GEM Fuel Mix Calculator

GEM is a blend of gasoline, ethanol and methanol, mixed so its stoichiometric AFR matches E85. That makes it a drop-in replacement for E85. The number in the name is the methanol percentage by volume, so GEM45 is 45% methanol.

I get questions about mixing GEM all the time, so I decided to whip up this calculator. You can use it to get an idea of the amount, by volume, of each component in GEM you’ll need for a batch size you enter (5 gallons by default, but you can change it!)

Use high quality race fuels and E98, there is no option for E85 in this calculator because street fuels are not exactly consistent.

Your blend

Methanol % (the GEM number) Batch size (gallons)

Fuel properties

	Stoich AFR	Density (g/cc)
Gasoline
Ethanol (E98–E100)
Methanol
Target (E85)

The blend's AFR is the mass-weighted average of each fuel's stoich AFR, so densities matter.

Calculate

Mixing tips

• Use pure ethanol (E98 or E100), not pump E85. Pump E85 already contains gasoline, and its ethanol content varies a lot by season and station.
• Measure by volume in marked containers, and mix in a sealed container.
• Recheck the results if your gasoline or ethanol supplier gives different stoich values, and enter them under Fuel properties.