Tyre Pressure Effect on Economy
Estimate how under-inflated tyres reduce fuel economy based on pressure deficit and loss rate.
Last updated:
What this tool does
This calculator estimates fuel economy at current tyre pressure by applying a user-specified percentage penalty per PSI of under-inflation. The user enters recommended pressure, current pressure, baseline MPG, and economy loss rate (%/PSI); the engine computes the pressure deficit, multiplies by the loss rate (capped at 50% total penalty), and returns adjusted MPG. The model assumes a linear penalty relationship and does not account for over-inflation effects, rolling resistance variability, or real-world driving conditions.
Compare scenarios
Continue with your figures
These calculators share inputs with this one. Change a value above and your figures travel with the link, in the part of the URL your browser never sends to a server.
Premium vs Regular Fuel Cost-Benefit
Fuel & EfficiencyCalculate whether premium fuel pays for itself through efficiency gains at your fuel prices.
Towing Fuel Penalty Calculator
Performance & EngineeringCalculate the extra fuel cost and economy penalty when towing a trailer over any distance.
Roof Box Drag Penalty
Performance & EngineeringCalculate the extra fuel cost from a roof box over a given distance based on aerodynamic drag penalty.
Cruise Control Fuel Savings
Fuel & EfficiencyCalculate annual fuel savings from using cruise control on motorway journeys.
Air Conditioning Fuel Cost
Performance & EngineeringCalculate the extra fuel cost of running your car's air conditioning over a given period.
Acceleration vs Fuel Economy Tradeoff
Fuel & EfficiencyCalculate the annual fuel cost of aggressive acceleration and hard driving
How the result changes
This input can't be charted across its full range — the formula has no valid result for most of it. Pick another input above.
Export Report
How would you like to export your results?
Share
Preview
Result
—
❮ ❯ switch fields·− + change value
0
Formula
How tyre pressure affects fuel economy
Under-inflated tyres increase rolling resistance, requiring more engine power to maintain speed and reducing miles per gallon. This calculator quantifies that penalty by multiplying the pressure deficit (recommended PSI minus current PSI) by a loss rate you specify—typically 0.2–0.6% per PSI based on published estimates—then subtracting that percentage from your baseline MPG. The result is an estimated adjusted MPG at your current tyre pressure.
The formula
The engine calculates:
Under-inflation (PSI) = max(0, recommended_psi − current_psi)
Economy penalty (%) = min(loss_per_psi × under-inflation, 50%)
Adjusted MPG = baseline_mpg × (1 − penalty ÷ 100)
Variables: recommended_psi is the manufacturer specification; current_psi is measured cold; baseline_mpg is observed fuel economy at correct pressure; loss_per_psi is the percentage drop per PSI under, derived from test-fleet data or published sources.
Where this method is most accurate
The linear penalty model holds best for moderate under-inflation (1–10 PSI below specification) on passenger cars at steady highway speeds. Accuracy depends on the loss-rate parameter; laboratory dynamometer studies report 0.3–0.5%/PSI, while on-road fleets may see 0.2–0.4%. The 50% cap prevents physically implausible results at extreme deficits. Over-inflation is not modelled; when current PSI exceeds recommended, the tool returns baseline MPG unchanged.
What this tool does not do
It does not measure actual tyre pressure, does not account for tyre type (summer vs. all-season vs. run-flat), does not model temperature effects on pressure, and does not consider load, speed, or driving style. The baseline MPG input is assumed correct; the calculator performs no independent fuel-flow measurement. Results are estimates for comparison, not certification of any vehicle's real-world economy.
Disclaimer
This tool is provided for educational and comparative purposes only. It does not constitute vehicle maintenance advice, safety guidance, or a guarantee of fuel savings. Actual economy depends on tyre condition, wheel alignment, vehicle load, aerodynamics, and driving behaviour. Always inflate tyres to the manufacturer's cold-pressure specification found on the door placard or owner's manual. MotorMath publishes the formula; users supply all parameters and interpret results at their own discretion.
Questions
- Where does the 0.4%/PSI default loss rate come from?
- Laboratory dynamometer studies and fleet data typically report 0.3–0.5% MPG reduction per PSI of under-inflation. The 0.4% midpoint reflects a consensus estimate; individual vehicles may vary by tyre size, weight, and aerodynamics.
- Why is the penalty capped at 50%?
- At extreme under-inflation (e.g. 20+ PSI below specification), the linear model breaks down and tyres risk structural failure. The 50% cap prevents mathematically valid but physically impossible economy estimates while signalling the need for immediate re-inflation.
- Does over-inflation improve economy further?
- The calculator treats current PSI above recommended as zero under-inflation and returns baseline MPG. Over-inflation reduces contact patch and may decrease rolling resistance slightly, but it also degrades traction, ride comfort, and tyre wear; the tool does not model those trade-offs.
- Should I measure pressure hot or cold?
- Manufacturer specifications are cold pressures (tyres at ambient temperature, not driven for at least three hours). Warm tyres read 2–4 PSI higher due to gas expansion. Enter cold-measured values for both recommended and current PSI to match the model's assumptions.
- How do I find my vehicle's recommended tyre pressure?
- Check the placard on the driver's door jamb, fuel-filler flap, or glove-box lid, or consult the owner's manual. The recommended PSI varies by front/rear axle and load; use the figure corresponding to normal driving conditions unless otherwise specified.
Spotted something off?
Calculations or display — let us know.
Report a Problem
Help us fix issues faster
Sources & Methodology
The calculator computes under-inflation as the difference between recommended and current PSI (floored at zero), multiplies by a user-supplied loss rate (%/PSI), caps the total penalty at 50%, then applies that percentage reduction to baseline MPG. The linear penalty model derives from SAE and manufacturer test data showing approximately 0.3–0.5% economy loss per PSI under-inflation in controlled conditions.
Related tools
Braking Distance Calculator
Calculate total stopping distance from speed, road friction and driver reaction time.
Power-to-Weight Ratio Calculator
Calculate hp/tonne and kW/tonne from horsepower and vehicle weight in seconds.
0-60 mph from Power-to-Weight
Estimate 0–60 mph time from engine horsepower and vehicle weight using the Hales empirical formula.
Quarter Mile Time Estimator
Estimate quarter-mile ET and trap speed from horsepower and weight using the Huntington formula.
Top Speed Estimator (Drag-Limited)
Calculate theoretical top speed from power, drag coefficient, and frontal area.
0-100 km/h Calculator
Estimate sprint time from power, weight and drivetrain using quarter-mile physics.
People also use
Monthly Car Payment Calculator
Financing & PurchaseCalculate your monthly car loan payment from loan amount, APR, and term in months.
Fuel Cost for a Journey
Fuel & EfficiencyCalculate the total fuel cost for a journey from distance, MPG, and fuel price per litre.
Loan vs Lease Comparison
Cost of OwnershipCompare total cost of buying on finance versus leasing over different terms and buyout scenarios.
EV vs Petrol Total Cost (5yr)
EV vs ICECompare total cost of ownership—purchase price, fuel, and depreciation—for EV vs petrol over 1–20 years.
Long Road Trip Total Cost
Practical & UtilityCalculate total fuel, lodging, food, and toll expenses for a multi-day road trip.
Depreciation % Retained Calculator
Resale & DepreciationCalculate what percentage of its original value a car has retained after depreciation.