Speedo difference on tyre change ?

Submitted: Thursday, Feb 01, 2007 at 00:37
ThreadID: 41837 Views:3119 Replies:7 FollowUps:9
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Just changed my original Dunlop Grandtrec AT22's 275/65/17 to CooperATR's LT265/70/17 on my '06 LC 100 Series.
I've looked at a previous link but the actual over-all diameters are different to what I am measuring.
Off the car the the original tyre had a O.D. of 790mm.
The Cooper has a O.D. of 800 mm. Not much, but does any one know what difference the speedo would be ?.
Is there a way to re-correct it ?.
Cheers
Glenn
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Reply By: Richard Kovac - Thursday, Feb 01, 2007 at 00:44

Thursday, Feb 01, 2007 at 00:44
Glenn

I Would make it 1.25% Faster (i think) LOL
not mush to worry about just drop 5 and you will save lives.

or try AISAT INSTRUMENTS PTY LTD Welshpool..
Richard
AnswerID: 218920

Reply By: Member - Doug T (W.A) - Thursday, Feb 01, 2007 at 00:45

Thursday, Feb 01, 2007 at 00:45
Glenn
It only stands to reason that there will be a difference

A 275/65/17 is 31" tall and a 265/70/17 is 31.6" tall so there is not a lot in it when you halve that

Doug
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Follow Up By: GaryInOz (Vic) - Thursday, Feb 01, 2007 at 06:07

Thursday, Feb 01, 2007 at 06:07
That is the "technical" answer, but the "real" answer is given in the manufacturers overall diameter measurement, even then it is not an accurate one as it doesnt take into account the "flat spot" contact ptch on the bottom of the tyre, or the effect of actually rolling.

Some brands are well known to be anything up to about 1.5" (35mm) less than what they should measure (BFG), whilst others are pretty much spot on, within about 3/8" (10mm) or soor actually larger (Simex).

Most useful number is revolutions per km, or revolutions per mile as it is a percentage difference between those two numbers.
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Follow Up By: GaryInOz (Vic) - Thursday, Feb 01, 2007 at 06:10

Thursday, Feb 01, 2007 at 06:10
Edit: not actualy the difference between revs per mile and revs per km, but the difference between the two tyres using one measurement yardstick.
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Reply By: Member - 'Lucy' - Thursday, Feb 01, 2007 at 00:51

Thursday, Feb 01, 2007 at 00:51
Evening Glenn

If you go to the following site, enter the data you have provided above you will find the exact difference re same.

Change the data to suit whatever it is you are trying to calculate and it will do that for you as well.

Site Link
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Follow Up By: Glenn WA - Thursday, Feb 01, 2007 at 01:16

Thursday, Feb 01, 2007 at 01:16
Thanks Lucy,
What an excellent site for tyre conversions.
And a great site for info
cheers
Glenn.
any ideas for correction ?
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Follow Up By: Member - 'Lucy' - Thursday, Feb 01, 2007 at 01:30

Thursday, Feb 01, 2007 at 01:30
Depends on how much it is out.

I have a second set (muddies) for our JEEP which changes things by the needle width of the speedo. (slower)

So I just drive with the needle on the + side of the 100 mark.

If it bothers you, or is to much for that method then either transmission or Instrument specialists can compensate the difference by fitting different gearing on the speedo cable or if it is electronic, then they 'tweak' the sensors somehow.

How costly is that process - I have no idea.

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Follow Up By: Truckster (Vic) - Friday, Feb 02, 2007 at 22:35

Friday, Feb 02, 2007 at 22:35
But as Gary states, these measurements vary from manufacturer to manufacturer.. some 33's (or pick a size) are much bigger than others.. you really need to measure model of tire v model of tire...
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Reply By: Member - Bucky (VIC) - Thursday, Feb 01, 2007 at 07:50

Thursday, Feb 01, 2007 at 07:50
Get a mate, and his GPS.
Set it up to read in km / hr, and go for a drive..
Works a treat, don't worry about getting anything re-calibrated, at worst note the difference, and dot the plastic on the outside of the speedo with white-out, to indicate 100, 80, 60, 40,
Presto all better

Cheers Mate
Bucky
AnswerID: 218938

Follow Up By: greydemon - Thursday, Feb 01, 2007 at 11:12

Thursday, Feb 01, 2007 at 11:12
I think this is a good idea for all vehicles, I did it on my previous vehicle and found that my speedo read very low. It didn't matter much at 60km/hr but by the time I got up to the maximum legal speed of 110km/hr I was actually only travelling at about 100. This error was the right way round to avoid trouble but it was nice to know that I could legally drive at an indicated 120k/h when on long trips and still be legal.

Why haven't I done this on my current vehicle. Inertia. Which is of course a polite way of saying bone idleness.
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Reply By: MickO13 - Thursday, Feb 01, 2007 at 09:25

Thursday, Feb 01, 2007 at 09:25
There is a bloke in Dandenong Vic who makes a unit that will solve your problems. He advertises in all the 4x4 mags. His name and company escapes me at the present but I'll have a poke thru some old mags and see if I can find his add.
''We knew from the experience of well-known travelers that the
trip would doubtless be attended with much hardship.''
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Follow Up By: MickO13 - Thursday, Feb 01, 2007 at 09:27

Thursday, Feb 01, 2007 at 09:27
Ration correction boxes. see this link. Cheers.

www.marks4wd.com/products

''We knew from the experience of well-known travelers that the
trip would doubtless be attended with much hardship.''
Richard Maurice - 1903

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Follow Up By: MickO13 - Thursday, Feb 01, 2007 at 09:29

Thursday, Feb 01, 2007 at 09:29
Try Ratio Correction boxes (To early in the morning after to much the night before!).
''We knew from the experience of well-known travelers that the
trip would doubtless be attended with much hardship.''
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Reply By: Member - Phil G (SA) - Thursday, Feb 01, 2007 at 20:44

Thursday, Feb 01, 2007 at 20:44
Get hold of a portable GPS and see how accurate your speedo is. No amount of calculations will give a better answer. I'll bet that your speedo is more accurate now than it was before.

Cheers
Phil
AnswerID: 219077

Reply By: Member - Glenn D (NSW) - Friday, Feb 02, 2007 at 21:09

Friday, Feb 02, 2007 at 21:09
Hows it going ,

I went from stock tyres to 285/75r16's .

Was very sus at first , checked with gps and were spot on ????

Glenn.
AnswerID: 219347

Follow Up By: Truckster (Vic) - Friday, Feb 02, 2007 at 22:37

Friday, Feb 02, 2007 at 22:37
wish mine was... out 15kph at 110.. eg 125
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