diff --git a/Machines/Apple/Macintosh/DriveSpeedAccumulator.cpp b/Machines/Apple/Macintosh/DriveSpeedAccumulator.cpp
index 5da17b9a3..fe1e5199b 100644
--- a/Machines/Apple/Macintosh/DriveSpeedAccumulator.cpp
+++ b/Machines/Apple/Macintosh/DriveSpeedAccumulator.cpp
@@ -26,22 +26,23 @@ void DriveSpeedAccumulator::post_sample(uint8_t sample) {
 		// Treat 33 as a zero point and count zero crossings; then approximate
 		// the RPM from the frequency of those.
 		int samples_over = 0;
+		int sum = 0;
 		const uint8_t centre = 33;
 		for(size_t c = 0; c < 512; ++c) {
 			if(samples_[c] > centre) ++ samples_over;
+			sum += samples_[c];
 		}
 
 		// TODO: if the above is the correct test, do it on sample receipt rather than
 		// bothering with an intermediate buffer.
 
-		// The below fits for a function like `a + bc`; I'm not sure it's
-		const float duty_cycle = float(samples_over) / float(samples_.size());
-		const float rotation_speed = 392.0f + duty_cycle * 19.95f;
+		// The below fits for a function like `a + bc`.
+		const float rotation_speed = (float(sum) * 0.052896440564137f) - 259.0f;
 
 		for(int c = 0; c < number_of_drives_; ++c) {
 			drives_[c]->set_rotation_speed(rotation_speed);
 		}
-//		printf("RPM: %0.2f (%d over)\n", rotation_speed, samples_over);
+//		printf("RPM: %0.2f (%d over; %d sum)\n", rotation_speed, samples_over, sum);
 	}
 }