speedo reads 40mph when only doing 30mph

[CHRIS J]

Hi
Have purchased a very nice 1968 Traveller so not sure if it has the correct speedo fitted.
The problem is it reads very high, Checked with sat nav at 30 the speedo reads 40 and at 50 the speedo reads 68
the fitted speedo is 4423/04 1408
Is the 1408 a calibration no. for revolutions per mile ?
And what would be the best way forward?
If anyone can give advice would be much appreciated.

[oliver90owner]

Could be a changed speedo, gearbox, or rear diff. Even wheels! Or any combination of those options.

Maybe someone will know the most typical answer to your ratio of speed:read.

[geoberni]

As Oliver said, could be any number of things.
Take a look at the Potteries Branch website and see if you can work out if any elements are the wrong item.
http://potteries.mmoc.org.uk/Identification.htm

The Speedo Info does have a lot of gaps in it.

[simmitc]

Identifying which components have been changed is a good starting point. However, if everything else about the car is working well and you don't want to change it, then the speedo you have can be recalibrated to suit your current setup - there are several firms who can offer this service. For example, Speedy Cables

[jaekl]

First, you need to confirm the odometer. It is geared directly, whereas the speedo needle is induced. If you don't have a way to confirm the odometer, record the indicated speed at various known speeds and confirm that the difference is proportional. These will tell you if the gearing up to the odometer is correct or not. If the difference is not proportional and gets increasingly inaccurate as the speeds increase, especially as the needle rounds the top of the dial, chances are the needle counterweight is missing.

[oliver90owner]

First, you need to confirm the odometer. It is geared directly, whereas the speedo needle is induced. If you don't have a way to confirm the odometer, record the indicated speed at various known speeds and confirm that the difference is proportional. These will tell you if the gearing up to the odometer is correct or not. If the difference is not proportional and gets increasingly inaccurate as the speeds increase, especially as the needle rounds the top of the dial, chances are the needle counterweight is missing.

Agreed, but before I posted I checked the ratios given - 1.33 snd 1.36 - which were not so far apart (basically within experimental error?). Odometer accuracy can be checked at any speed, of course.

A run down a motorway between, say 16km distance markers, should provide an appropriate odometer reading (10 miles for 16km). Google maps could be another reference from which to check the odometer. Some sat navs provide the remaining distance to destination.

Likely a van differential or a saloon gearbox changed in? Changes from original (cross ply?) tyres to lower profile radials can be considerable.

Cost of checking the differential ratio is zero and minimal effort, so I would start there. As are tyres and wheels.

[Monty-4]

A difference as large as that described would make me think the issue lies with the speedo - even going from a 4.22 to 3.7 differential wouldn't put you that far out I don't think.