NOTE: This page is a little dated since the release of ECMLink (V3). Much of this information is still accurate, of course, but the specifics of what you enter into the screens is different because this page assumes a V2 application. A better reference for the V3 application can be found in the AFRatioEst section of this page.
The A/F Ratio display item (AFRatioEst) causes more confusion than any other. But it's really very simple. It's basically taking mass airflow data reported from the datalog and dividing it by mass fuel flow data to arrive at an estimated A/F ratio. To calculate mass fuel flow, the display item uses injector pulsewidth data from the datalog and the true (more on this below) flowrate of your injectors along with the specific gravity of the fuel you're running.
What's confusing many times is that you have to TELL the A/F Ratio display item the flowrate of your injectors. It doesn't know and can't assume that the global fuel scalar is correct. You configure the A/F Ratio display item the same way you configure other display items. Right click on the A/F Ratio label in the graph legend and select “Props”. Fuel specific gravity data will be available from your fuel supplier. C16 race fuel, for example, will have a very different specific gravity than regular Exxon pumpgas.