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MG Midget and Sprite Technical - Type 9 Speedo Drive

1973, 1275, RWA
Just emerged from a long winter overhaul, Type 9 conversion, big valve head, HIF44 on Titan, Maniflow LCB & twin box RC40 all fitted and car is now running great. The speedo drive on the secondhand gearbox I fitted was missing so I was starting from scratch. Following archive advice regarding speedo drives, I fitted a 23 tooth gear wheel. Have just followed my son in his car I have established that at 3000rpm in 5th my car is doing 60mph but the speedo is reading 48mph. The 20mph per 1000revs fits the turbomidget calculation table for my set up. I'm running 165/70/13 tyres, 3.9 rear axle and 0.82 gearbox final drive. I know I need a smaller wheel, 22 or 21 tooth so any advice would be helpful. At 17 per gearwheel from Ford I would like to get it right rather buy a selection.

Cheers,
Ross
R Livingstone

I'm using a brown one (22T) and an MGC speedo. That was within the 10% 'ruling' when it worked, before the worm drive b*ggered off down the tailshaft. I've got 175/70/13s and the same diff and final drive as you. The MGC speedo goes up to 120 and has a different number on it, I can find out what it is later if that helps.

Have you trawled through the archives yet, both in tech, general and the twin cam section? remember your internet string or you'll get lost...

yikes how expensive are those gears?! I bought mine for 7 quid! Alas....
Rob Armstrong

Thanks Rob, I've ordered a 22T from Ford, now £21. Would you be able to advise the MGB speedo reference. There are a few available on ebay just now and it would give me another combination to try.
Ross
R Livingstone

the number on it is

SN 6142/09 and it's got 1000 written on it too

goes up to 120mph

my final drive is 21.2 mph per 1000 rpm with my tyres :)



Rob Armstrong

Rob, I'll try the 22T first and see what it reads before trying an MGB unit. I'm looking for a speedo/drive combination that gives around 20.5mph per 1000rpm in 5th. As mentioned previously the 23T and standard speedo combination reads 12mph low at 3000rpm for my set up.
Thanks again for your help,
Ross


R Livingstone

I've reactivated this thread because I found it in the archives while I was looking for something else.
I have a type 9 'box in my Mk3 Sprite with a 4.22 axle. My speedo is reading low, ie, 32 mph is actually 40mph etc. I was going to send my speedo away to have it recalibrated using the wheel rotation to turns of the speedo cable formula, but according to what I read above I can do it by buying the correct drive wheel from Ford. If this is so, can it be done in situ ( it's a bit tight in the tunnel) or would I be better sending my speedo head away as was my original idea? I have 175/70x13 tyres and the 'box is standard from a 1.6 Sierra.(0.82 final drive). Which size drive wheel would I need?
Thanks.
Bernie.
b higginson

Bernie

Box in the car? ... very difficult as you need to remove the cup/canister device that seals the hole and sets the gear against the drive worm in the box. As that comes out on an upward slant it is in the "awful but maybe" category :(

I have cut a hole in the passenger side tunnel opposite the speedo drive so I can access the drive gear quite easily 4" x 4" does it for me but there's quite a tight fit if you stay as small as my panel

When I have done what ever job I'm doing, it was usually changing drive gears to get nearest the bull on the speedo, I close the box with the steel cup/canister and close the tunnel with a small sheet of steel cut to suit, glued on with silicone sealant

Then carpet back on the tunnel, job done


Bill1

I did mine from underneath, with box installed. Maybe my box is lower than Bill's. But its difficult to get the cup thing out without distorting the edges. They will hammer flat again but you need to use some sort of sealant to avoid oil leaks.

Incidentally, I have always thought that the gears were colour coded. I have just counted a black one with 24 teeth. Somewhere I have a brown and a green one both with fewer teeth, although I cannot be sure how many on each until I find them!

Just thought - one must be in the car!
Guy

An electronic speedo may work out cheaper in the long run! !-)
HALL JOHN

Garmin now do one of those bike computers, but that works entirely off GPS. Supposed to be pretty accurate. It does all of the functions that the ordinary ones do working off a wheel sensor - speed, distance, trip etc. But adds the option to upload your journey to a pc mapping system. Presumably so you can see where it was that you have been - in case you weren't sure!
Guy

Thanks chaps. I have a 90 degree angle drive on my 'box and that was a swine to get the circlip in as the actual device got in the way. Anyway I'll have a look at the type 9 exploded drawing on google then make a decision as to what to do. I wish I had known about the different size wheel method before I put the damn gearbox in.

Bernie.
b higginson

Is the different size speedo drive wheel, ever as accurate as getting the speedo calibrated to the exact revolutions?
Lawrence Slater

Lawrence. That's what I've been wondering. To get it exact, which size wheel would I need. I've just been looking on the net at different applications of the type 9, mainly MGAs and I think that I will probably opt for my original thought of getting the speedo recalibrated, because all I have to do is get the head out and send it away. The drive wheel method may be cheaper, but possibly more of a ball ache and I don't fancy cutting another hole in the tunnel. I bought the angle drive to avoid that and running the cable through the car in the first place, so it would rather defeat the object.

Bernie.
b higginson

I think there's only 3 different speedo wheels available for the T9, a brown one a grey one and a black one, with 22, 23 and 24 teeth respectively

(https://www.burtonpower.com/catalogsearch/result/?q=speedo+drive)

The one I have in the latest gearbox over-reads like a lunatic and I can't remember which colour is is. I have a sad feeling it might be a brown one, disproving the info I gave above.....

It's wrong, whatever it is, paired with the MGC speedo that's got the number 1000 written on it (reads 90 at 70)


I've done mine in the K so that I can change it in situ, but it did mean a hole in the tunnel and a LOT of shouting while fighting the circlip.

Speedo calibration is probably the only way of getting it properly right, If I change the gear drive to a different one I'll let everyone know...
Rob Armstrong

Rob,
There's definitely also a green one (but I don't know where it is!) But I can confirm that black = 24 teeth

Whatever wheel I currently have in mine (possibly the missing green one!), it reads slow. i.e. when it says 60 I am in fact doing around 66 as checked against GPS
Guy

This thread was discussed between 16/06/2011 and 11/12/2011

MG Midget and Sprite Technical index

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