Unfortunately don't have "before" data in the RS, but I can speak to local blends here from other vehicles
Previous car: 2002 Grand Prix GTP (Supercharged 6-cyl, "Bulletproof" GM 3800 Series 2 Engine)
Gas mileage on Premium (93 Octane) before Ethanol mandate wiped out clear gas in my area:
21.2mpg calculated, info center in the car displayed 21.9-22.1mpg. This was 90% city, 10% highway driving
Only have a single 400-mile road trip to compare for more highway driving: 27.7mpg calculated, 27.0mpg on the infocenter at 99% highway, 1% city
Gas mileage on Premium (93 Octane) after Ethanol mandate: 12.2mpg calculated, 12.4mpg infocenter. 90% city, 10% highway driving
Road trip mileage: 15.4mpg calculated and spot on on the infocenter, 95% highway, 5% city driving on 3 road trips at 250-400 miles each
And now I'm starting to see the signs on the gas pumps change. Thankfully VA law states they must label the pump with the blend they use. Recent change has been the 10% sticker getting covered up with a sticker that states the gas may be up to 15% ethanol.
What in the ever-loving fuck?
Some simple data for you, pruned from
http://www.afdc.energy.gov/fuels/fue...ison_chart.pdf
Gasoline energy content: 116090 btu/gal. Dependent on additives, octane ratings commercially available between 81 and 110 R+M/2
Ethanol energy content: 76330 btu/gal. Generally accepted as 127 octane (dervied from the same R+M/2 formula)
Taken a step further, if you want 93 Octane in a 10% ethanol blend, you mix the following:
0.9 gallons of clear-gas 90 octane (104481 BTU)
0.1 gallons of ethanol 127 octane (7633 BTU)
and end up with 1 gallon of blend, with an approximate energy content of 112114 BTU (96.5% of the energy content of clear-gas)
But wait, you say, shouldn't I get 96.5% of my old gas mileage? In a perfect world, yes, you should.
However, since burning clear gas and burning ethanol emit different chemical signatures, you are tricking your vehicle in two ways:
1st, with the lower energy content (even at best, only 3.5% lower) of ethanol-blended gas, your car must increase fuel flow by 3.5% to compensate for the energy content difference. By increasing the fuel flow, your car must also take in more air to reach what it believes is stoichiometry (in clear-gas only, that's 14.7 parts air to 1 part fuel in the combustion chamber at time of detonation), BUUUUUUT ethanol has a different stoichiometric ratio, at roughly 9 parts air to 1 part fuel. This means your 14.7 parts of air to 1 part of ethanol blend is really LEAN (too much air) (14.7 * .9)+(9*.1)=14.13 parts of air to 1 part of fuel in true stoichiometry for 10% ethanol. Your fuel consumption is up 3.5%, your air intake is up 3.5%, but your air:fuel mixture is out of whack by ~4% to the lean side. This makes your oxygen sensors read too much air in the mix, which causes the car to push more fuel into the combustion chamber to compensate (by ~4.17%) to reach the 14.13:1 stoichiometric ratio needed. This skews your STFT and LTFT (Short Term, and Long Term Fuel Trim) tables, by ~7.5%.
I could go on, but I'm already hurting my own head with this. I've given you guys the first couple paragraphs of what I could expand into a doctoral thesis. Questions?
O_E