Hi all,

 

 

does anyone know of a link or have some information of the speedo variation from actual speed to the speed shown on the speedometre when upgrading your rim size?

 

i currently have 17'' rims on my 91 corolla and would like to know approximately how much the speedo is out by?

 

i think i speak for us all when i say, i do not want to be done by a speed camera.

 

any help would be great!

 

thanks in advance

its not the inch size of the rim, doesnt count at all.

 

it the rolling diameter of the tyre which affects the speedo.

 

try kumho tyres website for a calculator that compares the rolling diameter

to your old one.

Here you are this link is a Tire Calculator...You put in what your car comes stock and it will tell you what tire to use for your 17....

 

 

CLICK>>>> http://www.miata.net/garage/tirecalc.html :lolcry:

Quick calculation

 

If you want to know the difference in speed as a percentage (which is what it will be, not a constant number), you use the ratio of the old rolling diameter to the new diameter

 

D = rim diameter (in inches) + 2 * (tyre width in mm * sidewall profile (in percent) / 100)

 

So if you have 17" rims with 205/40 tyres, you'll have a D of 596.8mm

 

Chances are your original wheel/tyre combo was something like 175/65 on 14" rims, or 165/75 on 13" rims

 

In that case you'd have an old D of 583.1mm for 14", or 577.7mm for 13"

 

Thus, your speed ratio is then 102.35% if you originally had 14s, or 103.3% for 13s

So it's around a 2 to 3% increase in speed, thus your speedometer will be reading low by 2 to 3% (which equals 102.35 or 103.3km/h at an indicated 100km/h)

Nice reply Hiro, simply and to the point, I gaurantee that those 2-3% would actually bring the actual speed at 100km/h to 100, because most cars are only doing around 95 at 100.

http://www.carbibles.com/tyre_bible.html

 

about half way down.

 

or read the whole thing, its a good read :lolcry:

Quick calculation

 

If you want to know the difference in speed as a percentage (which is what it will be, not a constant number), you use the ratio of the old rolling diameter to the new diameter

 

D = rim diameter (in inches) + 2 * (tyre width in mm * sidewall profile (in percent) / 100)

 

So if you have 17" rims with 205/40 tyres, you'll have a D of 596.8mm

 

Chances are your original wheel/tyre combo was something like 175/65 on 14" rims, or 165/75 on 13" rims

 

In that case you'd have an old D of 583.1mm for 14", or 577.7mm for 13"

 

Thus, your speed ratio is then 102.35% if you originally had 14s, or 103.3% for 13s

So it's around a 2 to 3% increase in speed, thus your speedometer will be reading low by 2 to 3% (which equals 102.35 or 103.3km/h at an indicated 100km/h)

 

 

your a legend buddy!

just remember that the factory speedo buffer is there for wear and reading compensation,

its a bad idea to make it exact as it leaves no room for any slight error unfortunately.