I am guessing that if the speedo isn't working that will affect the mpg/average speed readouts on the MFA?
Yep. MPG is distance travelled per gallon of fuel. Distance travelled is derived from the speed (multiplied by time spent driving). The speed reading comes from the speed sensor on the speedo, which in turn comes from the cable connected to the gearbox.
If your speedo cable is bust, then the speedo won't work, and neither will the MPG, average speed or distance travelled.
The fuel usage reading comes from a vacuum sensor on the back of the speedo, which is connected via a vacuum pipe to the fuel inlet.
Getting to the back of the clocks is fiddly, but if you follow the guide on here, it's not too bad. Due to various clocks issues, I've had mine out 3 or 4 times, and can now get them out in about 20 minutes. Make sure you remember which screws go where, as there are at least three different types. And whatever you do, make sure you test the clocks fully before you put the dash back together...