Fuel economy gauge

[staneee]

My fuel economy gauge is not reading right.
It used to go off the scale (past 10MPG) when I accelerated hard and roughly read about 30MPG when cruising on the motorway.

Now it only gets to just below 20MPG when I accelerate hard, and when on the motorway it sits off the scale past 50mpg.

If I set the computer to recalculate when on the motorway it reads about 30MPG so it seems its just the gauge is at fault.

It seemed to have happened after I had a flat battery, Could something in the sw have been reset to Kilometers, so when the gauge thinks I'm doing half my actual speed, which means I'm doing twice as many miles to the gallon.

Any ideas?

Is there some diagnostic computer a garage can plug in and check for this?

[staneee]

just performed the dash display test and the gauge goes nicely from one side to the other. so the gauge is Ok.

Any thoughts? I saw some threads that the dash has some batteries for the SI, thsi can cause the MPG and temp gauges to do silly things. is this true?