Sentence:  One further advantage of the ‘J' type overdrive units is that the <WSD>speedo</WSD> drive gear on the rear drive shaft of the gearbox can be changed, thereby adjusting the <WSD>speedo</WSD>-drive ratio to suit the TR′s 15in wheels. 
Meaning of the word marked with <WSD> tags:  A speedometer, particularly one in a race car or other automobile.