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I had toyed for ages with the idea of fitting an overdrive to my Midget. Although this model is fitted with the higher ratio 3. I have also seen references to this conversion on a number of occasions, but have never seen any real details of what work is involved.
The term '3-rail' refers to the fact that this type of gearbox had 3 selector rails in this case in a separate housing bolted to the top of the gearbox. However, because the Spitfire was introduced into the USA earlier than the UK and other European countries some early USA Specification Spitfire 's were also fitted with the 3-rail type gearbox it is possible that some of these were re-imported into the UK. Only 'D'Type overdrive was fitted.
Print this page. This thread is from the archives. For those interested in this and not a T9 conversion.
This coveted option adds a fifth gear, lowering the engine rpm to about at 70 mph. Sales literature during the Seventies suggested dramatic fuel savings by using the overdrive. This is doubtful as the energy necessary to propel the MGB along the highway remains constant, no matter the speed of the engine.
The tranny bolts right up, so that part is easy. However, the body was never meant to take it so there are some modifications to be made. This is a semi-common modification and I have emailed some people who have done it, but I have never actually seen a car with it done so some of this is playing by ear.