my wheels argument

[djsaanda]

At the risk of sounding VERY stupid, i need confirmation of what u need to do when u change the size of ur wheels. i am currently running my calibra on 205/55/15's but as i need new tyres all round (v. badly) i thought of changing up to 17's(too poor to go for much more). But my dad thinks this is really complicated and verging on impossible. He's trying to put me off! what exactly do u have to do, ie. how much is it to get the speedo altered, and what else do u need to do?
Sounds a bit thick i know.
cheers.
WEBBY.

[Tom Williams]

in reality all you need do is change the wheels over, new tyres will come with a set of new wheels normally so you simply bolt them on

rollnig raduis isnt affected too much so you wont notice much difference in the speedo

one thing tho

make sure the wheels are 49mm offset

[djsaanda]

cheers 4 ur help, tom.
i thought this was the case but not being ultra clued up and with me dad bending me ear 'bout it, thought i had better put it over to the people in the know first.
right, off to show this to my dad and then splash the cash!

[Adam_Davis]

Any place worth their salt who you approach for new wheels should be able to tell what profile of tyre should match your new wheels so that your new rolling radius is not significantly different from your standard rolling radius which, as Tom says above, will alter your speedo reading.

Given your current wheel and tyre size your options on a 17 inch wheel are something like iirc 215/45/17 or 215/40/17.

Definitely go for the 17 inch wheels as they look really good and really improve the handling of your car.

[arbee]

to work this out you need to understand tyre sizing....
I think this is how to do it.....

you refer to your existing wheel/tyres as 205/55/15s

that is...
15 = external diameter of wheel/internal diameter of tyre in inches
205 = width of tyre in mm
55 = profile of tyre expressed as a percentage of tyre width

Therefore your total diameter (with new tyres) is...
15" + (2x (0.55x205))mm
or
381 + 225.5 = 606.5mm

You need to get as near to this radius as possible for minimal interferance with speedo and tyre scrub etc

We'll also pressume that you want to keep the same tyre width, tho you can work it out for others too.

So....(bit of algebra)....(where 'p' = unknown profile (decimal))


606.5mm = 17" + (2x ('p' x 205))mm
606.5 = 431.8 + (2x ('p' x 205))
174.7 = 2x('p' x 205)
87.35 = 205p
p = 42.6%

unfortunately i don't know of a manufacturer that produces a 17/205/42.6 tyre! so you would have a choice of a 40 or 45 profile

(alternatively try a different tyre width)

I would suggest using the 45 alternative in this case as it will fill the arches better and also because your speedo would be more accurate. we'll put the 45 back in to that formula to see what i mean

431mm + (2x(0.45x205)) = 615mm total diameter

lets compare the two diameters and substite this into pi (3.14 x diameter) to give the distance travelled per revolution

with the 15's = 3.14x606.5
= 1904mm

with the 17s =3.14x615
=1931mm

so where you speedo thinks your car is travelling 1.904 metres you are actually travelling 1.931 metres (an increase of just under 1%)

With original wheels and tyres most speedos over-estimate the speed by anthing from 5-10% so with the 17s the over-estimation would decrease to 4-9%. If this was the case your speedo can now be said to be 10-20% more accurate!

As I said you can play with these formulas to your hearts content and substitute any of the values to give your required width/profile. I'm no expert or mathmatician so if I've got something wrong then I wont be offended if you say! hope it helps tho!

cheers
arbee

[djsaanda]

mate, i am certainly no mathmetician. probably cant even spell the word right. but believe it or not (probably not!!) i have an A- level in maths and was considering a degree in it once! (stop bloody larfing.wot u are saying sounds right to me, but being a car thicko i am in no place to judge ur theory, which i am sure is correct. but ur maths is on the button i think( unless anyone wants to correct me).
sod this, i'm starting to babble. i'm off to just buy some...
being likely to buy some online, the best places i can find are 'tyresave' and 'wheel-base'. can anyone help me any further...
cheers.

[Coppen]

i saw u storming along in yah mighty 1.6 scort...

haha

Just buy some bloody wheels m8 and stop procrastinating!!!!



2nite and some tunes, my head hurts like a bitch this morning

[djsaanda]

yeah mate. and a mighty scort it is too (parp).
U know the cally's in the garage. why dont they deliver right parts to garages??? only sent the wrong sensor twice!
anyways, spect to get it back tomorrow with any luk. u have to give the scort too much punt to get it shiftin'!
c ya

[Coppen]

get some wheels la la la, get some wheels la la la, get some wheels la la la, get some wheels la la la, get some wheels la la la, get some wheels la la la, get some wheels la la la, get some wheels la la la,

u get the gist!!!!