diff --git a/agb-fixnum/src/lib.rs b/agb-fixnum/src/lib.rs
index be5a97be..f058b1b5 100644
--- a/agb-fixnum/src/lib.rs
+++ b/agb-fixnum/src/lib.rs
@@ -91,7 +91,7 @@ pub trait FixedWidthSignedInteger: FixedWidthUnsignedInteger + Neg<Output = Self
 }
 
 macro_rules! fixed_width_unsigned_integer_impl {
-    ($T: ty, $Upcast: ty) => {
+    ($T: ty, $Upcast: ident) => {
         impl FixedWidthUnsignedInteger for $T {
             #[inline(always)]
             fn zero() -> Self {
@@ -109,10 +109,37 @@ macro_rules! fixed_width_unsigned_integer_impl {
             fn from_as_i32(v: i32) -> Self {
                 v as $T
             }
-            #[inline(always)]
-            fn upcast_multiply(a: Self, b: Self, n: usize) -> Self {
-                (((a as $Upcast) * (b as $Upcast)) >> n) as $T
-            }
+
+            upcast_multiply_impl!($T, $Upcast);
+        }
+    };
+}
+
+macro_rules! upcast_multiply_impl {
+    ($T: ty, optimised_64_bit) => {
+        #[inline(always)]
+        fn upcast_multiply(a: Self, b: Self, n: usize) -> Self {
+            let mask = (Self::one() << n).wrapping_sub(1);
+
+            let a_floor = a >> n;
+            let a_frac = a & mask;
+
+            let b_floor = b >> n;
+            let b_frac = b & mask;
+
+            (a_floor.wrapping_mul(b_floor) << n)
+                .wrapping_add(
+                    a_floor
+                        .wrapping_mul(b_frac)
+                        .wrapping_add(b_floor.wrapping_mul(a_frac)),
+                )
+                .wrapping_add(a_frac.wrapping_mul(b_frac) >> n)
+        }
+    };
+    ($T: ty, $Upcast: ty) => {
+        #[inline(always)]
+        fn upcast_multiply(a: Self, b: Self, n: usize) -> Self {
+            (((a as $Upcast) * (b as $Upcast)) >> n) as $T
         }
     };
 }
@@ -131,8 +158,9 @@ macro_rules! fixed_width_signed_integer_impl {
 fixed_width_unsigned_integer_impl!(u8, u32);
 fixed_width_unsigned_integer_impl!(i16, i32);
 fixed_width_unsigned_integer_impl!(u16, u32);
-fixed_width_unsigned_integer_impl!(i32, i64);
-fixed_width_unsigned_integer_impl!(u32, u64);
+
+fixed_width_unsigned_integer_impl!(i32, optimised_64_bit);
+fixed_width_unsigned_integer_impl!(u32, optimised_64_bit);
 
 fixed_width_signed_integer_impl!(i16);
 fixed_width_signed_integer_impl!(i32);
@@ -1370,4 +1398,17 @@ mod tests {
             ]
         );
     }
+
+    #[cfg(not(debug_assertions))]
+    #[test]
+    fn test_all_multiplies() {
+        use super::*;
+
+        for i in 0..u32::MAX {
+            let fix_num: Num<_, 7> = Num::from_raw(i);
+            let upcasted = ((i as u64 * i as u64) >> 7) as u32;
+
+            assert_eq!((fix_num * fix_num).to_raw(), upcasted);
+        }
+    }
 }
diff --git a/justfile b/justfile
index d9a17a3d..05a12dce 100644
--- a/justfile
+++ b/justfile
@@ -20,6 +20,7 @@ test:
 
 test-release:
     just _test-release agb
+    just _test-release agb-fixnum
     just _test-release-arm agb
 
 doctest-agb: