Problem with performing fractions with Arduino UNO R3

Using Arduino Programming Questions
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Hello everybody,
I'm quite new with Arduino and programming with C-like code. So probably I made some mistakes in assigning of variables formats.

Following the complete code. I know that is performing 3 times the same calculation, but during the test of the distance sensor I noticed that something was wrong, so I decided to print out as much as possible and see where the problem arises.
So I started with declaring all variables unsigned long. And then after seeing that the results had no sense I decided to declare variables as float.

unsigned long CurrDist;
unsigned long PrevDist;
unsigned long CurrentTime;
unsigned long PrevTime;
unsigned long Vobst;
 float calc;

void setup() {
  Serial.begin(9600);
  Serial.println("Ready");
  delay(5000);
  PrevDist=800;
  PrevTime=millis();
}
void loop(){
    int in=analogRead(A0);
  if (in<=81) {CurrDist=800;}
  else if (in<=122){CurrDist=800-0.7317*(in-81)*10;}
  else if (in<=143){CurrDist=500-0.4762*(in-122)*10;}
  else if (in<=184){CurrDist=400-0.2439*(in-143)*10;}
  else if (in<=225){CurrDist=300-0.1220*(in-184)*10;}
  else if (in<=266){CurrDist=250-0.1220*(in-225)*10;}
  else if (in<=327){CurrDist=200-0.0820*(in-266)*10;}
  else if (in<=471){CurrDist=150-0.0347*(in-327)*10;}
  else if (in<=563){CurrDist=100-0.0217*(in-471)*10;}
  else if (in<=604){CurrDist=80-0.0244*(in-563)*10;}
  else if (in<=645){CurrDist=70-0.0244*(in-604)*10;}
  else {CurrDist=55;}

  CurrentTime=millis();
  float Vobst= long(PrevDist-CurrDist)/(long(CurrentTime-PrevTime))*1000;
  Serial.print ("ingresso= ");
  Serial.println(analogRead(A0));
  //Serial.println(pow(analogRead(A0),(-1,26)));
  Serial.print ("Distanza= ");
  Serial.print (CurrDist);
  Serial.print("[mm] \t Velocità=");
  Serial.print(Vobst);
  Serial.println("[mm/s]");
  Serial.print("current time=");
  Serial.print(CurrentTime);
  Serial.print("[ms]  previous time=");
  Serial.print(PrevTime);
  Serial.println("[ms]");
  Serial.print(long(PrevDist-CurrDist));Serial.print("/");Serial.print(CurrentTime-PrevTime);
  calc=(float(PrevDist-CurrDist))/long((CurrentTime-PrevTime));
  Serial.print("=");Serial.println(calc);
   Serial.print("0-1="); Serial.println(0-1);
  
  Serial.println("");
  
  PrevDist=CurrDist;
  PrevTime=CurrentTime;
  delay(5000);
}

The thing that seem very strange to me is that even for small negatives values the problem occurs:

-8/5071=846966.56

Thanks in advance