Premium fuel is pretty high in price.

I usually fuel regular.

And as some have noticed, the Cruze doesn't do well with regular when the temps exceed 100F.

I was thinking in my 13Gal fuel tank, to add 2 gallon of E85 fuel for every 11 gal of Regular fuel.

It should be exactly the same price as a tank of regular, but it'll result in E14 (higher knock resistance than regular, pretty similar to Premium fuel).

The price of a full 13GAL tank here locally at $2.69/gal is exactly $35.

This would be the same as a tank of Regular + E85, as E85 is sold here at the same price as regular (in your area too?).

If I would fill up with premium, at $3.09/gal, I would pay $40 for a tank.

$5 extra for a tank might seem silly, especially since Shell premium fuel usually has 5% better MPG than Shell regular (which brings the total price closer to $3 surplus per tank).

And I'm only expecting that 2 gal of E85 is enough to make the fuel similar to Hi-Octane fuel in terms of knock resistance, which is more of a guess than an actual measurment.

Open for discussion on more technical data on E85 octane rating!

So, if I would do 500 miles on a tank of regular fuel (13GAL),

I would do 525 miles on a tank of Premium (Hi Oct) fuel under the same conditions.

On the other hand, E85 is about 30% less efficient than regular fuel (according to some sources, more proof needed) , so in order to go just as far as 13 gallons of regular fuel, I would do:

11 gallons of regular (500/13*11) + 2 gallons of E85 (500/13*2*0.6), and that gives me 470 miles.

Now we can compare the range of all tanks (regular, Regular + E85, and premium), with the price paid:

Reg: 14.28 miles per dollar.

Prem: 13.12 miles per dollar.

Reg+E85: 13.42 miles per dollar.

When we look at the above numbers, and use Regular as our reference (base), and convert the miles/dollars to percentage, we get:

Reg: 100%

Prem: 92%

Reg+E85: 94%

Our current numbers indicate going Premium is the least economical, and going regular is the most.

And adding 2 gal of E85 in an 11GAL regular tank, will result in a 6% loss, however, there will be less heat soak/knock losses.

Using regular + E85 over Premium would make sense, if Premium gasoline prices are 10-15% (or higher) priced than regular fuel.

Otherwise (premium fuel prices are <10% surplus over regular), use premium gasoline.

Most people wouldn't go through the trouble of mixing E85 with regular, if your local pump doesn't serve E85; but if you have both fuels at the same pump near your home, why not?

Open for discussion!