Figure out exactly how much E85 and pump gas to add at the pump to hit your target ethanol blend. Works in gallons or liters.
Ethanol's effective octane rating (~113 AKI) lets turbocharged engines run more boost and timing without knock. But full E85 demands roughly 30% more fuel volume than gasoline, which maxes out stock pumps and injectors quickly. Blending captures most of the benefit at a fraction of the strain - which is why E30-E50 "blend tunes" are so popular on direct-injected platforms.
Fuel sold as "E85" is legally allowed to be anywhere from 51% to 83% ethanol, and stations blend seasonally - winter E85 often tests closer to E70 for cold-start performance. If you're mixing to a tight target without a flex fuel sensor, test the pump fuel with a cheap ethanol tester first, and enter the real number in the "Pump E85 Actual Content" field above.
Add the calculated volumes in either order - the tank mixes within a few miles of driving. The blend changes your air-fuel requirements: only run blends your tune supports, and if you're datalogging, watch fuel trims and HPFP pressure the first few pulls after a blend change. The AFR/Lambda calculator shows how stoichiometry shifts with ethanol content.
Ethanol is partially oxygenated, so it takes less air to burn it completely. Its stoichiometric air-fuel ratio is about 9.0:1, compared with roughly 14.7:1 for gasoline. Pure ethanol also holds less chemical energy per gallon than gasoline. The combined effect is that an engine running full E85 has to deliver on the order of 30% more fuel by volume to make the same power - the exact figure depends on the blend, since E70 needs less extra fuel than E83. This is the single most important consequence of switching to ethanol: stock injectors and pumps that were comfortable on pump gas can run out of headroom at high load, which is why blend tuning at E20-E30 is so popular. A lighter blend captures most of ethanol's knock resistance while keeping the fuel-flow demand within what a stock system can supply.
Say you have 5 gallons of E10 already in a 15-gallon tank and you want to land at E30 on a full tank. You have 10 gallons of space to fill. Your pump E85 tested at 80% ethanol (a realistic real-world reading rather than the 85% on the sign), and the pump gas is E10. The total ethanol you need in a full 15-gallon tank at E30 is 15 × 0.30 = 4.5 gallons of pure ethanol. The 5 gallons of E10 already contribute 5 × 0.10 = 0.5 gallons. So the 10 gallons you add must supply the remaining 4.0 gallons of ethanol.
Adding about 4.3 gallons of the 80% E85 and 5.7 gallons of E10 lands the full tank right at E30. Notice that if you had assumed the pump fuel was a true 85% instead of the tested 80%, you would have added too little E85 and come up short of your target - which is exactly why testing the actual content, or running a flex fuel sensor, matters when you are aiming for a tight blend.
E30 (30% ethanol) is the common sweet spot for direct-injected turbo cars: most of E85's knock resistance with far less fuel-system strain and fuel consumption penalty. Dedicated E85 tunes run E60-E85 but require upgraded fuel pumps and injectors on most platforms. Always match the blend to what your tune was calibrated for.
Legally, fuel sold as E85 can contain anywhere from 51% to 83% ethanol, and stations blend seasonally - winter E85 is often closer to E70 to aid cold starting. If your car lacks a flex fuel sensor, test the actual content with an ethanol tester before mixing to a tight target.
Ethanol's stoichiometric air-fuel ratio is roughly 9.0:1 versus about 14.7:1 for gasoline, and ethanol carries less energy per gallon. To make the same power the engine must inject substantially more fuel - on the order of 30% more for full E85 - which is why dedicated ethanol setups usually need larger injectors and a higher-flow fuel pump.
If your car has a flex fuel sensor the ECU reads ethanol content directly. Otherwise, use a graduated ethanol test kit: add a measured amount of water to a known volume of fuel, shake, let it settle, and read how much the water layer grew. The increase is the ethanol fraction, since ethanol mixes into the water and gasoline does not.
Use a volume-weighted average: resulting ethanol % = (Vol1 × %1 + Vol2 × %2) ÷ total volume. To solve for how much E85 to add to a known amount of pump gas, rearrange for the unknown volume. The calculator on this page does both the target-blend solve and the two-fuel mix automatically.
It can be if you assume a fixed content. Winter blends drop toward E70 for cold starting, so a tank you expected to be E85 might actually mix leaner on ethanol than your tune assumes. Either test each batch, run a flex fuel sensor, or leave margin in your tune so a few points of ethanol variability does not push you into knock.
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