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MG MGB GT V8 Factory Originals Technical - Speedometer error

The odometer on my newly purchased 79B V8 reads about 25% high (12.5 miles on a measured 10 mile highway run) And the speedo reads 80mph at 60 (one mile in 59.5 seconds). The code numbers on the speedo are correct for the car, the trans is stock 4spd OD, and the rear is stock MGB 3.9. I know that there are right angle drives, but I thought they were all 1:1.

Can anyone help with some ideas?

Thanks. Allan

You probably have the speedometer gears in your transmission for a higher geared rear end, not for the 3.9; smaller diameter tires shouldn't cause that great a disparity.
George B.

I have a 3.9 diff and calculated that going 3.08 (std v8 diff) would increase the ratio by about 27% so I think George is correct, it looks like you have the 3.08 speedo drive in.
frank swinton

The right angle drives are 1:1.
It sounds like the trans is from a 68-74 chrome bumper B. If you have a trans with a dipstick instead of a side fill - the speedo output will be incorrect for your car. Check in the archives under "tpm" (turns per mile) for a more in depth explanation.
The easiest solution will be to get a speedo shop to recalibrate your speedo, or make an adapter box.
Good luck.
Tom Sotomayor

The correct fix is to replace the speedometer drive gears; recalibrating the speedometer does nothing for the odometer. Building an adapter would be more expensive for most of us than putting the right gears in the transmission.
George B.

Thanks for your help guys. It sounds like Tom may be right, I know that the trans has been changed. It could be from a pre-74 car. I'm assuming that the speedo drive gear in the trans is not replaceable without opening the trans. Is that correct?

The driven gear for the speedo drive is accessible from the outside of the transmission. But, you may need to replace the drive gear (internal - on the trans output shaft) as well. This would entail doing a partial teardown of the transmission. If you have a spare side fill trans to get the parts from, it will be the most elegant/cost effective solution, but it will take a significant amount of effort.
Good luck.
Tom Sotomayor

I also have a new speedometer that reads in this range. This is number two. The first one indicated high. The new ones have a top end of 120 MPH. The original indicated 80 MPH. My MGB is a 1978 and came with a 80 MPH but after having a V8 installed I wanted to have the odometer to be set at zero. I wish I had the original to put back in ! New is not always better. I have the ring and pinion to slow the engine down on the freeway.(3.08:1) I wish I had the Ford rear end though the whineing is not worth the cost !
L Cook

This thread was discussed between 04/06/2002 and 10/06/2002

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