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

Long-Term Fuel Trim Calculator

2026-09-27 6 min

Learned trim verdict and MPG penalty from LTFT percent.

Learned Correction: 12.0 %
Fuel Penalty: 12.0 %; Extra fuel burned

Calculator

Long-Term Trim(%) Help

Warm cruise LTFT

Results update live as you type

Live statistics

Learned Correction

12.0 %

Fuel Penalty

12.0 %

Extra fuel burned

Results

Optimal

Learned Correction

12.0%

Fuel Penalty

12.0%

Extra fuel burned

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

Visualizations

Results overview

12Learned Correction

12Fuel Penalty

Learned Correction vs. Long-Term Trim

Learned Correction

12%

-3030

Formula

Treats learned correction as both fault evidence and economy loss.

Penalty ~= |LTFT|% (each percent of correction is a percent of fuel)

Alternative Form

LTFT over +/-10% deserves diagnosis even without codes

Variables

Symbol	Name	Unit	Description
LT	Learned trim	%	Chronic correction memory

Explanation

LTFT is the ECU permanent adaptation: +12% means it always adds 12% fuel — and economy always pays it.

Step-by-Step Calculation

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

Step 1

Record the inputs.

Long-Term Trim:12 %

Result: 1 inputs ready

Step 2

Apply the formula.

Penalty ~= |LTFT|% (each percent of correction is a percent of fuel)

Result: Primary result = 12.0 %

Step 3

Read every derived output.

Learned Correction:12.0 %

Fuel Penalty:12.0 %

Result: Learned Correction = 12.0 %

Final Answer

12.0

Worked Examples

See how the calculator works with different scenarios.

Scenario

MPG complaint with no codes yet.

Inputs

LTFT:+12%

Calculation

Penalty = 12%

Result

+12% fueling, ~12% economy loss.

Interpretation

Find the unmetered air or weak delivery — free MPG hides here.

Understanding Your Result

Your Result: Learned Correction: 12.0 %

Typical range: Under 5%: excellent. 5-10%: aging normal. 10%+: diagnose.

What This Means

Chronic fueling error and its economy cost.

Why It Matters

Explains mystery MPG drops with no check-engine light.

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

LTFT is the ECU long memory of fueling error.

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

•Reset (or relearn) after repairs
•Check at idle AND cruise
•Graph over time to catch drift early

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 causes high positive LTFT?

Vacuum leaks, dirty/failing MAF (under-reading), weak fuel pump, clogged filter, exhaust leak ahead of O2.

What causes negative LTFT?

Leaking injectors, high fuel pressure, saturated EVAP canister, or an over-reading MAF.

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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.

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.