Speedo inaccuracy v differential

[James D]

I have a 1996 reg, 1600k 6 speed to which has a 3.92 diff and 14 inch wheels.  My speedo is rock steady but an indicated 30mph is actually 35/36mph and an indicated 60mph is actually 70/71mph. 

 

The worm drive within the gearbox has been changed to try to compensate for this with a 25 tooth, but to no avail, however I have been reliably informed that if I changed to the 'standard' 3.62 diff that should have come with the 6 speed box that all my worries will be over and my speedo will be accurate or perhaps over read to which I am most happy with. 

 

Question posed, is the a decent resolve or could I have missed another solution that does not involve changing the speedo for digital or running a bicycle speedo for example?  

 

Kind regards,

 

James

[Jonathan Kay]

I'd choose the transmission ratios I wanted first then play with the instruments later...

Other options include:

1. A GPS speedometer, either a dedicated instrument or an app on a smart 'phone. 
2. Keeping the rude mechanicals and adding some accurate speed marks to the face of the speedometer with Letraset or similar.
3. Having the speedometer tweaked eg by John Ostick.

Jonathan

[James D]

Jonathan, thank you for your prompt response. I like the 3.92 diff I have hence my interest in your third point. Thank you I will pursue accordingly.

 

James

[Wrightpayne]

I'm sure there is a company in Nottingham that manufacture geared angle drives to correct speedo issues.

Ian

[Wrightpayne]

Speedograph Richfield LTD

[Jonathan Kay]

As you say. Two previous mentions in the archives, but no direct experiences.

Jonathan

[DJ.]

I may be reading it wrongly, but if you raise the differential gearing won't you be going even faster for a set gearbox speed. I think changing to a higher ratio would make it worse.

[Ged]

ISTR that there 4 tiny switches on the back of the speedo, adjusting these might get you a more accurate reading.

[Jonathan Kay]

I think that James' speedometer is mechanical...

Jonathan

[James D]

Indeed I have a mechanical Speedo. I just could not see how it could not be calibrated to suit rather than defer to a diff replacement. Hope a conversation with John Ostick may resolve but will see.... not unless there is another solution?

[Ged]

The drive from the gear box is mechanical, but my '96 Caterham had the 4 switches.

[James D]

Will check the speedo but went on what I was told by the garage. Out of interest would reducing the quantity of teeth on the speedo drive gearbox increase or decrease the speedo readings please?

[Richard Price]

Reducing the number of teeth on the speedo driven gear will increase the speed reading

[Duckpit]

John Ostick recalibrated mine when I went to 3.91 and it was accurate, I need to send to him again now I have 4.1. Its the simplest way

[LazerBrain]

I have a 1997 car with the same engine/gearbox/diff/wheel combination, and the speedo over reads so there must be the parts out there to make it behave like this. The car did have a kmh speedo at some point before I owned it (Caterham supplied a cert with its previous mileage) but otherwise I'm not aware of any changes that would have been made. The speedo is cable driven.

I fitted a bike speedo, to get something more accurate and less "bouncy" but can understand if you don't want to go down that route.

[James D]

All relatively fixed, had a 22 tooth speedo gear installed within 10 mins. Now an indicated 30mph is 31mph true gps speed and an indicated 70mph is 72mph true gps speed so happy bunny.

[Jonathan Kay]

:-)

Can you tell us who tweaked it, please?

Jonathan

[James D]

Sevens and Classics. Always received a great service, friendly, helpful and pleasure to deal with. So much so I have had four cars from them.

[Jonathan Kay]

Thanks

Jonathan