Hi, im only getting about 30mpg out of my 3tj, but not sure why.
I changed the air filter, plugs and set the exup valve, it runs really well and starts first time and ticksover fine
any ideas?

It is not a diesel corsa?

Dunno - is the gearing standard? Might be running higher revs than std for a nominal speed?

What colour were the old plugs ?

Give the carbs a clean make sure the choke is turning Off fully.
Recheck your MPG sums and do them for a full tank.

Std gearing, choke works fine