The I/M 240 test ("Inspection/Maintenance" and the "240" is the length of the full test in seconds, though clean cars will usually terminate early) measures emissions is grams/mile. I/M 240 was developed by the EPA and is the suggested test for non-compliant air quality areas. Setting up a I/M240 test cell is very expensive, so California developed the ASM (acceleration simulation mode) test, which requires much less capital investment and sold it to the EPA.

The ASM test "simulates" acceleration by testing the car at 15 and 25 MPH using more load than is required to maintain speed on a level road, and gas measurements are in PPM (.0001%) and percent.

There is not an exact correlation between I/M 240 emissions and ASM emissions. A car could pass one and fail the other, and I have some issues with the state of California regarding how they select the pass/fail "cutpoints" on the ASM test.

Duke