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MG Midget and Sprite Technical - Speedo Calibration

With my 5 Speed box plus angle drive the Speedo now shows 28mph when I am doing 30. If it was the other way round I would have left things as they are but I would like to have a true reading. Can I dismantle the speedo head, pull off the needle and put it back in the "correct" place. Alternatively is there a way of tensioning the return spring. If it needs specialist attention I will live with white dots on the speedo glass.

As a further comment I am using an angle drive fitted into the Ford box with the circlip and the standard Spridget cable to the speedo. There are no sharp bends, just a large loop under the bonnet and the needle is absolutely rock steady. With the 4 speed box there was always needle flicker.


Jan T
J Targosz

Jan- how many teeth do you have on your speedo gear at the gearbox--there are several available from memory 21-26 teeth----You need one with 2 less teeth than what you have in there at the moment.
willy
William Revit

HI Willy,

Thanks for the response. I have no idea how many teeth there are on the speedo gear and to find out would mean either removing the engine/gearbox or cutting a hole in the transmision tunnel! I should have checked the speedo impications before I did the 5 Speed conversion.

I fitted a 5 speed box to my MG TF and had the speedo reconditioned and calibrated by Richfields. It did need repair and the face was very faded. The one in the Spridget is in excellent condition and a spend of £100+ is an extravagance. Looks as though white dots on the speedo glass will be the answer. I note you live down under any may not know 20mph zones are now common in the UK. I will need to dot 20/30/40/50 on the speedo. I don't think I will need to bother about 60/70.

Hope you are not too smug about the rugby

Cheers

Jan
J Targosz

I have just partly dismantled a MGB speedo that has been in the attic for years, just to "see what's inside".

I note that the stop for the needle can be pushed down and the needle returns to its base position. I could pull the needle off its pivot and move it slightly so 30mph is correct but would this affect other speeds. Since my speedo is reading slightly under I wonder if I filed a wisker of material from the aluminium drum this would correct the reading. If it had been over reading I could have dropped a spec of super glue onto the disc.

Jan T
J Targosz

You can reposition the needle, but as Willy says you need to be sure of the ratio first. If you have that right, I would be surprised if the needle position is wrong unless the speedo has been dismantled and wrongly reassembled. I did strip mine down, generally to refurbish it, and found that putting the needle back is an absolute pain. The return spring is pre-tensioned so that the needle rests against the stop pin. Hence you need to position the needle below the stop pin by the correct amount to pretension the return spring. I have the dial on mine marked with the position for the needle. The next bit is really tricky. Having pushed the needle onto the spindle in the correct position, you then have to lift it over the stop pin without breaking it. I ended up slightly shortening the stop pin to make this easier.

I have seen some instructions which say that the drag disc behind the dial is marked with the correct position for the needle. This may well be the case, but because of the trip meter spindle and knob it's impossible to get the whole speedometer movement in and out with the dial in place. So you have to get the movement back in, then fit the dial and then the needle, by which time you can't see the drag disc. The frustration is that there is no substitute to repeated trips up the road to see if you have got it right, interspersed with multiple attempts to get the needle in the right place. That's why I have the mark on the dial. I really hope you don't have to do this!
Les Rose

Have you checked the speed reading against a GPS or radar speed signboard? My midget speedometer and odometer always read high with the ribcage so the new readings may actually be accurate with the Ford box.

I had the gearing changed to reduce the readings and popped the needle off and reinstalled it at 60 MPH. Both the speedometer and odometer still read a mile or so high at causing speeds but that is close enough. Beats the previous 12% Error with R155 tires. It must have been worse with the original R145 tires.
Glenn Mallory

This thread was discussed between 14/10/2023 and 27/10/2023

MG Midget and Sprite Technical index

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