My speedometer reads 60mph when I'm only going 40mph. Is there a gear change that I could make where the cable is connected? In fact what does drive the speedometer cable?
The speedo cable should go back to a small 'Smiths' box driven off the rear wheel?
The speedo drive boxes were available in a number of different ratios-so there's a chance your box drive ratio may not match the speedo if the reading is off, or it could be the that the speedo needs an overhaul as they often read fast when they are worn?
Look closely at the bottom of the speedo dial for some small numbers?
For a 1970 T100 MPH speedo those numbers should be
SSM5007/02 and probably
1600?
If so, then the matching drive box should be a Smiths
BG5330/287 2:1 ratio unit (2 cable turns to 1 turn of the rear wheel) You would need to remove the drive unit to find the number and ratio which is stamped on the back, however the ratio can be checked simply by disconnecting the cable from the speedo and carefully counting exactly how many cable turns there are in one turn of the rear wheel?