"Real time" fuel consumption measurement

Given ever higher price for gas it would be nice to be
able to monitor ones driving habbits to get the best MPG
posible.

In a EFI engine the fuel consumption is based on the on
time for the injector(s). Has anyone seen any DIYS circuits
for intergrating the injector duty cycle.

I have been playing with a laptop PC versio, but it isn't
too healthy to be watching a LCD while driving.

I am hoping to find a way to reduce this info to a single
analog meter so that all you have to do is maximise or
minimisethe needle position.

Terry