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Speedo Calibration

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Discussion Starter #1
just wondered peoples thoughts on this, who has had there's done and who think's its a good idea to do or leave as it is.

Its tricky to stick to 30 at times so if by accident you slip over slightly your'e probably really only doing 30 anyway so acts as a safe guard, or is it better to have an accurate reading for precision? Say there's a road you would usually do 35 on, I think you'd still do 35 on it after fixing the speedo, what do ya think?
 

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Cockup Specialist
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12,885 Posts
Funny you should ask that....and rather than start a new thread....

Just went out in the car today to test the true vehicle speed (using gear ratio and rpm)
vs reported vehicle speed by the speedo.
Rather than a 10%+ error quoted by some and the wiki.
My speedo appears to consistantly report the speed as 2MPH faster than the actual speed.

So If I took the wiki advice and allowed the full margin of error I would have a ticket for my troubles.
I guess with some cars, there is fast and there is genuinely fast :worms:

The following is for the 2.2 FN3, although all 2.2 diesels should have the same gearing.

Independent Gear ratios
1st 3.933
2nd 2.037
3rd 1.314
4th 0.975
5th 0.777
6th 0.653
Final drive 3.285

Combines Gear ratios
1st 3.933 * 3.285 = 12.919905
2nd 2.037 * 3.285 = 6.691545
3rd 1.314 * 3.285 = 4.31649
4th 0.975 * 3.285 = 3.202875
5th 0.777 * 3.285 = 2.552445
6th 0.653 * 3.285 = 2.145105
Wheel circumeference = ((225 * 0.45 * 2) + (17 * 25.4)) * pi() = 1992.712mm

Speed per 1000rpm per gear.
1000000 mm/km
1.609 km /mile
60 min/hr
1st 1992.712 * (1000 * 60) / 1000000 / 1.609 / 12.919905 = 5.751 MPH / 1000 RPM
2nd 1992.712 * (1000 * 60) / 1000000 / 1.609 / 6.691545 = 11.105 MPH / 1000 RPM
3rd 1992.712 * (1000 * 60) / 1000000 / 1.609 / 4.31649 = 17.215 MPH / 1000 RPM
4th 1992.712 * (1000 * 60) / 1000000 / 1.609 / 3.202875 = 23.201 MPH / 1000 RPM
5th 1992.712 * (1000 * 60) / 1000000 / 1.609 / 2.552445 = 29.113 MPH / 1000 RPM
6th 1992.712 * (1000 * 60) / 1000000 / 1.609 / 2.145105 = 34.641 MPH / 1000 RPM

Vehicle speed at 2000rpm Tacho (ACTUAL by gearing) vs Speedo (CONNED)
2nd 22MPH 24MPH
3rd 34MPH 36MPH
4th 46MPH 48MPH
5th 58MPH 60MPH

For those that want to test 0-60 times dont use the speedo use the RPM

1st 10,433 rpm :rofl:
2nd 5,403 rpm [smilie=romanjury.g:
3rd 3,485 rpm [smilie=ebil-thumbu:
4th 2,586 rpm :facepalm:
5th 2,061 rpm :laugh:
 

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You military fool you!
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321 Posts
You're also assuming that your tyre diameter doesn't change over time, which it does as your tyres wear. :D So if you have brand new tyres with 8mm tread, your speedo will read lower than if you have fully worn tyres at 2mm. More reliable are GPS readings of current speed.
 

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Cockup Specialist
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You're also assuming that your tyre diameter doesn't change over time, which it does as your tyres wear. :D So if you have brand new tyres with 8mm tread, your speedo will read lower than if you have fully worn tyres at 2mm. More reliable are GPS readings of current speed.
Wheel circumeference = ((225 * 0.45 * 2) + (17 * 25.4)) * pi() = 1992.712mm

Speed per 1000rpm per gear.
1000000 mm/km
1.609 km /mile
60 min/hr
1st 1992.712 * (1000 * 60) / 1000000 / 1.609 / 12.919905 = 5.751 MPH / 1000 RPM
2nd 1992.712 * (1000 * 60) / 1000000 / 1.609 / 6.691545 = 11.105 MPH / 1000 RPM
3rd 1992.712 * (1000 * 60) / 1000000 / 1.609 / 4.31649 = 17.215 MPH / 1000 RPM
4th 1992.712 * (1000 * 60) / 1000000 / 1.609 / 3.202875 = 23.201 MPH / 1000 RPM
5th 1992.712 * (1000 * 60) / 1000000 / 1.609 / 2.552445 = 29.113 MPH / 1000 RPM
6th 1992.712 * (1000 * 60) / 1000000 / 1.609 / 2.145105 = 34.641 MPH / 1000 RPM

Vehicle speed at 2000rpm Tacho (ACTUAL by gearing) vs Speedo (CONNED)
2nd 22MPH 24MPH
3rd 34MPH 36MPH
4th 46MPH 48MPH
5th 58MPH 60MPH

Wheel circumeference = ((225 * 0.45 * 2) + (17 * 25.4) - 12) * pi() = 1955.013 mm
[8mm tread down to 2mm tread = 6mm from radius or 12mm from diameter]

Speed per 1000rpm per gear.
1000000 mm/km
1.609 km /mile
60 min/hr
1st 1955.013 * (1000 * 60) / 1000000 / 1.609 / 12.919905 = 5.643 MPH / 1000 RPM
2nd 1955.013 * (1000 * 60) / 1000000 / 1.609 / 6.691545 = 10.894 MPH / 1000 RPM
3rd 1955.013 * (1000 * 60) / 1000000 / 1.609 / 4.31649 = 16.889 MPH / 1000 RPM
4th 1955.013 * (1000 * 60) / 1000000 / 1.609 / 3.202875 = 22.762 MPH / 1000 RPM
5th 1955.013 * (1000 * 60) / 1000000 / 1.609 / 2.552445 = 28.562 MPH / 1000 RPM
6th 1955.013 * (1000 * 60) / 1000000 / 1.609 / 2.145105 = 33.986 MPH / 1000 RPM

Vehicle speed at 2000rpm Tacho (ACTUAL by gearing) vs Speedo (CONNED)
2nd 22MPH 24MPH
3rd 34MPH 36MPH
4th 45MPH 48MPH
5th 57MPH 60MPH


Yes you are right...my error would change from 2 MPH to 2-3 MPH :rolleyes:
 

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Cockup Specialist
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12,885 Posts
Anyway, if I do a donut on the spot and my speedo says 70..
..what will the GPS tell me i am doing ?
 

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The quest for perfection!
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don't understand why honda decided to make the reading a couple of mph higher. I would prefer precise read out.
 

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You military fool you!
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Most, if not all newish cars read too high, this is to take into account the regulations which state that a speedo can read out too high by 0-10% but never under-read. My civic is always 3-4 mph out, and so was every car I've had since 1995, they were all about 2-4mph too slow. The only exception was a VW camper whose needle bounced about so much you had to take an average reading. ;)

The difference takes into account different tyre/wheel choices on the car, and the wear of the tyres. When I changed my alloys on my old Focus from having 15" steel wheels to 17" alloys the speedo got more accurate to within 1mph.
 
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