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MG MGB Technical - Gearbox and speedo combination

My February 1974 mgb roadster was originally supplied to the USA with a 4 speed no overdrive gear box. It has an 80mm speedo of the 1280 type.

On the original gearbox the speedo cable entered the gearbox via a right angled elbow fitting.

A number of years ago I changed the gearbox to a 4 speed with overdrive, of the top oil filler/ dipstick type. When I purchased this box it had a speedo cable fitted to it. That cable went straight in without an elbow. So when I fitted it in the car I did not use an elbow.

The speedo and mileometer under read by a substantial amount.

I guessed that I might need to change the speedo. However, from the tables I have seen, such as the one at www.teglerizer.com/mgstuff/mgspeedospecs.htm, the 1280 looks as if it may be correct. I am not sure when the change to the 1000 type speedo fell relative to the change to the side oil filler.

So the questions are A) should I be using the elbow fitting from the original gearbox or not? B) was there a period when the 1000 type speedo was used with the top fill ‘box?

Thanks

David
David Witham

David, The elbow is 1:1 ratio so leaving it out should not change anything. I thought the speedometer change to 1000 tpm was made at the same time the oil fill was moved to the side. In a recent thread on this subject Rob Edwards mentiomned a way to verify the tpm of the transmission in your car. I'll repeat it here for your information. Find a straight level drive or parking lot. Mark a section exactly 52.8 feet long. Remove the speedometer cable at the speedometer and fasten a tape marker flag on the cable. Have someone push the car and count the number of turns the cable flag makes between the 52.8' marks. Multiply the result by 100 and the answer is the tpm speedometer you need. You may also want to search the archives for additional information. Your top fill od transmission should have a black ID label on the bottom, however I guess it could have been changed. Hope this helps. Clifton
Clifton Gordon

Thanks Clifton

Pushing the car 53 feet sound like hard work. And then do it again because someone lost count!

Over here we have counter posts every 100m on our motorways so an alternative would be to drive the car for 8km measured against the posts and see how many miles the trip meter thinks the car has tavelled.

I guess if the box needs the 1000 tpm type then the trip will only read 39.1 miles.

If I understand your comments abount the elbow properly, you are saying I have a choice between using one or not. The cable currently has to bend very sharply to go into the box.

Thanks

David
David Witham

David, Sorry, I didn't realize you used metric markings on your roads. Using or not using the elbow should not make any difference in the speedometer readings. Clifton
Clifton Gordon

David,

Although the gearbox may be top filler, what OD is fitted? Black or blue lable under the rear of the unit?

Black lable go with 1280 tpm speedos, blue with 1000 tpm speedos.

Even so it is possible for people to change lable on ODs - the final check is to remove the speedo pinion from the OD ( one 7/16 head screw on a small clamp) - a white pinion = 1280, a red pinion = 1000 tpm.

Finally - it is possible that the speedo needs servicing!
Chris Betson

Chris, thanks for the information about the pinions. I assume they are not interchangable without changing the wheel that drives them?

Clifton, it is only the numbered marker posts on the motorways that are metric. Otherwise road distances are still in miles. Too much trouble to change them all.

Thanks

David
David Witham

This thread was discussed between 16/06/2003 and 17/06/2003

MG MGB Technical index

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