Speedometer read-out

[cavallino]

Would putting on tyres with a bigger or smaller diameter effect the read-out on your speedometer? How does the speedometer get it's information?

[graywolf624]

Would putting on tyres with a bigger or smaller diameter effect the read-out on your speedometer? How does the speedometer get it's information?

There are 2 different systems. Both get their speeds from your transmission. One does it mechanically and the other by electrical signals. Diameter will not change your speedometer. It will be changed if you change the tire wheel radius.(forget the technical term off tip of my head.) Basically when upgrading wheels or tires you want to keep the total space from the ground to the center of the wheel roughly equal. That being said the speedo will only be off slightly.. for real hell with the speedometer change your rear gears.

[TT]

Yes wolf is right of course.. but wolf, if you keep the distance between the center of the wheel and the ground the same, also the diameter won't change... the diameter is just the radius x2, isn't it?

The thing related to the indicated speed is, afaik, the distance covered by a complete rotation of the wheel.. related again to radius ence diameter..

When changing wheels you have to consider that.. of course if you wont go to radical, the global result won't change to much... because the rim will be bigger but the tire will have a smaller sidewall, so usually even with a couple of inches more in the rim, the total diameter ence the space the wheel cover in a complete rotation, could still be the same or almost.. if you pass from 15 to 19 inches well... of course the speedo will be a little wrong

I think I read a nice article related to that on overboost.com (or .net?)

[graywolf624]

also the diameter won't change... the diameter is just the radius x2, isn't it?

Sorry long day.. I read diameter and thought width.. It's been a long day.

[TT]

LOL, will you believe me if I'll tell you I had to read your post 10 times and write mine 2 times because I messed up everything too? LOL.. just to inform you, in math usually I always had notes between 2.5 and 3.5 (on a 1-6 scale... 1 beeing complete crap! ) so for me even simple concepts as square and circles are source of nightmares

[cavallino]

Thanks for the info guys. This is the situation: I currently have 225/60-16 tyres on and I want to change them to 235/45-17. Does the 60 mean 6 cm sidewall and the 45 mean 4.5 cm sidewall? If this is the case, then I think the difference would be about a 1 cm smaller diameter with the new tyres.

[TT]

no, 45 is 45% of 225 if my mind doesn't fail on me

[graywolf624]

no... tires work like this
the first number is the width of the tire
then the number after the / is the size of the sidewall as a proportion of the width.
So 235/45-17 would be a 17 inch rim
with 235 mm width tire and a side wall equal to .45 * 235mm

Still that is damn close.

[TT]

Oh I found the link.. I think this could help: http://www.overboost.com/story.asp?id=1024

[graywolf624]

there are also temperature, tread wear ,speed ratings, type of vehicle, and type of construction information on tires.

Here is more information on tires
http://auto.howstuffworks.com/tire2.htm

[exotic_bugatti]

try here ask.com

[sickx]

225/60 = 135mm sidewall x 2 = 270mm total sidewall height on tire.
16" wheel x 2.54 cm/in = 40.64cm diameter
TOTAL wheel/tire diameter = 676.4

235/45 = 105.75mm sidewall x 2 = 211.5mm total sidewall height.
17" wheel x 2.54 cm/in = 43.18cm diameter
TOTAL wheel/tire diameter = 643.3

Your total error will be 676.4/643.3 = 5.1% error.

[cavallino]

Thanks for the help.

[sickx]

Anytime...btw is that your 355 in your avatar?

[cavallino]

I wish it was