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MG MGA - 5-speed Sierra/speedo calibration

I have just installed the Hi-Gear gearbox. I am not intending to recalibrate the speedo for the present, and was just wondering if anyone has an idea what the corresponding speeds are compared with the original gearbox/diff setup ..I have a kilometre speedo, but can do a conversion from mph.
Gary Lock

Gary

Look at Hi-Gear's website and the MGA page, it gives the ratios for the before and after. If you have or can borrow a sat nav you can calibrate quite accurately.

http://www.hi-gearengineering.co.uk/

Steve
Steve Gyles

Steve, aren't the numbers shown on that site the drive train gear ratios? Does that translate to a specific relationship with the speedometer drive gearing between the two gearboxes?
Andy Bounsall

Thanks Steve. Maybe I wasn't that clear, but all I want is an idea what speed I am travelling at when the speedo says say 50mph, 60mph etc. (based on the standard Ford gearbox).
Gary Lock

When I did the self same thing I followes my wife in her new car and stuck red tape on the front of the spedo at different speeds. Its realy spot on and probably more acurate than the speedo after it was professionaly recalibrated. I know 5 speed boxes are not original but having done the conversion they should be compulsory. what a difference.

David
David swaine

Gary

It's 6 years or there abouts since I did the conversion and I cannot remember how much the speedo under read before I had it recalibrated, sorry.

I fitted a 3.9 diff this year and the speedo under read by 5-10%, but that does not help you much. However, I found the sat nav quite useful while the speedo was away - saved following the wife!!

Steve
Steve Gyles

When I did the T9 5 speed conversion on my MGA I didn't want to modify the original speedometer. I found that a early MGB speedometer that has the face marked 1040 is very close and as accurate as the original gauge with the 4 speed. The only downside is that the gauge is marked Smiths instead of Jaeger and there is a small difference in the odometer lettering.

I suppose the originality police wouldn't approve of the Smiths gauge but I expect they would be so involved clutching their hearts over the 5 speed they may not notice.

Dave
David Ahrendt

With a SatNav I found that 1400 rpm in 5th was 30mph the rest is just arithmetic I think. This is with a standard diff as far as I know

I hope that helps
Gordon Derham

I have just remembered I did a calibration last summer when we were all boasting about 100+mph cars.

This is what I came up with (its in the archives). the figures are based on a speedo that had been calibrated for the 5-speed box, so it should be simple to read across to an uncalibrated speedo:

" I run my 1958 1500 fitted with the 1800 engine, Sierra 5-speed box and standard 4.3:1 differential. From the data gathered so far I seem to be running about 47/48revs/1mph in 5th. This gives the following (*) so far tested and as best as the eye can judge. The rest are linear assumptions.

Data gathered:
40mph = 1880-1900rpm *
50mph = 2350-2400rpm *
60mph = 2820-2900rpm *
70mph = 3300-3350rpm *

Linear assumptions:
80mph = 3750-3850rpm
90mph = 4230-4350rpm
100mph = 4700-4800rpm "

Steve
Steve Gyles

If you are going to do it with RPM, you need to recalibrate your Tacho first.
dominic clancy

I'm interested to know if anyone has worked out the relationship between the speedo drive gearing on the original MGA gearbox to that on the Sierra box. If you knew that relationship, you could figure out your new speed versus what your old speedo was indicating, and it would be independant of which gear you are in.
Andy Bounsall

Gary

Your original gearbox and the 5 speed are both 1:1 in 4th. The final drive ratio on the Sierra is 0.82. If you remember your old speed vs rpm at any given point you can use the same rpm in 4th with the Sierra and that will give you your correction factor to about the same accuracy as you had before.

Another approach is to use a marked kilometer section on a local road or highway and time how long it takes to cover the distance at a constant indicated speed. Calculate what the actual speed was for that time/distance and compare it to the indicated to get the correction factor.

FWIW

Larry

58 A



Larry Hallanger

I just bought an extra speedo and tack. I also bought an early B electric tack. You mount the face of the MGA tack on the MGB. You have to drill two new small holes to mount it. Take the needle from the MGA and shave the metal off the mounting brass part to lighten it as close to the MGB. Then send it to MO_MA in New Mexico in the US and they put in a new board and controls and it looks like a MGA and works just like the real thing. The speedo is also recalerbrated and reads spot on. Unless someone looks real close, they won't even notice the two small extra holes on the tack.
JEFF BECKER

The five speed box is 1:1 in 4th gear, but the speedometer drive output gearing may be a different ratio, making the speedo cable run at different speed.

I do know all of the gear ratios involved in the standard MGA drive train and speedometer and tachometer drives. I do not (yet) know the output ratio of the speedo drive in any of the five speed gearboxes, but would be happy to post it if someone does know.
Barney Gaylord

Barney

I can supply half the story for the 5-speed box. The speedo drive shaft has 24 teeth (see attached photo).

The second photo also shows a broken speedo cable jammed inside. This caused an engine and gearbox removal a few years ago.

Steve


Steve Gyles

Yes Barney, that's what I was getting at but perhaps you stated it more clearly.

Nisongers procedure for collecting recalibration data has you count the inner speedo cable rotaions over a distance of 52' 9.5". On mine car, the inner cable rotated 10.5 times over that distance with the Sierra box installed. Unfortunately I didn't think to do this test before changing the gearbox. Doing such a before and after comparison on the same car (same rear-end, same tires, different gearbox) would've provided some useful data.
Andy Bounsall

I know where you are coming from Andy. We could all do with degrees in Hindsight!

Steve
Steve Gyles

Andy, - That measured distance is 1/100 mile. Skip the decimal point over 2 places on your 10.5 turns, and you get 1050 turns per mile. A standard 1040 MGB speedo would be within 1% (if you can find or build a cable with proper end fittings to match).
Barney Gaylord

Ahhh that's how that works! Thanks Barney. So IF my original 1440 MGA speedo still has the original internal gearing (which I'm not sure that it does), and IF it was accurate with the MGA gearbox (which I'm not entirely sure it was), then with the Sierra box my speedo is registering...doing the math... about 37% below the actual speed. The odometer would also be off by the same factor, correct?
Andy Bounsall

This thread was discussed between 03/06/2007 and 06/06/2007

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