Proof of concept encoder analysis. Still to not have a solid gear ratio but it's clear the encoder is very likely 11 PPR. Rotations take ~650 pulses, maybe 660 but it's hard to tell because the gearing makes stopping inconsistent.

ino/motor_encoder_poc/motor_encoder_poc.ino

@@ -4,7 +4,8 @@
  *  SKU - JGY-370
  *  DC12V100RPM - SKU-GS00127-05
  * 
- *  Gear ratio: 40:1
+ *  Gear ratio: 40:1(?) 60:1(?)
+ *  11 PPR Encoder - (!)
  * 
  *  Red——Motor power + (exchange can control rotating and reversing)
  *  Black——Coding power- negative (3.3-5V) polarity cannot be wrong
@@ -25,24 +26,64 @@
 #define MOTORB 11
 
 volatile int posi = 0; // specify posi as volatile
+const long maxTime = 10000;
+const int maxPulses = 660;
+const int speed = 40;
+volatile long start = 0;
+volatile bool done  = false;
+volatile bool stop = false;
+volatile int incoming;
+volatile float rpm;
 
 void setup() {
-  Serial.begin(9600);
+  Serial.begin(57600);
+  Serial.flush();
   pinMode(ENCA, INPUT);
   pinMode(ENCB, INPUT);
+  pinMode(MOTORA, OUTPUT);
+  pinMode(MOTORB, OUTPUT);
+  
   attachInterrupt(digitalPinToInterrupt(ENCA), readEncoder,RISING);
+
 }
 
 void loop() {
+  int pos;
+  if (Serial.available() > 0) {
+    incoming = Serial.read();
+    start = -1;
+    posi = 0;
+    done = false;
+    stop = false;
+  }
   // Read the position in an atomic block to avoid a potential
   // misread if the interrupt coincides with this code running
   // see: https://www.arduino.cc/reference/en/language/variables/variable-scope-qualifiers/volatile/
-  int pos = 0; 
+  
   ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
     pos = posi;
   }
 
+  if (abs(pos) == maxPulses) {
+    stop = true;
+  }
+  if (start == -1) {
+    delay(1000);
+    analogWrite(MOTORA, speed);
+    digitalWrite(MOTORB, LOW);
+    start = millis();
+  } else if ((stop || millis() - start >= maxTime) && !done) {
+    digitalWrite(MOTORA, LOW);
+    digitalWrite(MOTORB, LOW);
+    Serial.print("Final: ");
     Serial.println(pos);
+    Serial.print("Time:  ");
+    Serial.println(millis() - start);
+    rpm = (float) 60000 / (float) (millis() - start);
+    Serial.print("RPM:   ");
+    Serial.println(rpm);
+    done = true;
+  }
 }
 
 void readEncoder(){