incorrect spedo

[mcpartland]

I have a 1956 split screen moggiethat has a slightproblem. the speedo does not record the correct speed. When I am doing 30 mph it only shows 15 mph 40 mph shows20 mph. I have tryed a couple of other silver faced clocks, but they are all the same. the car has an original gearbox and is on 14 inch standad cross ply wheels, i have also replaced the cable checked the speedow drive in the gear box it looked fine.Does any body have a cure thanks for your help

[Multiphonikks]

Hi


Could you tell us the number that's written on your Speedo? It's the serial number of the speedo and will help identify which version it is

It could be that you're using a later speedo on a car which is using an earlier ratio gearbox/diff. The would tie in with your car reading low.

Failing the serial number, how about a nice piccie of the speedo?

Cheers

Nikki

[Willie]

The serial number is on the face of the speedo so you do not have
to dismantle anything.

[Multiphonikks]

As my partner would say, Willie, "Egg-salad-sactly!" (She's American - it's not her fault )

[mcpartland]

I HAVE LOOKED TO FIND THE SERIAL NUMBER,THE ONE IN THE CENTRE IS SN4407/OO-15O4 (OR COULD BE 1S04)

Thanks for yoyu help

Jimmy

( one morris minnor split screen rolling renevation project and a 167 morris traveller project with rust and wood problems)

[Leyland]

Hi
According to my info, the speedo you quote i.e. SN4407/00 was fitted to Travellers with a 4.555:1 final drive, and chassis No 572907-953663
Hope this helps
Leyland

[mcpartland]

Leyland

can you tell me the serial number of the oe that should befitted to a 1956 split screen

thanks

jimmy

[Willie]

MCPARTLAND... your present speedo is for the 4.555-1 rear axle
ratio. Your car was originally a 5.375-1 axle ratio and if still the same
then you need to refer to your chassis numbers so:
chassis numbers 448801 to 579227 =speedo no. SN4401/9.
" " " " " " " 579228 to 990289 =speedo no. SN4406/08
(these apply if your car is a 2 door/4 door/or a convertible.

[mcpartland]

Willie

MY CHASSIS NUMBER STARTS 114307-- IT SAME WITH A INVIOCE FOR A REBUILT 948 GEARBOX WITH AN 803 REMOTE ANDSTEP REVERSE GEAR SELECTION BOX NUMBER M14294 THE ENGINE IS A 950 BUT I AM NOT TO SURE WHERE TO FIND THE REAR AXLE NUMBERS

THANKS

JIMMY

[Willie]

JIMMY....chassis number 114122 dates from around September 1951.
number 114502 is only slightly later so your car appears to be from
that period. Since you have a heavily modified car re the engine
and gearbox it will be difficult to decide where your problem lies
because you do no know at this stage if the diff ratio was changed too.
There could also be a problem with the speedometer drive gears in
the modified gearbox, as the ratios did vary. Perhaps it is a clue that
your speedo is markedly innacurate when it is the type for a 4.55-1
ratio? Suggests that the ratio is the original 5.375-1 I think.

[Multiphonikks]

You could of course, do it the old way - as laid out in this wonderful smiths speedo book I have.

In my wonderfully unexperienced ( ) minor ownership of a year and some I'd say that this is probably your best bet since we're not entirely sure what gearbox and diff you have... the solution I think may require a bit of planning but it's easily enough done and you can then give the readings to your local callibration specialist so they can gear the speedo your car came with correctly!

1)Make a cardboard pointer - fairly thick as it's going to have to go over the speedo drive from the gearbox.

2)Take out your speedo - it's really easy to do - undo the screw each side of the speedo from inside the gloveboxes and then prise it out - removing the speedo drive cable as you do so.

3) Ensure that your tyre pressure is correct for the tyre - and measure the total radius of your wheel.

4)put a stiky lable on one rear tyre corresponding to a mark on the ground - the mark points down to the mark on the ground.

5) Get someone to sit in the car and while it's pushed, count the number of times the cardboard pointer you made turns round as you push the car forward six turns of the axle (Ie, six complete turns of the rear wheels)

6) Then you can use the following formulae:

FORMULA

1680N
______ = T.P.M. No.
R

Where N = Number of turns made by the inner shaft for 6 turns of rear wheel and R- Radius of rear wheel in inches meausred from centre of hub to ground.

The TPM number is also found on your speedo - however, if we can work out what the TPM number is we can then work out what speedo you should have for this special setup - then you could even give that to a recalibration specialist.

Oh, for anyone who is interested - below is an exact quote (must scan this book when I get a scanner!)

Disconnect flexible drive from Speedometer. With gears in Neutral, count number of turns of inner shaft for six turns of the rear wheels when vehicle is moved forward in a straight line. Measure rolling radius of the rear wheels with tyres at correct pressure- centre of hub to ground. Apply figures in formula below.

FORMULA

1680N
______ = T.P.M. No.
R

Where N = Number of turns made by the inner shaft for 6 turns of rear wheel and R- Radius of rear wheel in inches meausred from centre of hub to ground.


EXAMPLE:

Cardboard pointer on inner shaft rotates 9 1/8 times as vechicle is pushed forward 6 turns of rear wheel. Rear wheel radius = 12 1/4" Flex turns per mile.


= 1680 x (9 1/
_____________
(12 1/4)


= 15330
_______ = (approx.) 1251 = T.P.M
(12 1/4)



Number shown on speedometer should correspond within 32 either way with the T.P.M Number obtained from the above.

[Willie]

JIMMY.. a further complication! The date of your chassis suggests
that your car was an original side valve engine which used a
4.55-1 ratio diff. The easiest way to solve this problem is to let
the speedo specialist re calibrate your one. You need to give them
certain information and they can then sort out your problem. If you
search through the back posts on this site your will find useful info
on recalibration, I think under 'useful tips'.
In your original post you said that the gearbox was original, later you
say you have bills for a rebuilt 948 box with earlier gear lever????