Octane blend calculator

Octane is treated as a volume-weighted average. Once the tank is full, the fuel already in it and the two pump volumes must add up to the capacity and average out to the target:

Estimating what’s left in the tank (from the range, or for the MR2 and CR-V from the fuel gauge), then eliminating \(V_2\), gives the four quantities the calculator reports:

1. \[ V_0 = \frac{d}{m} \]
   —
2. \[ V_{\text{fill}} = C - V_0 \]
   —
3. \[ V_1 = \frac{O^{\star} C - O_0 V_0 - O_2 V_{\text{fill}}}{O_1 - O_2} \]
   —
4. \[ V_2 = V_{\text{fill}} - V_1 \]
   —

Both pump volumes must lie between zero and \(V_{\text{fill}}\), so the reachable targets are bounded by filling entirely from the lower or the higher grade:

1. \[ O^{\star} \ge \frac{O_0 V_0 + \min(O_1, O_2)\, V_{\text{fill}}}{C} \]
   —
2. \[ O^{\star} \le \frac{O_0 V_0 + \max(O_1, O_2)\, V_{\text{fill}}}{C} \]
   —

\(d\)

Range left, miles

\(m\)

Average fuel economy, MPG

\(g\)

Fuel gauge reading, percent full (MR2 and CR-V)

\(C\)

Tank capacity, U.S. gallons

\(O^{\star}\)

Target octane for the full tank

\(O_0,\ V_0\)

Octane and volume of the fuel already in the tank

\(O_1,\ V_1\)

Octane and volume from pump 1

\(O_2,\ V_2\)

Octane and volume from pump 2

All octane ratings are AKI. Real blends can be slightly nonlinear, so treat the result as an estimate; it doesn’t certify fuel quality or suitability. Always follow the vehicle manufacturer’s fuel requirements.