As pointed out any small air leaks will make it read lean. Slip on collectors are a big culprit for sure. Also as pointed out idle means little to overall performance. Also don't start shooting for an "ideal number" once you use it. ALL combos are diffierent and will usually require something other than that "optimal number" ALWAYS jet for performance then note the O2 readings. Don't get caught up in chasing a number and not pay attention to what the car is doing. One last tidbit Stoich is different for different fuels. FWIW we run the AEM stuff on the dragster and it works fine for us.