diff --git a/go/vehicle-gateway/internal/realtime/repository.go b/go/vehicle-gateway/internal/realtime/repository.go
index 9c64c844..1b1eaba5 100644
--- a/go/vehicle-gateway/internal/realtime/repository.go
+++ b/go/vehicle-gateway/internal/realtime/repository.go
@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"sort"
+	"strconv"
 	"strings"
 	"time"
 
@@ -155,6 +156,9 @@ func mergeFields(snapshot *Snapshot, fields map[string]any, eventMS int64) {
 		snapshot.FieldTimesMS = map[string]int64{}
 	}
 	for key, value := range fields {
+		if key == envelope.FieldTotalMileageKM && !positiveNumber(value) {
+			continue
+		}
 		if eventMS >= snapshot.FieldTimesMS[key] {
 			snapshot.Fields[key] = value
 			snapshot.FieldTimesMS[key] = eventMS
@@ -162,6 +166,28 @@ func mergeFields(snapshot *Snapshot, fields map[string]any, eventMS int64) {
 	}
 }
 
+func positiveNumber(value any) bool {
+	switch typed := value.(type) {
+	case float64:
+		return typed > 0
+	case float32:
+		return typed > 0
+	case int:
+		return typed > 0
+	case int64:
+		return typed > 0
+	case uint16:
+		return typed > 0
+	case uint32:
+		return typed > 0
+	case string:
+		parsed, err := strconv.ParseFloat(strings.TrimSpace(typed), 64)
+		return err == nil && parsed > 0
+	default:
+		return false
+	}
+}
+
 func cloneFields(fields map[string]any) map[string]any {
 	if len(fields) == 0 {
 		return map[string]any{}
diff --git a/go/vehicle-gateway/internal/realtime/repository_test.go b/go/vehicle-gateway/internal/realtime/repository_test.go
index 3d440fa0..880720ab 100644
--- a/go/vehicle-gateway/internal/realtime/repository_test.go
+++ b/go/vehicle-gateway/internal/realtime/repository_test.go
@@ -94,6 +94,47 @@ func TestRepositoryOnlineStatus(t *testing.T) {
 	}
 }
 
+func TestRepositoryDoesNotOverwritePositiveMileageWithZero(t *testing.T) {
+	repo, closeFn := newTestRepository(t)
+	defer closeFn()
+	ctx := context.Background()
+
+	if err := repo.Update(ctx, envelope.FrameEnvelope{
+		Protocol:     envelope.ProtocolJT808,
+		VIN:          "VIN001",
+		EventTimeMS:  1000,
+		ReceivedAtMS: 1100,
+		Fields: map[string]any{
+			envelope.FieldTotalMileageKM: 12345.6,
+		},
+	}); err != nil {
+		t.Fatalf("Update() error = %v", err)
+	}
+	if err := repo.Update(ctx, envelope.FrameEnvelope{
+		Protocol:     envelope.ProtocolJT808,
+		VIN:          "VIN001",
+		EventTimeMS:  2000,
+		ReceivedAtMS: 2100,
+		Fields: map[string]any{
+			envelope.FieldSpeedKMH:       22.0,
+			envelope.FieldTotalMileageKM: 0,
+		},
+	}); err != nil {
+		t.Fatalf("Update() error = %v", err)
+	}
+
+	merged, err := repo.GetMerged(ctx, "VIN001")
+	if err != nil {
+		t.Fatalf("GetMerged() error = %v", err)
+	}
+	if merged.Fields[envelope.FieldTotalMileageKM] != 12345.6 {
+		t.Fatalf("zero mileage overwrote positive value: %#v", merged.Fields)
+	}
+	if merged.Fields[envelope.FieldSpeedKMH] != 22.0 {
+		t.Fatalf("new speed should still merge: %#v", merged.Fields)
+	}
+}
+
 func TestHandlerReturnsMergedSnapshot(t *testing.T) {
 	repo, closeFn := newTestRepository(t)
 	defer closeFn()