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Superspec Speedo Over-Reading


alanrichey

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As has been mentioned frequently on the forum the Superspec speedos all over-read, by up to 20%. The reason is that the MT75s that Richard bought were destined for a LHD Transit and were fitted with the 22-tooth (yellow) speedo drive gearwheel. If you do the maths, we actually need a 26-tooth gear, although one 'expert' told me Ford only went up to a 25-tooth. However, these seem to be incredibly rare now and basically unobtainable.

 

Here is the offending article and it struck me that is a prime candidate for a 3D printer to produce a bespoke 26-tooth version, Anyone know of a Company that could reproduce one of these but with more teeth ? (The metal sleeve could probably be removed and used again)

 

Or should I treat myself to a 3D printer ? :)

 

2017_10_09_15_29_38.jpg

 

Al

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Not sure about that, although I see what you mean. As far as we know, the MT75 speedo system was specifically designed to fit a number of different cars and the gear wheels were produced with teeth numbers ranging from 18 to 25 to allow for different ratios. I can't see Ford requiring a different drive system for every type of car. Maybe the tolerances are enough so it can cope with slightly different pitches.

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Isnt a simpler solution to print a new speedo dial face?

My mate who has a JBA bought a set of Cosworth dial faces that matched his very odd combination

of gearbox and diff, to fit his original Sierra dials, after re-calculation the difference needed.

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Plastic can be a fickle material - although it is cheap and readily available. You will need high reliability for your gear-wheel, so I wouldn't recommend getting a 3D printer just to print a gear. If you do, I think you will go through a high number of print iterations and failures before you succeed. But if you are willing to experiment and have plenty of time, then it's worth trying.

-steve

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Your answer seems very logical Alan, I just can't get my head around various teeth numbers without a difference in diameter.

 

yeah i am thinking the same. I can't believe that just changing the number of teeth would help. If you had more teeth they would be smaller and the drive cog wouldn't mesh or less teeth and the drive cog would clunk from too much backlash. I would guess that the gearbox is fitted with a specific drive cog for the application and a matching plastic cog of a specific size.

 

I do remember though that the one on my dads transit (mt75/pinto) was a blue drive cog, not that that really helps as such.

 

I can't do the maths but if it's a left hand drive box would it be set up for Km/hr?

 

Would a Km converter work the correct way or make it worse? http://www.brooksbarn.co.uk/Mechanical-speedo-converter-kph-to-mph_AQQH4.aspx

 

Alternatively fit really big wheels/tyres to get the correct ratio :)

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Nice idea, but the error is not as large as the Km/miles converter.

 

You will find that back in 2005 there were lots of people discussing this as all the Superspec kits came with the Yellow 22-tooth drive (Standard on the Granada). What they should have come with is the grey 25-tooth drive from a Transit. Quite a few seem to have swapped the drives without any problem (that was when the 25-tooth one was available from Ford) to get it through the SVA.

 

See Post #5 at http://www.rhocar.org/index.php?showtopic=21355&hl=%2Bspeedo+%2Bdriveas an example.

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Just seen this quote in an old post:

 

"...The MT75 was designed for many cars with different axle and wheel size. So if you look at the mechanical design the speedo drive cog engages a helically cut gear in the box and can hence can have a variable number of teeth..."

 

Does that make sense ?

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Just seen this quote in an old post:

 

"...The MT75 was designed for many cars with different axle and wheel size. So if you look at the mechanical design the speedo drive cog engages a helically cut gear in the box and can hence can have a variable number of teeth..."

 

Does that make sense ?

ah yes that makes sense now as i presume the plastic cogs are slightly larger or smaller diameter and push in more or less depending on their relative sizes to change the ratio.

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My understanding is that the worm gear on the output shaft is the same for all ratios.

The output gears had various numbers of teeth to give the required ratio.

The gears were made to have a lot of slack to allow easy swapping depending on fitment, helped by the fact that they pass very little power.

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  • 1 month later...

A place on the Interweb can 're calibrate Speedos pretty cheap all you need to do is a few steps to help them work out the 're calibration (on there web site) I can not remember the name of the company but should come up on google.

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