Originally Posted by Rmay635703
Originally Posted by Wolf359
Technically you could just figure it out the cost based on the btu content of the fuel although some claim they get slightly better mileage
BTU content ignoring octane is a very poor way to calculate fuel economy,
if it was accurate there wouldn't be the,
"My car looses 33% of its gas MPGs on e10"
folks on every site.
https://www.greencarcongress.com/2007/12/study-finds-cer.html
I've never lost anywhere near 20-30% when I have run e85, in hot weather my economy is within 1mpg, cold worse.
Vapor Pressure inflection point, cooling effects and octane can have large effects on fuel economy, e85 tracks positively on all those metrics.
That said when fuel is expensive and e85 is a buck a gallon cheaper I usually ran e30 because my fuel economy would remain unchanged compared to RUG. Scientifically e20-e30 nets much better economy than BTus alone would account for anyway which explains it.
I've run ethanol fuels a long time and view it as a worthwhile test, if you have a flexfuel car, start tracking your MPGs then switch to e20 30 or 85, if those fuel are much cheaper and don't drop economy off a rock go for it. At that it becomes a simple math problem for or against.
It's not equivalent to E0-E15 gasoline because it's not really gasoline. It's generally more efficient per BTU because the ECU can advance the timing. It's like 105+ AKI octane if it has the maximum ethanol. There lies the rub. Slightly less ethanol will result in slightly higher fuel economy, but the maximum performance comes from higher ethanol. You're adding oxygen to the mixture that's easier to compress and I understand it helps to cool down the charge air. So overall there's more oxygen, and an engine is really just an oxygen pump.
There's going to be a sliding scale where there's the tradeoff between performance and fuel economy. I think if the ECU was limited to advancing the timing no more than for 91 AKI octane, the fuel economy would be proportional with the energy content. But it's not that simple because of the increased timing. If one could use 105 AKI octane (usually leaded I think) racing fuel and an ECU could advance the timing, that should also be more efficient relative to the energy content compared to 91 octane.