[mwebb]

"Sorry but there is such a gauge, a Vaccum Gauge...

Rich
"

yes of course ,
however
the vacuum gauge has already been invented and in order to use it in the car , you need to run a vacuum line and mount the gauge somewhere ,

but a simple wire from the "load sensor" to a small high impedance voltmeter either analogue or digital samples the signal with the most authority that the ECM / PCM uses for calculating how much fuel to add at almost any given time ,
not counting decel fuel cutoff or clear flood or open loop WOT fuel enrichment conditions ,

better still, but much less simple ;
to just use the calculated load PID in scan data .....
lower is better for fuel economy ....
but most people will not want to have a laptop on the passenger seat as they do most of their driving , and it is awkward and distracting to look to the right seat all the time ...