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Speedo inaccuracy


Sue Dinnim

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Just a quick thought knowing that I see a lot of posts about the accuracy of Caterham speedo's.

If the speedo over reads how fast you are going would that not mean it shows you are doing more miles than you actually are? If so where does that leave you on a limited mileage insurance policy because you will have done less miles than your speedo says.

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On the principle of only needing a speedo in built up areas - ie not making progress - would it not be a great idea to have one that you culd switch off remotely. Plod won't twig it and it is there for MOT's etc - just don't show them the magic switch.

 

This would then not cover areas of Golf Lima Foxtrot (be it roads or track)!

 

In reality, the speedos are not that inaccurate unless mine is unusual. My point of reference is the 'slow down' signs that are triggered by radar. Assuming that they are correct, the lights go out when I decelerate (nasty idea) down to the displayed figure - although I havbe only tried this in 30 and 40 mph zones.

 

Low tech luddite - xflow and proud!

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Hmmmm

 

I have an electronic speedo (std caterham) with 16" 205/45's

 

Up to 70 is is the same as the sigma bike speedo readings.

 

After that things get a little strange. the speedo under reads.

 

Check for a posting by me a week or so back for details of the different readings

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Well I started out down the route of being miffed about losing 'real' miles on my limited mileage policy - and fitted a bike-speedo here...

 

...but then I found out that the increments on my policy for each extra mileage band were not horrific (£20 ish for each extra 2.5k above 5k on my MSM policy - until I broke the unlimited threshold) and stopped worrying about it.

 

I did fiddle with the dip-switches on the back of the speedo fairly early on too - and with my 185/60R13 RE720s, the speedo is pretty much bang on (to the extent that I've 'lost' only a few hundred miles over the 13k+ I've owned the car. However, with my 175/55R13 CR500s, I couldn't get much better than a 7%ish inaccuracy (this was measured using the odometers on the speedo and bike comp - much more accurate comparison than at any particular speed - but as Mav has found out, the accuracy can vary according to speed anyway - so my figure is a weighted-average).

 

 

 

Keep BC free and open for ALL. Membership No. 43xx

 

Alcester Racing 7's Equipe - 🙆🏻

 

Alcester-Racing-Sevens.com

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Right,

 

I was told (by Mike Molloy):

If you look at the back of the speedo under the rubber grommet, there should

be a row of 4 dip switches labelled 1 - 4.

Switch 1 sets the speedo cal to high or low ranges; switches 2 - 4 set the

odometer cal.

 

I would suspect that your car is still set to the configuration that would

have got it through the SVA check wearing CR322s.

This would be the "low" setting:

Sw1 = OFF, Sw2 = ON, Sw3 = OFF, Sw4 = OFF.

 

Try changing to the "high" setting

Sw1 = ON, Sw2 = OFF, Sw3 = ON, Sw4 = ON.


 

On 175/55R13 CR500's this appears to have resulted in:

The dip settings you gave me are an improvement - but aren't perfect unfortunately.

 

I just did 64.13 miles according to my bike computer - this came out as 66.3 miles on the odo (i.e. 3.4% overreading - I was getting about 7-8% before).

 

The speedo was a bit harder to get an accurate crossreading - but 40mph (indicated) appeared to be about 38.5 (3.9%) and 60 was about 56.5 in reality (6.2%). I'll try and get a better reading sometime soon on one of the bypasses. These figures are an improvement over the average of 10% I had before.


 

Hope this helps - but there are so few settings that you can't go too far wrong...

 

Keep BC free and open for ALL. Membership No. 43xx

 

Alcester Racing 7's Equipe - 🙆🏻

 

Alcester-Racing-Sevens.com

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  • 3 weeks later...
  • Area Representative

*confused*Speedo dip switches

 

Myles, is this the only information you have for setting the dip swicthes? Do you (or anyone else) know where one get the full details?

 

There are other combinations of the switches, so do they make any difference?

 

Currently, my speedo is reading 40 at 33ish! And wavers generally!

 

Adil

 

 

Edited by - Adil on 5 Jul 2004 16:38:51

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  • 9 years later...

Quoting evotell: 
My speedo is reading 80KM when in reality the cars doing 120KM according to my GPS. 😳That's outrageous, any suggestions would be most appreciated.

Terry, if you have the new type of Caterham/Caerbont electronic speedometer, then you can use this procedure to adjust the calibration here.

 

When in calibration mode, note the value that is flashing (the number of pulses per mile or kilometre), then determine the number of pulses that the value should be by taking current_setting multiplied by displayed_speed divided by actual_speed.

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What sort of speedo do you have ..... older = mechanical drive, in which case it's down to the speedo drive gear in the side of the gearbox.

 

Newer = various types of electronic speedo. One with 4 DIL switches (like my 2002 car), one with ten switches, and one with nothing internal, but the ability to programme a 4-digit code from the rubber button in the display glass.

 

On mine and a 2003 car I worked on last month, the 4 switches were all set the wrong way. Reversing them brought the accuracy in line with a GPS speedo.

 

plenty of threads on these settings if you do a search .... perhaps for 'speedo and setting' or 'speedo and switch'

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Quoting SM25T: 
one with nothing internal, but the ability to programme a 4-digit code from the rubber button in the display glass.

 

Ooh!! Mine has no dip switches so I'd just assumed it was not adjustable, however it *does* have said rubber button on the front.

 

Do you have any reference docs on how to reprogram this type?

 

Mine reads 60mph at 50, which is good for reference, but is a 20% over-read which would be nice to be able to correct.

 

Thanks in advance!

 

*smile*

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If anyone wants them, I've got the docs (I believe from Caterham) with the settings for the 10 DIP switch version of the speedo (MPH only), and the electronic newer style - sensor on rear wheel (MPH and KMH)

 

Just BM me with which one you'd like.

 

I used them plus a dual carriageway, to get my electronic one so that it matched it's speed reading against Tom Tom & Garmin Satnavs.

 

Obviously, you use at your own risk etc etc 😳

 

Perhaps it's something that should live on BlatChat as a reference point, as this topic comes up fairly regularly 🤔

 

Cheers

 

Mike

 

Edited by - michaeljclark on 15 Apr 2014 11:32:05

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Quoting evotell: 
My speedo is reading 80KM when in reality the cars doing 120KM according to my GPS. 😳That's outrageous, any suggestions would be most appreciated.
What year/model is your car? If you have the latest programmable speedo (no dipswitches), I can send you a list of the 6-digit program codes for a variety of tyres and wheel sizes. (I think the list came from CC originally.)

 

JV

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Quoting John Vine: 
Quoting evotell: 
My speedo is reading 80KM when in reality the cars doing 120KM according to my GPS. 😳That's outrageous, any suggestions would be most appreciated.
What year/model is your car? If you have the latest programmable speedo (no dipswitches), I can send you a list of the 6-digit program codes for a variety of tyres and wheel sizes. (I think the list came from CC originally.)

 

JV

 

Hi John

Thanks for your offer to send me the appropriate code. I spoke with Derek at Caterham and he sent me a code that I have inserted into the speedo. Once I drive it we will see. 😬

 

Thanks

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