E10 has 3% less energy.
E10 should give you 3% lower MPG. This is expected.

However, E10 adds more oxygen to the fuel. The exhaust O2 sensors see this increase in oxygen, and say to the engine "you can add more fuel to use up that oxygen".

So not only are you getting less energy in each gallon, now, your car is injecting more fuel at the same time. This "feedback loop" will cause some cars to see 10% lower MPG.

If your car gets more than a 10% drop in MPG when using E10, you should look deeper for other causes such as wind, terrain, and temperature.

It turns out at E30, 30% ethanol, most cars give the same, or better MPG than E10.
If your mileage is the same, you might as well save a few cents with E30, and send some pocket change to a mid-west farmer vs. mid-east shiek.... but unless you live in the just handful of cities that have E30 pumps, you need to fudge this blend on your own by mixing gas or E10 with E85. It's not exactly easy, nor an exact science either.

Most cars with electronic fuel injection can handle up to 50% ethanol without issues.
It is best to move the ethanol % up or down in small steps if possible. It takes cars more than 30 miles / 30 minutes to adjust to a new ethanol percentage, and your MPG, and car's performance will suffer, sometime dramatically during the adjustment process.

I've used 6 or 7 tanks of E85 in my normal car. It runs very, very well on E85.
There is more power, and it climbs hills at lower RPM.
It starts easily at 15'F.
But it idles rough if I change the ethanol by more than 25% in one "jump".

-John

P.S. California uses 5.75% ethanol which was an exact replacement, oxygen wise, for MTBE. Calif. will move to 10% ethanol soon.

Even if your MPG goes down ( and it will ) 10% ethanol produces 30% to 50% lower emissions than pure gas, so it's more of an environmental thing, than a MPG thing!