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MG Midget and Sprite Technical - D I Y Speedo Calibration

I have recently fitted a 3.9 diff to the Sprite. Initially I thought I would send the Speedo head away to be recalibrated and to cure the flickering pointer. However I solved the latter problem by fixing a P clamp to the cable and securing this to one of the pedal box bolts - this is how it should be. After driving past several of the solar powered speed indicator signs I now find that changing from the original diff to a 3.9, when the Speedo is showing 32 mph I am actually doing 30. This will give me a 2 mph cushion.

Jan T
J Targosz

My frog speedo (with new cable) still flickers a lot. Generally swinging through about 10mph range with the lower end approximating to my real speed, as followed on a GOS phone App. I.e. It swings between 50 and 60 when I am actually doing around 52.

I can live with that but it would be nicer if it was steadier! I thunk I will try the extra P- clip support though. Thanks for the tip Jan.
GuyW

My Frog bounces around too. About a 10 mile range so I drive by the tach. 1800 RPM in 4th is 35 mph, 3000 RPM in 5th is 62mph. This is with the Datsun 5 speed and a 3:90 rear end and 185/70 13 tyres. When I did the gearbox conversion I got the correct drive off of the gearbox to match my setup. It also came with a list of speeds at given RPMs. I can live with the bouncing speedo since my left arm covers it most of the time and I can't really see it. I've also checked the tach/speedo systems against those speed indicator signs and it's close enough to the actual speed. So I'm glad to see that others are using this method.
Martin

There was a relatively brief time when it was quite common to use a bike speedometer triggered off a magnet stuck to a rear wheel or the propshaft. I fitted one up for my car and it worked well. But time and technology fairly quickly overtook that idea and I doubt if anyone still uses that system!

As others, I rely on a bit of simple mental maths when it matters, but the rest of the time I just drive on the engine note and feel of the car. One of the nicest experiences driving these cars on a summer's night is to turn off the interior dash lights and, top down of course, drive on empty country roads just by the tune of the engine.
GuyW

Guy, the P clip secured to the pedal box really helps reduce needle 'float'
Bob Beaumont

Ok, I have added one now. Will expect an improvement next time I am out in it!
GuyW

Jan, your 2mph 'cushion' at 30 will be 4mph at 60. Still better than our Fiat Ducato which is about 8% fast.
Les Rose

Well a trip out yesterday to Kirkby Lonsdale shows a much improved speedometer needle! It now oscillates through less than about 5mph rather than the previous 10. Much less distracting!
Who would have thought the addition of a single P-clip could make that much difference.
GuyW

Sounds like I'll have to give the P clip a go too.
Martin

I came across this problem originally with a type 2 VW Camper. The clutch cable had to be secured in a certain place to stop judder. The clip on mine was missing and pulling away was a challenge until I replaced the clip. I think securing the cable ensures the inner cable rotates smoothly and irons out any cable oscillations. My frog has always had one but I forgot to reconnect it to the pedal box on one occasion and wondered what had happened to the speedo! Normal service was resumed after i realised my error but not before the cable had been removed and re greased and various connections tightened etc.
Bob Beaumont

This thread was discussed between 19/07/2021 and 23/07/2021

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