Speedometer help please

[Donnyrhino]

I have a 1971 mgb gt 4 sync with overdrive on 14" wire wheels with 165/14 tyres fitted. My problem is when I'm traveling at 70mph ish my Speedo is showing 55mph to 60mph, when I'm traveling at 30mph my Speedo is showing 22mph to 25mph. I have bought a new speedometer sn-6144/20A 1280 (the same as the one I have fitted) plus a speedo cable & new gear for the gearbox end.I have been told by someone my Speedo is for a 1968 -1973 4 sync non overdrive mgb plus as my overdrive plate is blue its for the rb mgb. So firstly what's the difference in the speedo's & if I send the new speedo back & changed it for a later rb cars would it read correctly?
Sorry for the long post but I'm going do Lally with it
Thanks

[Vic Butler]

The rubber bumper speedometer is completely different and won't fit the dash. 1280 is the turns per mile of the speedometer. You need to find out that figure for the overdrive speedometer if different or whether the speedometer drive gears are correct.

[Ian F]

A couple of things:
Firstly how are you measuring your "true" speed? A GPS speedometer whilst generally accurate will probably always read less than an original Speedo (whilst traveling along a level road). This is because an original Speedo must, by law, always read accurately or over-read the true speed - most manufacturers therefore set them to read slightly high.
Secondly it sounds as though your B might have a mixture of components fitted? This being the case you need to identify all the components which may affect your road speed/engine speed relationship. A Speedo specialist should then be able to get an accurate instrument for you.

Apologies if you know all of this already!

Ian F

[Peter Cresswell]

It sounds to me that sometime in it's life your car has had a gearbox swap. The chrome and rubber bumper cars had a similar looking gearbox and overdrive unit, but with one important difference - the speedo drive gear fitted to the on the output side of the overdrive! These have 21 teeth for the chrome bumper cars with a matching pinion (blue) for the 100mm (4in) speedos marked 1280 on the face. These overdrives had a black solenoid cover plate. The rubber bumper cars had pinion of 20 teeth and the matching pinion is red and the speedo is smaller at 80mm (Triumph sourced?) and has 1000 marked on the face. So if your solenoid cover is blue you have a mismatch of speedo drive to your speedo of about 20% under reading which is the error you have.
Your choices are:

1. Put up with it and if you have a modern phone there is a app you can download that gives you a fullscreen MPH reading using GPS - but this is only accurate on level ground.
2. Contact either Speedy Cables (https://speedycables.com/) or JDO Instruments (http://www.jdo1.com/) to see if your current speedo can be recalibrated to suit the drive.
3. Get another gearbox/overdrive unit with a black solenoid cover and swap them. You will need to take the engine and gearbox out to do this - and make sure you make how the gearbox crossmember goes together as it is a nightmare of combinations.
4. Get the Overdrive unit speedo gear changed by a specialist - you need a variety of special tools to strip down an overdrive and test it once it is put back together. (I have had great service from OD Spares in Rugby - see http://www.odspares.com/)

If you have fitted the speedo pinion for a chrome bumper car to your gearbox I expect it will soon mash it up as the drive gear is steel and the pinion is nylon.
I nearly made the mistake of fitting a low mileage gearbox with blue cover overdrive to my car last year when I was looking for a replacement gearbox/overdrive. Fortunately the seller put me right before it was too late!

[Donnyrhino]

I've been reading some posts from American owners & they are mentioning gear adaptors to change 1280 yo 1000 & visa versa. Has anyone heard of these? I remember when I had a Japanese Pajero I used an adapter to change it from mph to mph.
Thanks