diff --git a/fuellog-api/internal/service/refuel_record_test.go b/fuellog-api/internal/service/refuel_record_test.go
new file mode 100644
index 0000000..a37c941
--- /dev/null
+++ b/fuellog-api/internal/service/refuel_record_test.go
@@ -0,0 +1,211 @@
+package service
+
+import (
+	"context"
+	"sort"
+	"testing"
+	"time"
+
+	"gorm.io/gorm"
+
+	"github.com/kennethcheng/fuellog-api/internal/model"
+)
+
+// fakeVehicleRepo / fakeRefuelRepo 是用于单测的 in-memory 假实现,
+// 不接 SQLite,直接塞到 service 的接口里。
+type fakeVehicleRepo struct{ v *model.Vehicle }
+
+func (f *fakeVehicleRepo) Create(context.Context, *model.Vehicle) error { return nil }
+func (f *fakeVehicleRepo) Get(_ context.Context, id uint64) (*model.Vehicle, error) {
+	if f.v != nil && f.v.ID == id {
+		return f.v, nil
+	}
+	return nil, errNotFound
+}
+func (f *fakeVehicleRepo) List(context.Context) ([]model.Vehicle, error) { return nil, nil }
+func (f *fakeVehicleRepo) Update(context.Context, *model.Vehicle) error   { return nil }
+func (f *fakeVehicleRepo) Delete(context.Context, uint64) error          { return nil }
+func (f *fakeVehicleRepo) ClearDefault(context.Context, *gorm.DB) error  { return nil }
+func (f *fakeVehicleRepo) SetDefault(context.Context, *gorm.DB, uint64) error { return nil }
+
+// fakeRefuelRepo:用切片装记录,支持 ListByVehicle(忽略时间窗,业务测试够用)。
+type fakeRefuelRepo struct{ records []model.RefuelRecord }
+
+func (f *fakeRefuelRepo) Create(context.Context, *model.RefuelRecord) error { return nil }
+func (f *fakeRefuelRepo) Get(_ context.Context, id uint64) (*model.RefuelRecord, error) {
+	for i := range f.records {
+		if f.records[i].ID == id {
+			return &f.records[i], nil
+		}
+	}
+	return nil, errNotFound
+}
+func (f *fakeRefuelRepo) Update(context.Context, *model.RefuelRecord) error { return nil }
+func (f *fakeRefuelRepo) Delete(context.Context, uint64) error             { return nil }
+func (f *fakeRefuelRepo) ListByVehicle(_ context.Context, vehicleID uint64, _, _ *time.Time) ([]model.RefuelRecord, error) {
+	out := make([]model.RefuelRecord, 0)
+	for _, r := range f.records {
+		if r.VehicleID == vehicleID {
+			out = append(out, r)
+		}
+	}
+	// 与真 repo 一致:按 refuel_time DESC,ID DESC
+	sort.Slice(out, func(i, j int) bool {
+		if out[i].RefuelTime.Equal(out[j].RefuelTime) {
+			return out[i].ID > out[j].ID
+		}
+		return out[i].RefuelTime.After(out[j].RefuelTime)
+	})
+	return out, nil
+}
+func (f *fakeRefuelRepo) PrevFullRefuel(context.Context, uint64, time.Time) (*model.RefuelRecord, error) {
+	return nil, errNotFound
+}
+
+var errNotFound = sentinel("NOT_FOUND")
+
+type sentinel string
+
+func (s sentinel) Error() string { return string(s) }
+
+func newRefuelSvc(records []model.RefuelRecord) *refuelService {
+	v := &model.Vehicle{ID: 1, Name: "test", FuelType: "gasoline"}
+	return &refuelService{
+		recRepo:     &fakeRefuelRepo{records: records},
+		vehicleRepo: &fakeVehicleRepo{v: v},
+	}
+}
+
+func TestStatistics_TwoConsecutiveFull(t *testing.T) {
+	t1 := time.Date(2025, 1, 1, 0, 0, 0, 0, time.UTC)
+	t2 := time.Date(2025, 1, 10, 0, 0, 0, 0, time.UTC)
+	recs := []model.RefuelRecord{
+		{ID: 1, VehicleID: 1, RefuelTime: t1, Odometer: 10000, Liters: 40, PricePerLiter: 8.0, TotalAmount: 320, IsFull: true},
+		{ID: 2, VehicleID: 1, RefuelTime: t2, Odometer: 10500, Liters: 40, PricePerLiter: 8.0, TotalAmount: 320, IsFull: true},
+	}
+	svc := newRefuelSvc(recs)
+	stats, err := svc.Statistics(context.Background(), 1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if stats.TotalRecords != 2 {
+		t.Errorf("TotalRecords = %d, want 2", stats.TotalRecords)
+	}
+	if !floatNear(stats.TotalLiters, 80) {
+		t.Errorf("TotalLiters = %v, want 80", stats.TotalLiters)
+	}
+	if !floatNear(stats.TotalAmount, 640) {
+		t.Errorf("TotalAmount = %v, want 640", stats.TotalAmount)
+	}
+	if !floatNear(stats.AvgPrice, 8.0) {
+		t.Errorf("AvgPrice = %v, want 8.0", stats.AvgPrice)
+	}
+	// mileage = 500km, liters = 40, 40/500*100 = 8.0 L/100km
+	if !floatNear(stats.AvgConsumption, 8.0) {
+		t.Errorf("AvgConsumption = %v, want 8.0 (plan example)", stats.AvgConsumption)
+	}
+	// cost = 320/500 = 0.64
+	if !floatNear(stats.CostPerKm, 0.64) {
+		t.Errorf("CostPerKm = %v, want 0.64", stats.CostPerKm)
+	}
+}
+
+func TestStatistics_HalfTankDoesNotPollute(t *testing.T) {
+	t1 := time.Date(2025, 1, 1, 0, 0, 0, 0, time.UTC)
+	t2 := time.Date(2025, 1, 10, 0, 0, 0, 0, time.UTC)
+	t3 := time.Date(2025, 1, 15, 0, 0, 0, 0, time.UTC)
+	recs := []model.RefuelRecord{
+		{ID: 1, VehicleID: 1, RefuelTime: t1, Odometer: 10000, Liters: 40, TotalAmount: 320, PricePerLiter: 8.0, IsFull: true},
+		{ID: 2, VehicleID: 1, RefuelTime: t2, Odometer: 10500, Liters: 40, TotalAmount: 320, PricePerLiter: 8.0, IsFull: true},
+		{ID: 3, VehicleID: 1, RefuelTime: t3, Odometer: 10600, Liters: 10, TotalAmount: 80, PricePerLiter: 8.0, IsFull: false}, // 半箱油
+	}
+	svc := newRefuelSvc(recs)
+	stats, err := svc.Statistics(context.Background(), 1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if stats.TotalRecords != 3 {
+		t.Errorf("TotalRecords = %d, want 3", stats.TotalRecords)
+	}
+	if !floatNear(stats.TotalLiters, 90) {
+		t.Errorf("TotalLiters = %v, want 90 (40+40+10)", stats.TotalLiters)
+	}
+	// 关键断言:半箱油不污染油耗采样,仍应是 8.0(只对 #1 → #2 计算)
+	if !floatNear(stats.AvgConsumption, 8.0) {
+		t.Errorf("AvgConsumption = %v, want 8.0 (half-tank must be excluded)", stats.AvgConsumption)
+	}
+	// avg price 全部三条:8.0, 8.0, 8.0 -> 8.0
+	if !floatNear(stats.AvgPrice, 8.0) {
+		t.Errorf("AvgPrice = %v, want 8.0", stats.AvgPrice)
+	}
+}
+
+func TestStatistics_OnlyHalfTanks(t *testing.T) {
+	// 全是半箱油,标准油耗应为 0(没有 is_full 序列)
+	t1 := time.Date(2025, 1, 1, 0, 0, 0, 0, time.UTC)
+	t2 := time.Date(2025, 1, 5, 0, 0, 0, 0, time.UTC)
+	recs := []model.RefuelRecord{
+		{ID: 1, VehicleID: 1, RefuelTime: t1, Odometer: 10000, Liters: 20, TotalAmount: 160, PricePerLiter: 8.0, IsFull: false},
+		{ID: 2, VehicleID: 1, RefuelTime: t2, Odometer: 10500, Liters: 20, TotalAmount: 160, PricePerLiter: 8.0, IsFull: false},
+	}
+	svc := newRefuelSvc(recs)
+	stats, err := svc.Statistics(context.Background(), 1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !floatNear(stats.AvgConsumption, 0) {
+		t.Errorf("AvgConsumption = %v, want 0 (no is_full sequence)", stats.AvgConsumption)
+	}
+	if !floatNear(stats.CostPerKm, 0) {
+		t.Errorf("CostPerKm = %v, want 0 (no is_full sequence)", stats.CostPerKm)
+	}
+}
+
+func TestStatistics_MultipleFullSamples(t *testing.T) {
+	// 三次加满,应有两个油耗样本
+	t1 := time.Date(2025, 1, 1, 0, 0, 0, 0, time.UTC)
+	t2 := time.Date(2025, 1, 10, 0, 0, 0, 0, time.UTC)
+	t3 := time.Date(2025, 1, 20, 0, 0, 0, 0, time.UTC)
+	recs := []model.RefuelRecord{
+		{ID: 1, VehicleID: 1, RefuelTime: t1, Odometer: 10000, Liters: 40, TotalAmount: 320, PricePerLiter: 8.0, IsFull: true},
+		{ID: 2, VehicleID: 1, RefuelTime: t2, Odometer: 10500, Liters: 40, TotalAmount: 320, PricePerLiter: 8.0, IsFull: true},
+		{ID: 3, VehicleID: 1, RefuelTime: t3, Odometer: 11000, Liters: 40, TotalAmount: 320, PricePerLiter: 8.0, IsFull: true},
+	}
+	svc := newRefuelSvc(recs)
+	stats, err := svc.Statistics(context.Background(), 1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !floatNear(stats.AvgConsumption, 8.0) {
+		t.Errorf("AvgConsumption = %v, want 8.0 (3 full samples avg)", stats.AvgConsumption)
+	}
+	if !floatNear(stats.CostPerKm, 0.64) {
+		t.Errorf("CostPerKm = %v, want 0.64", stats.CostPerKm)
+	}
+}
+
+func TestStatistics_EmptyVehicle(t *testing.T) {
+	svc := newRefuelSvc(nil)
+	stats, err := svc.Statistics(context.Background(), 1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if stats.TotalRecords != 0 {
+		t.Errorf("TotalRecords = %d, want 0", stats.TotalRecords)
+	}
+	if stats.LastRefuel != nil {
+		t.Errorf("LastRefuel should be nil, got %+v", stats.LastRefuel)
+	}
+}
+
+func floatNear(a, b float64) bool {
+	const eps = 0.05
+	if a == b {
+		return true
+	}
+	diff := a - b
+	if diff < 0 {
+		diff = -diff
+	}
+	return diff < eps
+}
diff --git a/fuellog-api/pkg/decimal/money_test.go b/fuellog-api/pkg/decimal/money_test.go
new file mode 100644
index 0000000..2475cc9
--- /dev/null
+++ b/fuellog-api/pkg/decimal/money_test.go
@@ -0,0 +1,119 @@
+package decimal
+
+import (
+	"math"
+	"testing"
+)
+
+const epsilon = 0.001
+
+func almostEqual(a, b float64) bool { return math.Abs(a-b) < epsilon }
+
+func TestPricePerLiter(t *testing.T) {
+	cases := []struct {
+		name           string
+		total, liters  float64
+		expect         float64
+	}{
+		{"plan example 176.33/23.67", 176.33, 23.67, 7.45},
+		{"simple 320/40", 320, 40, 8.0},
+		{"simple 256/32", 256, 32, 8.0},
+		{"zero liters", 100, 0, 0},
+		{"negative liters", 100, -1, 0},
+		{"precision 23.67/3.18", 23.67, 3.18, 7.4434}, // 23.67/3.18 = 7.4433...
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			got := PricePerLiter(c.total, c.liters)
+			if !almostEqual(got, c.expect) {
+				t.Errorf("PricePerLiter(%v,%v) = %v, want %v", c.total, c.liters, got, c.expect)
+			}
+		})
+	}
+}
+
+func TestConsumptionL100km(t *testing.T) {
+	cases := []struct {
+		name           string
+		liters, mileage float64
+		expect         float64
+	}{
+		{"plan example (40/500*100)", 40, 500, 8.0},
+		{"32/500*100", 32, 500, 6.4},
+		{"zero mileage", 40, 0, 0},
+		{"negative mileage", 40, -1, 0},
+		{"rounding 2dp", 7.123, 100, 7.12}, // 7.123/100*100 = 7.123 -> round 2dp
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			got := ConsumptionL100km(c.liters, c.mileage)
+			if !almostEqual(got, c.expect) {
+				t.Errorf("ConsumptionL100km(%v,%v) = %v, want %v", c.liters, c.mileage, got, c.expect)
+			}
+		})
+	}
+}
+
+func TestCostPerKm(t *testing.T) {
+	cases := []struct {
+		name           string
+		amount, mileage float64
+		expect         float64
+	}{
+		{"plan example 256/500", 256, 500, 0.512},
+		{"zero mileage", 100, 0, 0},
+		{"negative mileage", 100, -1, 0},
+		{"typical 200/400", 200, 400, 0.5},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			got := CostPerKm(c.amount, c.mileage)
+			if !almostEqual(got, c.expect) {
+				t.Errorf("CostPerKm(%v,%v) = %v, want %v", c.amount, c.mileage, got, c.expect)
+			}
+		})
+	}
+}
+
+func TestMileage(t *testing.T) {
+	cases := []struct {
+		name           string
+		current, prev  float64
+		expect         float64
+	}{
+		{"normal increase", 11000, 10500, 500},
+		{"equal (no travel)", 10500, 10500, 0},
+		{"odometer rollback (typo)", 10400, 10500, 0},
+		{"zero prev (first record)", 10000, 0, 10000},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			got := Mileage(c.current, c.prev)
+			if !almostEqual(got, c.expect) {
+				t.Errorf("Mileage(%v,%v) = %v, want %v", c.current, c.prev, got, c.expect)
+			}
+		})
+	}
+}
+
+func TestAvgFloat64(t *testing.T) {
+	cases := []struct {
+		name   string
+		xs     []float64
+		expect float64
+	}{
+		{"empty", nil, 0},
+		{"single", []float64{8.0}, 8.0},
+		{"two", []float64{7.0, 9.0}, 8.0},
+		{"three typical", []float64{6.0, 7.0, 8.0}, 7.0},
+		{"rounding", []float64{0.123, 0.456, 0.789}, 0.46}, // avg = 0.456
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			got := AvgFloat64(c.xs)
+			if !almostEqual(got, c.expect) {
+				t.Errorf("AvgFloat64(%v) = %v, want %v", c.xs, got, c.expect)
+			}
+		})
+	}
+}