Every formula, worked example, and free calculator for MPG, fuel cost, car depreciation, engine horsepower, tire size, gap insurance, trade-in value, dealer markup, DUI cost, and every major car ownership calculation — all verified and in one place.
Calculate real-world MPG, fuel cost per mile, monthly fuel budget, and compare metric vs. imperial fuel efficiency.
MPG is the most widely used measure of fuel economy in the United States. The calculation requires only two data points: miles driven and gallons consumed. The most accurate method is the fill-to-fill method — fill your tank completely, reset your trip odometer to zero, drive your normal mix of roads, then fill up again. Divide the miles driven by the gallons needed to refill.
MPG = Miles Driven / Gallons Used
Example: 378 miles driven, 13.5 gallons used = 378 / 13.5 = 28.0 MPG
Fuel cost per mile = Fuel price per gallon / MPG
Example: $3.60/gallon at 28 MPG = $3.60 / 28 = $0.129 per mile
City driving delivers lower MPG because of frequent stopping and starting, engine idling, and lower average speeds where aerodynamic drag is minimal but acceleration losses are significant. Highway driving is more efficient because the engine operates at steady-state load. The EPA combined MPG is calculated as a weighted harmonic mean — not a simple average — using 55% city and 45% highway weighting.
Combined MPG = 1 / (0.55 / City MPG + 0.45 / Highway MPG)
Example: 22 city, 32 highway: 1 / (0.55/22 + 0.45/32) = 26.2 MPG combined
In Canada, Europe, and Australia, fuel efficiency is measured in liters per 100 kilometers (L/100km). Unlike MPG, a lower number means better efficiency. You can convert directly between the two systems using the constant 235.21.
L/100km = (Liters used / Kilometers driven) x 100
Example: 42 liters used over 480km = (42/480) x 100 = 8.75 L/100km
Convert MPG to L/100km: L/100km = 235.21 / MPG
Convert L/100km to MPG: MPG = 235.21 / L/100km
Example: 28 MPG = 235.21 / 28 = 8.4 L/100km
Knowing your monthly fuel cost is essential for budgeting and comparing vehicles. The formula is straightforward but most drivers underestimate their annual mileage. The average American driver covers 13,500 miles per year according to the Federal Highway Administration's 2024 data.
Monthly fuel cost = (Monthly miles / MPG) x Fuel price per gallon
Example: 1,100 miles/month, 28 MPG, $3.60/gallon = (1100/28) x 3.60 = $141.43/month
Annual fuel cost = Monthly fuel cost x 12 = $1,697/year
| Vehicle Class | City MPG | Highway MPG | Combined |
|---|---|---|---|
| Compact car | 28–32 | 36–42 | 31–36 |
| Midsize sedan | 24–28 | 32–38 | 27–32 |
| Midsize SUV | 19–24 | 26–30 | 22–26 |
| Full-size pickup (V8) | 14–17 | 18–22 | 16–19 |
| Minivan | 19–22 | 26–28 | 22–24 |
| Hybrid compact | 42–50 | 44–52 | 43–51 |
| Plug-in hybrid | 55–80 MPGe | 42–50 | 50–65 MPGe |
Calculate how much your car loses in value each year, estimate trade-in worth, and determine classic car values.
Depreciation is the single largest cost of car ownership, yet most buyers ignore it. A new car loses roughly 20-25% of its value the moment you drive it off the lot — this "first-year cliff" includes both the immediate purchase premium and the first full year of age-related depreciation. After year one, depreciation typically runs 10-15% per year on the remaining value, following a declining balance pattern.
Straight-Line: Annual depreciation = (Purchase price - Salvage value) / Years
Declining Balance: Value after n years = Purchase price x (1 - rate)^n
Example: $35,000 car at 15%/year: After 3 years = 35,000 x (0.85)^3 = $21,484
After 5 years = 35,000 x (0.85)^5 = $15,517
| Brand Tier | Year 1 Loss | Year 3 Value | Year 5 Value |
|---|---|---|---|
| Luxury (BMW, Mercedes) | 30–35% | 48–52% of MSRP | 35–40% of MSRP |
| Japanese mainstream (Toyota, Honda) | 15–20% | 60–68% of MSRP | 48–57% of MSRP |
| American mainstream (Chevrolet, Ford) | 20–28% | 52–60% of MSRP | 40–50% of MSRP |
| Electric vehicles | 20–40% | 45–65% of MSRP | 32–55% of MSRP |
| Trucks (pickup, heavy) | 15–22% | 62–70% of MSRP | 52–62% of MSRP |
Trade-in value is lower than private party value because the dealer must recondition the vehicle, carry it in inventory, and earn a profit on resale. The gap between private party and trade-in value is typically 10-20% of the vehicle's worth. Dealers start with wholesale auction values (Manheim Market Report), then adjust for local demand, mileage, and condition.
Trade-in estimate = Private party value x (0.80 to 0.90)
Mileage adjustment: For every 1,000 miles above 15,000/year, subtract $50-$150
Condition impact: Excellent = full value | Good = -5% | Fair = -15% | Poor = -25%
Classic cars (typically 20+ years old) do not follow standard depreciation curves. Their value is driven by rarity, originality, provenance, and collector market demand. A "numbers-matching" car — where the engine, transmission, and body all carry the original factory serial numbers — can be worth 2-3x a rebuilt equivalent. Condition grading follows the Hagerty scale from 1 (Concours) to 6 (Parts car).
Calculate engine horsepower, torque-to-HP conversion, quarter-mile performance, and engine displacement.
Horsepower and torque are closely related but describe different things. Torque is a rotational force — it tells you how hard the engine can push. Horsepower is a measure of how fast the engine can do work over time. At 5,252 RPM, horsepower and torque numbers are always equal. Below 5,252 RPM, torque is numerically higher; above it, horsepower is higher.
HP = (Torque in ft-lb x RPM) / 5,252
Torque = (HP x 5,252) / RPM
Example: 310 ft-lb torque at 4,200 RPM: HP = (310 x 4200) / 5252 = 248 HP
Example: 400 HP at 6,000 RPM: Torque = (400 x 5252) / 6000 = 350 ft-lb
One horsepower equals 33,000 ft-lb per minute (James Watt's original definition). One revolution involves traveling 2*pi radians. Converting: HP = (Torque x RPM x 2*pi) / 33,000. Simplifying: HP = (Torque x RPM) / (33,000 / 2*pi) = (Torque x RPM) / 5,252.11. The constant 5,252 is mathematically exact from Watt's definition combined with rotational mechanics.
If you have quarter-mile trap speed and vehicle weight data, you can back-calculate rear-wheel horsepower using the Hale formula — commonly used by drag racers when a dynamometer is not available.
HP = Weight (lbs) x (Trap speed (mph) / 234)^3
Example: 3,400 lb car, 100 mph trap speed: HP = 3400 x (100/234)^3 = 265 HP
ET method: HP = Weight / (ET / 5.825)^3
Example: 3,400 lb car, 13.5 sec ET: HP = 3400 / (13.5/5.825)^3 = 148 HP (wheel HP)
The horsepower advertised by manufacturers is measured at the crankshaft (crank HP or bhp). Drivetrain losses — transmission, driveshaft, differential — absorb 10-20% of crank power. A rear-wheel-drive car typically loses 12-15%; a front-wheel-drive car loses 13-16%; all-wheel drive loses 17-22%. Rule of thumb: multiply wheel HP by 1.15 to estimate crank HP on a RWD vehicle.
| Drivetrain Type | Typical Power Loss | Wheel HP = Crank HP x |
|---|---|---|
| Rear-wheel drive (RWD) | 12–15% | 0.85 to 0.88 |
| Front-wheel drive (FWD) | 13–16% | 0.84 to 0.87 |
| All-wheel drive (AWD) | 17–22% | 0.78 to 0.83 |
| 4-wheel drive (4WD) | 18–23% | 0.77 to 0.82 |
Decode tire sidewall codes, calculate overall tire diameter, compare tire sizes, and determine speedometer error from different tire sizes.
The numbers on a tire sidewall encode critical dimensions in a standardized format. A tire marked 225/60R16 contains everything you need to calculate total diameter, sidewall height, and speedometer impact from size changes.
Sidewall height (mm) = Section width (mm) x (Aspect ratio / 100)
Example 225/60R16: Sidewall = 225 x (60/100) = 135mm
Total diameter (mm) = (Wheel diameter (in) x 25.4) + (2 x Sidewall height)
Example: (16 x 25.4) + (2 x 135) = 406.4 + 270 = 676.4mm (26.63 inches)
Tire circumference (mm) = pi x Total diameter = 3.14159 x 676.4 = 2,124.8mm
Changing your tire size affects your speedometer reading because the speedometer is calibrated to the original tire's rolling circumference. A larger tire covers more ground per revolution, making the speedometer read low (you are going faster than it shows). A smaller tire does the opposite.
Actual speed = Indicated speed x (New tire diameter / Original tire diameter)
Speed error % = ((New diameter - Original diameter) / Original diameter) x 100
Example: Original 26.6", new tire 27.4" diameter:
At 60 mph indicated: Actual = 60 x (27.4/26.6) = 61.8 mph (3% faster than shown)
Every tire has a load index and speed rating stamped on the sidewall after the size code. Load index 91 means each tire can carry up to 1,356 lbs. Speed rating H means the tire is rated for sustained speeds up to 130 mph. Never install tires with a lower load index or speed rating than specified by your vehicle manufacturer.
| Load Index | Max Load (lbs) | Speed Rating | Max Speed |
|---|---|---|---|
| 85 | 1,135 | S | 112 mph |
| 89 | 1,279 | T | 118 mph |
| 91 | 1,356 | H | 130 mph |
| 94 | 1,477 | V | 149 mph |
| 97 | 1,609 | W | 168 mph |
| 100 | 1,764 | Y | 186 mph |
Understand GAP insurance coverage, dealer markup above invoice, and the total financial picture of a car purchase.
GAP (Guaranteed Asset Protection) insurance covers the "gap" between what you owe on your auto loan and your car's actual cash value (ACV) at the time of a total loss. Because cars depreciate faster than loans are paid down in the early years, many drivers are "upside down" — owing more than the car is worth — for the first 2-3 years of a loan.
GAP coverage amount = Loan balance - Actual Cash Value (ACV)
ACV = Purchase price x (1 - depreciation rate)^years
Example: $32,000 loan, car now worth $26,000: GAP covers $6,000
When is GAP needed: When Loan-to-Value (LTV) ratio exceeds 100%
LTV = Loan balance / Vehicle ACV x 100
GAP insurance is worth buying when you: financed more than 80% of the vehicle's value, took a loan term of 60 months or longer, made a down payment of less than 20%, purchased a vehicle with rapid depreciation (luxury or EV), or rolled negative equity from a previous loan into a new loan. Once your loan balance falls below the vehicle's ACV, you no longer need GAP coverage.
The dealer invoice price is what the dealer theoretically paid the manufacturer, but this is not the dealer's true cost. Holdback payments (2-3% of MSRP paid back to dealers quarterly), dealer cash incentives, and volume bonuses mean the true dealer cost is typically 4-10% below invoice. Understanding this gives you a realistic floor for price negotiation.
Dealer markup % = (Selling price - Invoice price) / Invoice price x 100
MSRP to Invoice estimate: Invoice = MSRP x 0.92 to 0.98 (varies by brand)
Market Adjustment (ADM): Added above MSRP during high demand periods
Example: MSRP $42,000, invoice $39,500, selling for $44,500:
Markup above invoice = ($44,500 - $39,500) / $39,500 = 12.7% above invoice
Calculate the full financial impact of a DUI conviction and understand the true total cost of owning and operating a vehicle.
A DUI carries severe financial consequences that extend far beyond the initial fine. The total out-of-pocket cost for a first-offense DUI in the United States typically ranges from $10,000 to $25,000 — and in high-cost states like California or New York, costs can exceed $30,000 when all factors are included. The insurance premium increase alone accounts for 30-50% of the total cost and persists for 3-7 years depending on the state.
Total DUI Cost = Attorney fees + Court fines + DMV fees + DUI school
+ Ignition interlock + Insurance premium increase x Years
+ License reinstatement + Tow and impound + Lost wages
Typical range: $10,000 – $25,000+ for first offense (state-dependent)
| DUI Cost Component | Typical Range | Notes |
|---|---|---|
| Attorney fees | $2,000 – $10,000 | Higher for contested cases or trials |
| Court fines and penalties | $1,000 – $3,000 | Plus county/state surcharges |
| DUI education program | $250 – $800 | Mandatory in most states |
| Ignition interlock device | $1,000 – $2,500 | Installation + 6-24 month rental |
| License reinstatement | $100 – $500 | Varies by state |
| Tow and impound fees | $300 – $1,200 | Often immediate at arrest |
| Insurance increase | $3,000 – $12,000 | Over 3-7 year surcharge period |
The IRS standard mileage rate of $0.67 per mile (2025) is designed to capture all costs of operating a personal vehicle. Most drivers dramatically underestimate what their car actually costs because they only think about the car payment and gas. The true annual cost of owning a new midsize sedan in 2026 averages $9,500-$11,500 per year according to AAA's annual "Your Driving Costs" study.
Annual cost = Depreciation + Loan interest + Insurance + Fuel
+ Maintenance + Tires + Registration + Taxes + Parking
Per-mile cost = Annual cost / Annual miles driven
IRS 2025 standard rate: $0.67/mile (all costs included)
Example: $11,000 annual cost / 13,500 miles = $0.815/mile true cost
All formulas and reference data on this guide are sourced from authoritative automotive, government, and industry standards: