Cosmic37

Dear Scoobynetters, Relating to a UK 98 Impreza turbo AWD with 17" wheels a tyre dealer has suggested the tyres are wrong. Two dealers have said standard wheel/tyre size for this car is 205/50 16 implying a rolling circumference of 1865mm (and one even said it should be 205/50 15 although they didn't know a rolling circumference).
1) Firstly, confirmation of standard size would be much appreciated.
2) Secondly, I am told that the current 215/40 17s give a circumference of 1840 mm and I would be better off with 205/45 17s to give a circumference of 1879 mm. Is there anyone out there who can advise on how significant the circumference is? A friend of mine says 215/40 17s are better because they provide more grip (wider tyre) and the car will hug the road better. Any opinions and reasons would be much appreciated...
3) The tyre place says all this will affect speedometer reading and potentially four wheel drive and ABS. Now. I'm sure that is true to some extent but I would be interested on views on whether that is SIGNIFICANT? I mean the difference in cirumference is about 1% which does not seem much. Plus a larger circumference surely means the car is going faster for a given wheel rpm and therefore the speedometer is reading lower than reality; not an ideal state of affairs if you want to err on side of caution). I don't know much about ABS and 4WD but I would have thought that these systems would be capable of automatically compensating for a 1.5% change in circumference?

Once again, any advice on any or all of these 3 questions would be gratefully received.

rgv_stu

its usually fine
the speedo will read low anyway, and in theory it will be more accurate
abs / 4wd wont be affected at all
the only thing to worry about is rubbing and you shouldnt get any problem with 215/40/17

Jerry B

Normal tyre wear will account for a difference of more than 1% between new and worn anyway.....

This may help with sizing:
http://www.tyretraders.com/index2.html?lmd=38696.626725

Cosmic37

My own calculations:

rolling circumference, C = pi * (rolling diameter, D)
D=(wheel diameter,d)+2*(tyre thickness, t)
ie C=pi*(d+2*t)
But t=(tyre width, w)*(profile percentage factor,p)/100
So, C=pi*(d+2*[w*p/100])


Real Life Examples (taking pi=3.142 and 25.4 mm in an inch):
1) {REFERENCE} 205/50 16"
d=16*25.4 mm
w=205 mm
p=50
Then C=3.142*([16*25.4]+2*205*50/100)=3.142*(406.4+2*205*0.5)
=3.142*(406.4+205)=3.142*611.4=1921 mm

2) 215/40 17"
d=17*25.4=431.8 mm
w=215 mm
p=40
Then C=3.142*(431.8+2*215*0.4)=1897 mm (98.75% of REFERENCE)

3) 205/45 17"
d=431.8 mm again
w=205 mm
p=45
Then C=3.142*(431.8+0.9*205)=1936 mm (100.78% of REFERENCE)

4) 215/45 17"
d=431.8 mm again
w=215 mm
p=45
Then C=3.142*(431.8+0.9*215)=1965 mm (102.29% of REFERENCE)


My reckoning agrees with www.miata.net/garage/tirecalc.html so it looks like the dealer gave me some wrong numbers!!! I bet he didn't realise I was about to go and do the calculations! hahaha

Boost II

iTrader: (1)

205/50/16 are indeed standard.

Subaru/Prodrive fit 17" wheels with 205/45/17. These have the nearest possible circumference to the 16" wheel and it works out a tiny bit bigger.

The next closest possible circumference to the 16" wheel is 215/40/17 and it works out a tiny bit smaller.

In reality both wheels are close enough to the original that there will be no noticable effect on speedo and gearing etc.

Since 205/45/17 is a fairly unusual size there is not a massive choice of tyres and they tend to be expensive. Most people use 215/40/17 which are cheaper and a bit wider to help with grip/protecting your alloys from the kerb.