Speedo calibration improved - can I get it any better?

[Smithy77]

Hi all,

I have done lots of reading of the archives etc which has helped me improve the accuracy, but I'm wondering if I can get it any closer to bang on. Previously, the speedo seemed to read 10mph too fast pretty much constantly between say 40-80mph. Now having played with the settings it is now reading 5mph too fast across the same speed range. Testing was done using a speedometer app on my phone.

First off, I have an electronic speedo with the 4no. dip switches on the back. I understand the other varieties of electric speedo either have 10no. dip switches or no switches with the calibration set using the trip button on the front.

Q1. On the Caerbont website they have fitting/calibration instructions for the 10 dip switch speedo and the no dip switch speedo variants, but nothing for the 4 dip switch version like mine - why is this?

So the Caerbont website was of little use, but this old thread was very useful, specifically the following:

If you look at the back of the speedo under the rubber grommet, there should be a row of 4 dip switches labeled 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. - See more at: http://www.lotus7.club/forum/techtalk/speedo-inaccuracy#sthash.DOzWSx6u.dpuf

So looking at my switch positions, I found that they were originally set to the 'low' setting as noted above. I have now switched all switches to the opposite setting to give me the 'high' setting as above. This improved the accuracy to 5mph too fast - some would be happy with that but is it too much to want better accuracy?

Q2. Is it possible to fine tune the calibration on this type of speedo, or are there just two settings being 'high' or 'low'?

With 4 switches one would think that there must be other settings, somewhere in between 'high' or 'low', by flicking the switches in any other combination....

[Mark w]

I spent 7 hours on the motorway on Thursday ( driving back from Dijon ) which gave me plenty of time to think about things ! 

At an indicated steady 80 mph on the speedo my Tom Tom was showing 78 . I thought that was pretty good for a Caterham speedo which are normally way off ! 

[Smithy77]

2mph out - see that I'd be happy with!! That's about normal for any other production car

[CharlesElliott]

As far as I know, Caerbont made the speedo for Caterham with limited calibration options.

I'm sure if you spoke to them they could make it spot on, depends whether you want to pay the £100 odd they would charge to do it.

[Jonathan Kay]

2mph error is pretty good.

But needle on dial speedometers are a very poor design for display. As it happens there's a fix that improves both the accuracy and the speed and ease of reading. Find the correct dial position for the speeds of interest, eg 30 mph, 50 mph, 70 mph using a GPS. Then mark the dial with a radius line using Letraset or similar. It's much easier to spot the needle in relation to a line like that than to a tag mark and number on the outside. This can also help when a small steering wheel hides some of the dial.

Jonathan