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MG Midget and Sprite Technical - fitting a MGB Speedo

i'm looking into changing the speedo MGB or similar. what is the standard tpm on a midget's speedo?

Dan Cusworth

1000, 1376 or 1472 depending on which ratio diff is fitted
David Smith

thanks, i'm going into a world of pain!

to many variables tyres - diff - ford gearbox and 1000 tpm speedo!
Dan Cusworth

it's only simple arithmetic, which are your constants and which are the variables? The speedo drive is taken off the gearbox output shaft so the 'box is irrelevant surely? Just need the rolling radius of the tyre size, and the diff ratio, compute the shaft tpm and send the speedo off for recalibration, Robert is your uncle.
David Smith

JDO instruments in keighley reclibrated mine quite cheaply. It reads exact according to gps.

how easy is it to remove the ford speedo gear?
Dan Cusworth

why do you want to remove the transmission gear?

as Shawn and David pointed out, you can get the speedo re-calibrated, no need to change anything in the transmission

Norm Kerr

its on the bench & surly i would need to collect the amount of teeth on the speedo, along with tyre height & diff ratio?
Dan Cusworth

I run the Ford box on a black nylon speedo gear (this just plugs into the hole on the opposite side of the box to the speedo output drive), costs a couple of quid. The gears come in several colours, blck, blue, green, grey and I think brown. All change the speedo calibration due to slightly differing teeth numbers, drive angle and diameters affecting how they sit on the helical output shaft spiral drive gear bythe look of it. Far too clever for me to work out :(

I am using the same sized MGB speedo as that fitted normally to The Spridges, no idea what year or series but I needed a 120 mph one and a very kind mate came to the rescue

As many of you know I also rely on my Sigma Cycle Computer for accurate speed measurement on which one rotation of the speed sensor (a small powerful magnet on the brake drum) operates a reed switch after one rotation of the tyre circumference. Simples!

I find that with the black drive gear the MGB speedo runs slightly fast - 30 on my cycle gauge is 32 on the MGB one. This dissimilarity increases with speed but always errs to keep me legal.


It tries ;)

I have used the Ford drive gear with its square drive slot opened out by running a hot piece of BMC (wider square) speedo cable into the drive hole to make it fit

Cheapo? Me?

Oh yes, not half...
Bill 1

Thanks for that, so the speedo drive comes out from the opposite side, i'll take a look. At present, i have a datsun 'box with a 90deg drive fitted, i will try to get that to fit, using the ford digital sender adaptor ebay #Item number: 160682238443. I will turn a lamda sensor blanking bolt down, as im a tight ar5e!
Dan Cusworth

If you haven't seen these things yet here's a few of my stock

I keep a few because "you never know"

the black one has been in Lara for a couple of years now by back before Cjristmas when I put the gearbox in last time it didn't engage with the square hole so I put in its understudy.

The gear is held in by a pressed steel, domed plug which is like a welch washer but with a flange that you get a grip under to pull out

I don't have to take the box out to do mine because I cut the tunnel to allow an access panel and stick the cover to the tunnel with a line of silicone sealant to prevent fume ingress.

I run my speedo cable into the car (behind the crossmember and under the carpet, outta sight!) through another hole on the driver's side which also has a silicone sealed cover panel. The cable then rises to the dash area behind the side trim panel via a cut out at the front and round smoothly to the speedo head


W Mohan

OK, bid and won a speedo on ebay arrived in post today, on unpacking, it has shrunk! either I didn't know they are smaller on MGB or i've been sold a pup?!

Dan Cusworth

What a sod! There're two sizes I understand, think you need an early one but have no idea which it is.

Mine's a B one though, goes up to 120

Bet Dave O'N knows though.

IIRC MGBGTV8s and later r/b models had smaller main dials?
David Smith

Dan Cusworth

eBay it Dan, someone always wants MG stuff

Might get more than you paid for it too, set a decent start price

Yes, rubber bumper MGBs had a smaller (3") speedo (and tacho). You want a chrome bumper one.

Although, that said, sod the MGB one, get an MGC speedo, reads to 140 mph - even more fun trying to get off the clock.
Andy Walbran

My Midget 1500 clock goes to 140mph... is that original? If so maybe that's what you should go for.
Bob T

<< I run the Ford box on a black nylon speedo gear (this just plugs into the hole on the opposite side of the box to the speedo output drive), costs a couple of quid. The gears come in several colours, blck, blue, green, grey and I think brown. >>

Bill1sdgpm: Are these are the gears you mention? I ask as a supplier over here claims that "These are the driven gears for all Ford standard transmissions and all Ford transfer cases with speedometer cables attached", however the type 9 experience here is minimal so I have some doubts as to the accuracy of that statement. Picture shows their 16 to 20 tooth gears, they also list 21 and 23 tooth versions.


Richard Reeves

No Richard those gears are not like the type 9 gears at all.

(except they are made of plastic)

Here's another picture of mine

up above I posted pictures of these posing as WMohan whoever he is :)


This thread was discussed between 14/11/2011 and 28/11/2011

MG Midget and Sprite Technical index

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