It's pretty accurate the way it's measured presently. Whether you measure the air going in or the fuel injected, the ratio has to be much the same i.e. 14.7 otherwise the stuff doesn't ignite properly so a quick calculation and the job's done.

As there's already the equipment there to measure the air flow (Manifold Pressure x rpm x constant) then it's pointless putting more unnecessary sensors in the fuel line, just something else to pay for and go wrong

Hi Deck,
Yes the "miles to empty" stays steady. I double checked the int mpg again as I was going along a level road at a steady 40 with foot not moving on the gas pedal and the reading was all over the place again.

Well that's not right. It's odd that your "range to empty" is steady as it uses the fuel load and instant mpg to work that out.

As the inputs are MAP sensor, rpm sensor and car speed then there must be something up with one of those inputs. Do you have any other issues like hesitation, stalling at idle which would point to map sensor? Any problems with indicated speed, fluctuating, inaccurate pointing to speed reading problems?