Not getting desired Output

[Amnesiax]

Hi, I have an assignment to create a code that tracks the amount of trips that can be taken on a set of tires before they wear out. For now, my input is 8 miles and 14 inch radius for the wheels. The correct output is supposed to be 607 trips, yet I am receiving 557 trips. I've looked at the calculations, but I'm not sure of what I'm doing wrong. My output is:
How far is a one-way trip in miles from your house to the Slate Rock and Gravel Quarry? 8
What size tires would you like to purchase (12, 14, or 16-inch radius)? 14
The distance you entered for a round-trip, in inches, is 1013760
The circumference of the tire you selected is 87.9646
The amount of total trips you can make is 557 trips.

 #include <iostream>
#include <cmath>

using namespace std;


constexpr int inches_per_mile{63360};
constexpr double rate_of_wear{0.0000005};

int miles_to_inches(int miles)
{
    int inches;
    inches = miles * inches_per_mile * 2;
    return inches;
}

double radius_circumference(int radius)
{
    double circumference;
    circumference = 2 * M_PI * radius;
    return circumference;
}

int num_trips(int circumference, int inches)
{
    int numOfTrips, tireRadius, trips, numOfRotations;
    double radiusWear, wearPerTrip;
    numOfRotations = inches / circumference;
    wearPerTrip = numOfRotations * rate_of_wear;
    tireRadius = circumference / (2 * M_PI);
    radiusWear = tireRadius * .25;
    numOfTrips = radiusWear / wearPerTrip;

    return numOfTrips;
}

int main ()
{
   int miles, inches, tireRadius, totalTrips, trips;
   double circumference;
   
    cout << "How far is a one-way trip in miles from your house to the Slate Rock and Gravel Quarry? ";
    cin >> miles;
    cout << endl;

    cout << "What size tires would you like to purchase (12, 14, or 16-inch radius)? ";
    cin >> tireRadius;
    cout << endl;

    

    inches = miles_to_inches(miles);
    circumference = radius_circumference(tireRadius);
    totalTrips = num_trips(circumference, inches);
    

   

   cout << "The distance you entered for a round-trip, in inches, is " << inches;
   cout << endl;

   cout << "The circumference of the tire you selected is  " << circumference;
   cout << endl;

   cout << "The amount of total trips you can make is " << totalTrips << " trips. ";
   cout << endl;

   return 0;
}

[dutch]

Where are your code tags?

[Amnesiax]

Sorry, thanks for pointing that out.

[jonnin]

most likely if its not what you expect, you have messed up mixing integers and doubles, and some piece of math has truncated a decimal that is messing up the result. should numOfRotations be a double? It is assigned a division result.

lets see... there are 5280 feet in a mile. there are 12 inches per foot. that is 63360, * 8 = 506880 inches in an 8 mile ride.
why does have *2 in inches per mile? round trip? If so, its right. I see what you did there, but its not well named (its not inches to mile conversion, its doing something else).

tire-radius could be corrupted from int/double. When I run it with your example input and print tire-radius in the function, at line 30, I get 13 instead of 14 due to floating point round off... !!

[Amnesiax]

When I changed numOfRotations to a double, it had no impact on the output. It was still the same.

[Amnesiax]

The *2 is to make it round-trip, instead of one-way.

[Amnesiax]

So could I possibly use float in order to get the correct tire-radius?

[Amnesiax]

Thank you very much, I figured it out due to the information you provided.