how to solve high usage of fuel in g reg nissan micra?

Several possible causes that come to mind. Dont overlook the obvious, have you got a fuel leak? could be in any of several places between the tank and the carburettor. Next thing is to check the air filter, if its to clogged up the mixture will never be right and you will be sucking fuel. Last thing to check is the mixture itself, adjustment on the side of the carburettor.
Failing all of these, there are heaps of other things, from too high an idle setting, to brakes rubbing, even a clogged or damaged exhaust can increase fuel usage. Being G reg it could of course be a combination of these things. Start with the simple and obvious things, like leaks, blocked/old filters and the like, then move on to the trickier stuff.
Let me know how you go

Mike B
January 2005