Generally older GA aircraft (as mentioned before) have ASIs in MPH. I teach on Cessna's and of the 12 aircraft in the fleet, three have MPH on their ASIs. Although not entirely correct we use the KT numbers and add 10 to get a rough speed in MPH. For a Cessna 150 this works out well. We also have a number of Arrows and of those two are the Arrow II variant with ASIs in MPH again. The others are newer and are calibrated in KT. For those Arrows the POH has everything in MPH so we use the proper figures from the appropriate POH.