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Speedo discrepancy


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#1 dublinxu1

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Posted 25 July 2020 - 02:25 PM

So,  I decided to take the dash out to change to LED bulbs for the instruments (thanks Dave and JD btw).

While I had the dash out I figured I would get the speedo calibrated as it has read too high ever since i have had the car.

 

Yesterday I went to Otto Instruments here is Brisbane and they put it on the calibration machine and told me it is reading perfect.

 

The thing is when sitting on 100 kph the speedo reads 70 mph, which is 7.5 mph too high. On every speedo calculator

I use, I have the correct gears on the speedo cable and drive gear in the box.

 

205/60/13 tyres

3:36 diff

aussie 4 speed

white gearbox drive gear

yellow 24 teeth speedo gear

 

Don`t know what is going on, any thoughts?

 

Brian

 

http://www.centauria...speedocalc.html

 

Calculator I am using


Edited by dublinxu1, 25 July 2020 - 02:26 PM.


#2 Dr Terry

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Posted 25 July 2020 - 04:40 PM

Don't worry about speedo calculators. The best way is to work out how much 'correction' you need, in this case about 7.5%.

 

Each tooth you change from a 24 tooth gear will change your reading by around 4% or 4km/h  @ 100km/h.

 

Try a 25 or 26 tooth gear, this will get you very close.

 

Dr Terry



#3 yel327

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Posted 25 July 2020 - 04:52 PM

As Dr Terry says. Remember the “correct” gears for a certain diff ratio are also for a certain tyre diameter. Also when you look at road speed vs rpm figures in published Engineering documents sometimes it is confusing but once you understand how cross plies behave at speed it makes more sense. This affects speedo accuracy too. Go and look at old toad tests as well, they normally publish speedo accuracy which are often out and aren’t always linear either.

#4 dublinxu1

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Posted 25 July 2020 - 05:11 PM

OK thanks guys, as they only sell a 25 tooth and not a 26 tooth one that will be close enough.

I'm  guessing that the crossply tyres expanded at speed yel?



#5 V-SLR5000-P

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Posted 25 July 2020 - 05:16 PM

7.5mph on 70mph is more like 10%, but it doesn't really matter. Dr Terry's advice is good, you need a bigger driven gear and a 26 tooth would be better than 25.

 

just saw your post 26 tooth not available, looks like a 25 is the go.


Edited by V-SLR5000-P, 25 July 2020 - 05:18 PM.


#6 yel327

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Posted 25 July 2020 - 05:50 PM

OK thanks guys, as they only sell a 25 tooth and not a 26 tooth one that will be close enough.
I'm guessing that the crossply tyres expanded at speed yel?


From the specs it looks like their growth is far more than radials.

#7 rexy

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Posted 27 July 2020 - 04:53 PM

How did you check your true road speed?

i use my portable GPS and have just learned to read the speedo differently....



#8 dublinxu1

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Posted 27 July 2020 - 05:39 PM

How did you check your true road speed?

i use my portable GPS and have just learned to read the speedo differently....


yep  with a GPS, had the speed all worked out just thought I would try to get it spot on.






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