The adjustments could be made in the
mpguino when cylinders are shut off, but only if the same cylinders are shut off every time, the monitored cylinder is never off, and - perhaps most critical - the ecu tells you when it is in power-save mode. Do these cars typically indicate this mode with a LED or such?
If they vary the # of cyl (say, a V8 cuts to 6 for some economy, then 4 for super economy), or if they skip different cylinders to keep the wear even, then you'd have to monitor each cylinder's pulse to get any kind of accuracy.