Fuel Metrics

1.Overview

The fuel metrics widget combines instantaneous economy, cumulative consumption, cumulative cost, and average economy into a single slot. The core calculation is fuel flow rate (L/h) — get that right and everything else is right.

Because the data available over OBD2 varies by make and model year, three fallback methods are provided. Newer vehicles get the most accurate source; older ones still get a usable approximation via Speed-Density.

2.On-screen items

The slot displays the following — exact items shown depend on slot size:

3.Fuel flow calculation (three-tier fallback)

The app picks a source automatically in this priority order, based on what the vehicle supports:

① PID 0x5E — Engine Fuel Rate (most accurate)

Reads the fuel flow value the ECU has already calculated internally. Since it comes from actual injection quantity, it is the most accurate option and requires no user tuning.

Supported on: relatively recent vehicles (some 2010s+ models)

② Estimated from MAF (Mass Air Flow)

Uses the air mass flow from PID 0x10 and assumes a stoichiometric air-fuel ratio to back out fuel mass. Practical accuracy in low-load conditions.

fuelRate (L/h) = MAF (g/s) × 3.6 ÷ AFR (14.7) ÷ density (0.75 kg/L)

Supported on: any vehicle with a MAF sensor

③ Speed-Density estimation (for vehicles without MAF)

Estimates the air mass flow itself from MAP, RPM, intake air temperature, displacement, and volumetric efficiency (VE), then derives fuel flow from there. This is the only mode that requires user tuning.

Air density ρ = MAP / (287.05 × (IAT + 273.15))
Volumetric flow V = displacement × RPM / (2 × 60)   // 4-stroke
MAF estimate = ρ × V × VE
Fuel flow = MAF / AFR ÷ density × 3600

Dynamic VE correction: VE is corrected by base VE × (1 + 0.13 × load%), where load% is derived from MAP / atmospheric pressure. Idle vacuum maps to 0% load, NA WOT to 100%, and boost is clamped at 120%.

AFR enrichment under boost: AFR = 14.7 / (1 + enrichment × min(boost/40, 1)). Enriches the mixture proportional to boost, approximating the fuel enrichment turbo engines apply at high load.

4.Deceleration fuel-cut detection

In Speed-Density mode the app approximates fuel cut during engine braking. When all of the following conditions are met, fuel flow is throttled to the idle ratio (default 0.5×):

• Fuel-cut feature enabled (fuelCutEnabled = true)
• Vehicle speed > 5 km/h
• MAP < 30 kPa (high vacuum)
• RPM > 1500 (high engine speed)

5.Detail settings

Settings split into "global" (tied to the vehicle, shared across all slots) and "per-slot" (display customization).

Global (vehicle parameters)

VE, boost enrichment, and fuel cut only appear in Speed-Density mode (irrelevant when PID 0x5E or MAF is available).

Per-slot (display)

Design principle: physical, vehicle-bound parameters (displacement, VE, enrichment) are global; user preferences (pricing, currency, alerts) are per-slot.

6.Update rate and session handling

7.Tips

Improving accuracy

• First, check "calculation mode" in the detail settings to see which source is active
• If PID 0x5E is available, no tuning needed — it comes straight from the ECU
• MAF mode usually does not need tuning either (stoichiometric assumption is accurate in cruise)
• Only Speed-Density mode requires VE tuning. Compare cumulative consumption to actual fill-ups: too high → lower VE; too low → raise VE

Turbo vehicle starting point

• VE: 0.85
• Boost enrichment: 15%
• Fuel cut: ON

NA vehicle starting point

• VE: 0.80 (typical)
• Boost enrichment: 0%
• Fuel cut: ON

Pricing and cost alerts

• Gallon mode targets the US market — pairs naturally with MPG as the display unit
• Alert intervals let you set notifications "every $5 / $10 / $20" for long drives

8.FAQ