40 or 45??

patty · Post by **patty** » Wed Aug 20, 2003 1:03 am

i have 215 40 R17 yohokama AVID 5036 tyre on tien suspension but it is a bit bumby

J1 · Post by J1 » Thu Aug 21, 2003 1:34 am

hmm i think bridgestone made a tyre in 225/40/17 for the NSX but then it was over $500+ a tyre... not exactly worth the money in my opinion...?

i've got 225/45/17 but i get guard rubbing for some reason... i'm still using stock springs/shocks!! anyone got any suggestions as to how i can fix this?? i was planning on lowering my car too but now reconsidering cos of the rubbing...

GPXXX · Post by **GPXXX** » Sun Aug 24, 2003 2:24 am

i can think of 2 possible reasons:

1) the inner guards are loose (i had this problem at full lock to the right on my left front corner)

2) the offset on your wheels could be way too deep...

Blaze · Post by **Blaze** » Tue Mar 02, 2004 11:58 pm

It may be uncomfy for some viewers to see this thread being dug up after left behind for a while.
But I just want to clarify (or maybe i m wrong) that 40 or 45 thing we are talking here. It seems you all relate 40 or 45 to be the value of the height, not the ratio of height.
Isn't this value supposed to be a height ratio in relative to the width of tyre?In other words, 205/50/R16 tyre will have the height ratio of 50% of 205mm for the recommended 16"x7" wheel.
Coz i plan for new tyres, that's all. Please correct me if i am wrong. Thanks.

ruchi · Post by **ruchi** » Wed Mar 03, 2004 12:09 am

Blaze you are correct.

Wheels and tyres are a real mismatch of measurements: rim diameter is measured in inches, tyre width in millimeters, and tyre height is a percentage of the tyre width.

Adriano · Post by **Adriano** » Wed Mar 03, 2004 12:22 am

sorry isnt tyre width in millimeters?

ruchi · Post by **ruchi** » Wed Mar 03, 2004 12:35 am

Thanks for picking me up on that... see what happens when you post messages late at night!

I have fixed it up in my original post.

fto12345 · Post by **fto12345** » Wed Mar 03, 2004 1:04 pm

Hey,

Does any one know how to calculate what the difference in speedo will be if you have an after market wheels of different size?

If there is a formular or some thing, that would be great.

Thanks a lot!

ruchi · Post by **ruchi** » Wed Mar 03, 2004 1:48 pm

1) Calculate the rolling circumference of both the standard setup and the new setup.
2) Divide the circumference of the new setup by that of the standard setup.
3) Multiple what your speedo says by that number.

e.g.

standard FTO setup - 205/50 on 16"rims
Rim Size = 16" or 40.64cm
Tyre height = 50% of 205mm = 102.5mm (10.25cm)
As the tyre adds height to both the top and the bottom of the rim, we need to multiply this figure by two (10.25 x 2 = 20.5cm)
So the diameter of the standard setup is 40.64cm + 20.5cm = 61.14cm
Mulitple the diameter by pi (or close enough to) 3.142 and we get a circumference of 192.10cm (61.14 x 3.142)

Lets use a 225/45/17" setup, as some one mentioned above. This would give us a rolling circumference of:
rim size = 17" (43.18cm)
Tyre height = 225 x .45 x 2 = 202.5mm (20.25cm)
overall diameter = 43.18 + 20.25 = 63.43cm
overall circumference = 63.43cm x 3.142 = 199.3cm

standard setup = 192.10
new setup = 199.30
ratio of new setup to old is: 199.30 / 192.10 = 1.037

So when the speedo reads 100kmh, with a 225/45/17" setup you'd actually be doing 103.75 (or enough over the limit to get a speeding fine in VIC)

As a side note, in this example the car would also sit a little over 1cm higher off the ground.

GPXXX · Post by **GPXXX** » Wed Mar 03, 2004 2:12 pm

i'm running 225/45/17s and my speedo is running exactly as shown on the speed traps along Western Hwy... i guess it also depends on what sort of tyre pressures you are running too...

ruchi · Post by **ruchi** » Wed Mar 03, 2004 2:35 pm

Speedos are NOT 100% accurate, tyre pressure can change the rolling circumference marginally, as can temperature and tread wear.

But this highlights how stupid the speeding laws we have in VIC are, as 3kmh over the limit is just way to slim a margin when factors such as the above can mean you are speeding, even though your speedo says otherwise.

fto12345 · Post by **fto12345** » Wed Mar 03, 2004 4:52 pm

That's bizzar....
So it means that the faster you are going the more you have to add on to your sppedo....

So for example, let's just say for some setup gives you the coefficient of 1.040.
Now, if you are going 60km/h then you are actually going at 62.4km/h.
Now, if you are going at 100km/h then you are actually going at 104km/h.

So the faster you go the more you add on.....
Why is that? I'm obviously not a rocket scientist!

Thanks,

ruchi · Post by **ruchi** » Wed Mar 03, 2004 5:04 pm

because it is a percentage - i.e. to multiply something by1.04 is the same as it being 104%

Same as GST being 10%, so at $50 you add an extra $5 and at $100 you would add an extra $10.

FTO338 · Post by **FTO338** » Thu Mar 04, 2004 12:21 am

What i'm aware of about the speedo is that most manufacture actually purposely wind up the speedo by few km's

Another word your speedo might said 100km, but in fact you doing 97-98km.

For those who had/got an european car like BMW or Merc...etc. You would agree with me cause theres two speedo in the car, one is the analogue like any other car, the other is a digital on the trip computer.

In the owner manual it said the digital speedo will display the true speed, while the analogue is few km above for safety reason. Is kinda stupid cause i just read the digital one cause i know its the true speed, & i'm sure all the other owner does the same too.

My FTO also had a digital speedo to display the true speed as well & the analogue seem to be about 2km out.

GPXXX · Post by **GPXXX** » Thu Mar 04, 2004 12:34 pm

Is that from your RSM Kev?

funny, coz whenever i go past the speed traps along the Western Hwy, my speedo and the speed readout from the overhead display are showing exactly the same speed... guess i'm just lucky? hehe..

FTO338 · Post by **FTO338** » Mon Mar 08, 2004 10:10 pm

Yep is RSM, well i guess u lucky hey heheheehehe did u connect via ECU or your Speedo?