Litres consumed per 100 kilometres; lower is better. Standard metric efficiency unit.

Energy lost deforming tires as they roll. Low-pressure or wide tires raise it.

Automotive Calculators

OBD2 Fuel Trim Calculator

2026-09-27 6 min

Total fuel correction and headroom to the fault threshold from STFT and LTFT.

Total Correction: 13.0 %; STFT + LTFT
Headroom to +/-20%: 7.0 %; Margin before codes

Calculator

Short-Term Trim(%) Help

Bank 1 STFT

Long-Term Trim(%) Help

Bank 1 LTFT

Results update live as you type

Live statistics

Total Correction

13.0 %

STFT + LTFT

Headroom to +/-20%

7.0 %

Margin before codes

Results

Optimal

Total Correction

13.0%

STFT + LTFT

Headroom to +/-20%

7.0%

Margin before codes

Planning estimate from standard automotive formulas. Validate with dyno, timeslip or datalog data when precision matters.

Visualizations

Results overview

13Total Correction

7Headroom to +/-20%

Total Correction vs. Short-Term Trim

Total Correction

13%

-3030

Formula

Sums instant (STFT) and learned (LTFT) corrections into the total fueling error the ECU sees.

Total = STFT + LTFT

Alternative Form

+/-10% normal; +/-10-20% investigate; beyond +/-20% sets codes

Variables

Symbol	Name	Unit	Description
%	Correction	%	Positive = adding fuel (lean condition)

Explanation

Positive trims mean the ECU adds fuel (unmetered air, weak pump); negative means removing it (leaking injector, high pressure).

Step-by-Step Calculation

Follow each step to understand how the result is calculated from your inputs.

Step 1

Record the inputs.

Short-Term Trim:5 %

Long-Term Trim:8 %

Result: 2 inputs ready

Step 2

Apply the formula.

Total = STFT + LTFT

Result: Primary result = 13.0 %

Step 3

Read every derived output.

Total Correction:13.0 %

Headroom to +/-20%:7.0 %

Result: Total Correction = 13.0 %

Final Answer

13.0

Worked Examples

See how the calculator works with different scenarios.

Scenario

Cruise trim check on a high-mileage car.

Inputs

STFT:+5%

LTFT:+8%

Calculation

Total = +13%; headroom = 7%

Result

+13% total correction.

Interpretation

Lean drift — smoke-test for vacuum leaks before it codes.

Understanding Your Result

Your Result: Total Correction: 13.0 %

Typical range: 0-10%: healthy. 10-20%: diagnose. 20%+: code territory.

What This Means

Total fueling error and distance to a fault code.

Why It Matters

The single best OBD health number — catches leaks and delivery faults early.

Professional Interpretation

Treat the primary result as a planning estimate, then validate on a dyno, at the track, or with logged sensor data. If measured numbers disagree by more than ~10%, re-check inputs (units are the usual culprit) before changing parts.

Recommendations

•Double-check units on every input before trusting the result
•Compare against the worked example below to sanity-check magnitude
•Validate with measured data (dyno, timeslip, datalog) when it matters

Learn — fuel economy, deeply

Fuel trims are the ECU confession of metering problems.

1 · Where the gallon goes (energy path)

Engine heat

60–70% lost

Coolant + exhaust carry most energy away. Hybrids recover some via Atkinson cycle + regen.

Driveline

5–15% lost

Transmission, differential, accessories. CVTs and 8–10 speed autos lose less.

Aero drag

∝ v²

Doubles from 55→78 mph. Roof boxes hurt more than trunks.

Rolling + brakes

10–20%

Tires, weight, stops. Regen braking (hybrid/EV) reclaims city braking energy.

4 · Measure like a pro (the 3-tank protocol)

Fill to first click at the same pump, record the odometer (Tank 0).
Drive normally for 2–3 tanks; always refill to first click at the same pump.
MPG = total miles ÷ total gallons across all tanks — averaging kills pump-click noise.
Cross-check with the trip computer; a persistent >10% gap means the computer needs calibration — or the car needs service.
Log with an OBD-II dongle for per-trip MPG if you want to isolate commute vs. highway behavior.

Practical Tips

•Read at warm idle AND cruise
•Log both banks to split bank-specific faults
•Fix trims before chasing MPG

Common Mistakes

Mixing units (inches with mm, psi with kPa)

Convert every input to the unit shown on its field before calculating.

Trusting a single output blindly

Cross-check with the worked example and, when it matters, a dyno or datalog.

Assumptions

This calculator makes the following assumptions to provide estimates:

Steady-state model: Formulas assume stabilized conditions; transients, heat soak and traction limits are excluded.

Standard factors: Empirical constants (BSFC, VE, drivetrain loss) use typical values — your combination may differ.

Limitations

Be aware of these limitations when using this calculator:

Estimate, not measurement: Predictions carry real-world scatter of roughly ±5–15% depending on the model.

No transient effects: Shift time, wheelspin, spool lag and heat soak are not modeled here.

Frequently Asked Questions

Answers to common questions about this calculator.

What are normal fuel trims?

Within +/-10% total at warm cruise. Positive = lean condition compensated; negative = rich.

High trims at idle but fine cruising?

Classic vacuum leak (unmetered air matters most at low airflow). Smoke-test the intake.

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Related Guides

Deepen your understanding with these educational resources.

Automotive Guides

Build planning, measurement protocol and validation how-tos

Glossary

Technical terms used in mortgage calculations explained in plain language.

MPG: Miles per gallon. Distance travelled per gallon of fuel; higher is better. US gallon = 3.785 L.
L/100 km: Litres consumed per 100 kilometres; lower is better. Standard metric efficiency unit.
MPGe: Miles per gallon-equivalent. EV metric: miles travelled on 33.7 kWh (energy of 1 gal of gasoline).
CAFE: Corporate Average Fuel Economy — US fleet-average efficiency standard enforced by NHTSA.
Rolling resistance: Energy lost deforming tires as they roll. Low-pressure or wide tires raise it.
Aerodynamic drag: Air resistance growing with the square of speed — why highway MPG falls fast above 60 mph.
Hypermiling: Driving techniques (coasting, steady speed, anticipation) that maximise real-world MPG.
OBD-II: On-board diagnostics port. Scanners and dongles read live MPG, fuel trim and fault codes.
Fuel trim: ECU fuel correction percent: STFT instant, LTFT learned. Beyond +/-10% signals trouble.

Authoritative References

Trusted sources and standards referenced by this calculator.

EPA — FuelEconomy.gov DOE — Fuel-Efficient Driving Tips NHTSA — Driving Safety & Efficiency SAE International — Fuel Economy Standards Consumer Reports — MPG Truth IRS — Standard Mileage Rates

Applicable Standards

EPA Fuel Economy Testing (40 CFR Part 600)CAFE — Corporate Average Fuel Economy Standards (NHTSA)WLTP — Worldwide Harmonised Light Vehicle Test ProcedureSAE J1321 / J1711 — Fuel consumption test proceduresISO 2575 — Road vehicles symbols & efficiency markings

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Last Updated:

2026-09-27

Version:

2.0.0

References:

License:

Free to use for educational and professional purposes.