Author Topic: TYRE DIAMETER AND SIZE QUESTIONS??? Re speedo calibration (Read 21018 times)

Cameron 77C3 · « **on:** October 23, 2006, 06:43:32 PM »

Has anybody got published data on what the running diameter on a 245/60/15 tyre is. I have measured it but I would like to know the published figure.

Second question. What DIAMETER size wheel did C3's come out with, 15" or 16"?

CAM:grr

nifty · « **Reply #1 on:** October 23, 2006, 07:15:36 PM »

Quote

Originally posted by Cameron 77C3
Has anybody got published data on what the running diameter on a 245/60/15 tyre is. I have measured it but I would like to know the published figure.

Second question. What DIAMETER size wheel did C3's come out with, 15" or 16"?

CAM:grr

easy to work out,.you have a 15 inch rim,with a 245/60 tyre.... so side wall of tyre is 60% of width.
now if only i had a calculater to make it easy for me.

c3's i think came with 15's

Cameron 77C3 · « **Reply #2 on:** October 23, 2006, 07:41:23 PM »

So 60% of 245mm = 147mm and that = 5&3/4"

So I add 15" + 5 3/4" + 5 3/4" = 26&1/2"

So my tire on rim measure 26&1/2".

Many Thanks.

Chromie or smokem, whats that equation again for the speedo gear teeth number?

I'm at home and the equation is at the factory. DOOHH!!

Chromie · « **Reply #3 on:** October 23, 2006, 07:48:50 PM »

Hi Cam;;-)45;-)
All C3's had 15" X 8" rims with the exception of 1968 which had 15" X 7" rims
Tyre size varied through the C3's from 210 X 60 X 15 up to 255 X 60 X 15
and I believe there were some that were 70 series.;-);-);-);-)
So cam why do you want to know
And nifty is correct with his tyre formula;42

Vette Obsessed · « **Reply #4 on:** October 23, 2006, 07:48:55 PM »

Quote

Originally posted by nifty
Quote
Originally posted by Cameron 77C3
Has anybody got published data on what the running diameter on a 245/60/15 tyre is. I have measured it but I would like to know the published figure.

Second question. What DIAMETER size wheel did C3's come out with, 15" or 16"?

CAM:grr

easy to work out,.you have a 15 inch rim,with a 245/60 tyre.... so side wall of tyre is 60% of width.
now if only i had a calculater to make it easy for me.

c3's i think came with 15's

This isn't the published figure but from memory the maths to arrive at a figure goes like this:
245mm/100 = 2.45
2.45mm x 60 = 147mm

147mm x 2 = 294mm

294mm + wheel diameter = actual total wheel diameter.

I'm sure if I've overlooked something one of you guys will pull me up on it. ;42

Chromie · « **Reply #5 on:** October 23, 2006, 07:51:37 PM »

hi cam the formula is
63360 X axle ratio
-----------------------[divided by]
3141.6 X tyre diameter in inches
the figure you get for that is then times by 8. This gives ya the number of teeth needed on the speedo driven gear.
Ok Cam;53

Chromie · « **Reply #6 on:** October 23, 2006, 07:54:15 PM »

Various tyres even though have the same tyre sixe actually can be quite different in height;50
I found that out the hard way;48
when I set a Corvette up with Comp T/A and then fitted yokohama
Bloody different by almost 1"
;30
i WAS NOT HAPPY JANE:20

Cameron 77C3 · « **Reply #7 on:** October 23, 2006, 07:54:35 PM »

Good man Chromie. I'm just double checking the gear that I'm changing on the speedo cable. I have Yoko's on her and they measured with the tape measure 26"

Thanks squire. You're a legend;42

Chromie · « **Reply #8 on:** October 23, 2006, 07:55:50 PM »

yOU ARE SOOOOOOO WELCOME CAM :24
sO YOU SHOUTIN THE DRINKS AT DARLING HARBOUR PERHAPS;mmm

Cameron 77C3 · « **Reply #9 on:** October 23, 2006, 07:58:41 PM »

Sounds fair, since you're buying lunch. :22

Chromie · « **Reply #10 on:** October 23, 2006, 07:59:49 PM »

Done deal cam:spook
I'll tell my other half she is payin;mmm

Cameron 77C3 · « **Reply #11 on:** October 23, 2006, 08:01:30 PM »

Funny....I'm telling Michelle the same thing.;43

Chromie · « **Reply #12 on:** October 23, 2006, 08:02:44 PM »

arhhhhhhhhh great minds think alike:}
Just hope neither of them read the forum;mmm

Chromie · « **Reply #13 on:** October 23, 2006, 08:03:19 PM »

so obsessed you staying very quiet;40

Cameron 77C3 · « **Reply #14 on:** October 23, 2006, 08:05:42 PM »

Quote

Originally posted by Chromie
so obsessed you staying very quiet;40

We do in the tech threads. We keep the jokes to a minimum here.;27

Vette Obsessed · « **Reply #15 on:** October 23, 2006, 08:38:53 PM »

Quote

Originally posted by Chromie
arhhhhhhhhh great minds think alike:}
Just hope neither of them read the forum;mmm

Don't worry Chromie, I'll bring a printed copy of this thread to Darling Harbour.

Looks like I'll get a free lunch (Chromie) AND drinks (Cam) on the 5th. ;41 ;42

nifty · « **Reply #16 on:** October 24, 2006, 10:44:05 AM »

hey chromie....i was once told if i jacked the rear wheels off the ground,and rotated them twice,and counted the number of turns the tail shaft turned,that i would have a fair idea of the gear ratio....
eg. tyres rotate twice,but tailshaft goes just over 4 turns... so probably a 4:11 ratio... anybody tried it? is it gospel?

Chromie · « **Reply #17 on:** October 24, 2006, 11:05:27 AM »

Diff ratio can be measured by
1/ Put a white pen mark of the differential flange (where drive shaft from transmission attaches)
This white mark has to be both on the differential flange and the differential housing so you will know exactly when the pinion has done one full turn (it will be many turns but this will tell you exactly how many turns).
2/ Put a white mark on both rims and backing plates as you want to do only one exact full turn of both rear wheels at the same time.
While rotating the rear wheels count the number of times the drive shaft rotates
1 wheel rotation will end up with just over 3 rotations of the drive shaft is you have a 3.08 ratio
Hope this helps:24
MAKE SURE VEHICLE IS JACKED UP AND CAR STANDS ARE USED CORRECTLY

77CVT · « **Reply #18 on:** October 24, 2006, 11:06:07 AM »

Rotate the tyre once, count the turns of the tailshaft. eg. just on 3 turns = 3.08/3.07 ratio

Chromie · « **Reply #19 on:** October 24, 2006, 11:08:58 AM »

Yes but both rear wheels have to end up with the white marks lining up exactly on both rear wheels or it will not be exact:drink