IDC will ALWAYS be comparable at a given RPM.
What you really need though, is a read out of how long the injector is open for, per minute.
The less it's open per minute, at any given speed, the lower your fuel consumption. But then you can only compare at the same speed. (This removes engine RPM from the mixture)
Then if you want to remove time, you need to replace time with distance, and suddenly, you have a working Fuel Economy gauge.