin]

The aspect ratio is and width are the variables, the wheel diameter is a constant, but the other two affect the circumference, and thus the figure displayed on your Speedo for any given rotation…..

How can they affect circumference? The circumference is calculated from the diameter!

circumference = 2πr

r being .5diameter

 

[Mark A]

Diamer and radius both change when you change wall height.

as per earlier post it is a % of width

 

[rorywquin]

Diamer and radius both change when you change wall height.

as per earlier post it is a % of width

Doesn’t matter you still use only the diameter (or radius being 1/2 diameter) to work out the circumference.

 

[Flyinspanner]

How can they affect circumference? The circumference is calculated from the diameter!

circumference = 2πr

r being .5diameter

its the TOTAL radius which will differ for :-

245 x35 x 18 and 245 x45 x18

and will differ between

215 x 45 x 18 and 245 x45 x18

your idea of it only holds true if you don’t use tyres - then you can just use the wheel diameter.

a tyre adds to the overall diameter, and so to the rolling circumference.

(it’s a maths/physics thing).

get a tape and measure your wheel rim diameter …. Then measure the total diameter from one side of tyre to the other = different.

 

[rorywquin]

its the TOTAL radius which will differ for :-

245 x35 x 18 and 245 x45 x18

and will differ between

215 x 45 x 18 and 245 x45 x18

your idea of it only holds true if you don’t use tyres - then you can just use the wheel diameter.

a tyre adds to the overall diameter, and so to the rolling circumference.

(it’s a maths/physics thing).

get a tape and measure your wheel rim diameter …. Then measure the total diameter from one side of tyre to the other = different.

I never said rim. I totally agree that different profiles will have different diameters.

 

[Flyinspanner]

Isn't the speedometer reading taken from the gearbox drive to the driven wheels? So if only the front tyres are being changed then the speedometer will still read correctly.
The only issue may be, as stated in another post, that the cruise control may drop out or a fault flagged if the ABS sensors detect a large discrepancy in speed of rotation between the front and rear wheels.

The reading is of the number of rotations per second, and the gearing to the speedometer is set up for the ‘standard’ wheels.
(as they know what the rotational circumference will be, and that distance multiplied by the number of rotations per time period, will be the speed in mph.

you are correct that just altering the front tyres won’t affect this, but usually this is done by people swapping all 4 to something else when ‘upgrading’.

if you then alter the rotational circumferenc (by altering wheel /tyre size) then you will bring a margin of error into the set gearing for your Speedo.

 

[the FLASH]

I am a bit disappointed that no-one here has grasped how to calculate the overall diameter of a given tyre size.
You have first to convert the rim size to millimetres: an 18" rim is 18 x 25.4 = 457.2 mm
Most folk correctly realise that tyre height is calculated as a percentage of tyre's width.
In this case 40% of 225 ( 90 ) versus 40% of 235 ( 94 )mm.
The overall wheel+tyre diameter is rim diam + ( 2 x tyre height ). This seems obvious, but...
For 235/40 18 tyre overall diameter is 457.2 + 188 = 645.2 mm
For 225/40 18 tyre overall diameter is 457.2 + 180 = 637.2 mm
Hence the 225 tyre has a diameter 637.2 / 645.2 or 98.76 % of the 235 tyre.
The speedo should over-read by a negligible extra 1.24%.
{We don't need to convert diameter to a circumference as this is a constant - all just multiply by Pi.}

 

[Ron240]

I am a bit disappointed that no-one here has grasped how to calculate the overall diameter of a given tyre size.

I am a bit disappointed that you jumped straight into a 17 month old thread to give us a lecture with your very first post, rather than properly introducing yourself.

 

[axelcls]

I am a bit disappointed that no-one here has grasped how to calculate the overall diameter of a given tyre size.
You have first to convert the rim size to millimetres: an 18" rim is 18 x 25.4 = 457.2 mm
Most folk correctly realise that tyre height is calculated as a percentage of tyre's width.
In this case 40% of 225 ( 90 ) versus 40% of 235 ( 94 )mm.
The overall wheel+tyre diameter is rim diam + ( 2 x tyre height ). This seems obvious, but...
For 235/40 18 tyre overall diameter is 457.2 + 188 = 645.2 mm
For 225/40 18 tyre overall diameter is 457.2 + 180 = 637.2 mm
Hence the 225 tyre has a diameter 637.2 / 645.2 or 98.76 % of the 235 tyre.
The speedo should over-read by a negligible extra 1.24%.
{We don't need to convert diameter to a circumference as this is a constant - all just multiply by Pi.}

Hi and welcome ! Don’t be disappointed and don’t disappoint


Sent from my iPhone using Tapatalk

 

[Naraic]

I am a bit disappointed that no-one here has grasped how to calculate the overall diameter of a given tyre size.
You have first to convert the rim size to millimetres: an 18" rim is 18 x 25.4 = 457.2 mm
Most folk correctly realise that tyre height is calculated as a percentage of tyre's width.
In this case 40% of 225 ( 90 ) versus 40% of 235 ( 94 )mm.
The overall wheel+tyre diameter is rim diam + ( 2 x tyre height ). This seems obvious, but...
For 235/40 18 tyre overall diameter is 457.2 + 188 = 645.2 mm
For 225/40 18 tyre overall diameter is 457.2 + 180 = 637.2 mm
Hence the 225 tyre has a diameter 637.2 / 645.2 or 98.76 % of the 235 tyre.
The speedo should over-read by a negligible extra 1.24%.
{We don't need to convert diameter to a circumference as this is a constant - all just multiply by Pi.}

You're a smart arse. I gave the answer in post #10.

 

[the FLASH]

You're a smart arse. I gave the answer in post #10.

I'll gladly own up to being smart; but if you think your post 10 says anything useful, you're the ass!

 

[Blobcat]

I'll gladly own up to being smart; but if you think your post 10 says anything useful, you're the ass!

Keep up with that attitude and your user name will be what you’ll be gone in…

 

[NJS5]

As Mioba said "Easy tiger."