The break even point in price differs with each car and local supply prices. If the local vendors are smart the E85 price point that gives the best over all revenue to the vendor is with E85 priced about 20-22% lower than regular gasoline. Some vendors cannot get that low due to poor purchasing decisions and in some cases local laws make it illegal to price that low due to "minimum markup laws".
On most cars the difference in fuel mileage between E85 and gasoline can range from a low end of 5% less fuel mileage on a well tuned conversion (ie not a FFV but a user converted car) to typically about 15% less for a Detroit flex fuel vehicle, although a few of them are horrible and get as low as 20%-25% less fuel mileage.
Different makes and models also vary in how they adjust for the new fuel. Some immediately adjust to optimum tune for E85 when switching from gasoline and others take a few tanks to get dialed in.
One other note, is that some cars get better over all fuel economy and lowest cost per mile on mixtures of gasoline and E85. The sweet spot often shows up around 60% E85 with regular gasoline, so play around with splash blending in the tank by putting a few gallons of gasoline in the tank then filling with E85.
Just like gasoline, E85 fuel blend changes seasonally to ensure good starting in cold weather. In the winter months fuel marketed as E85 may only be 70% ethanol in the northern states to get good cold weather starts, and only reach 85% ethanol blend for a few weeks in the heat of summer. In southern states where it is always warm you may never drop below 80% ethanol year round.
If the car is a high compression street rod, or turbocharged or blown muscle car you will almost always win price wise with E85 because the proper fuel is pump premium. Properly tuned in a high compression or boosted application E85 will give from 5% to almost 20% more power than pump premium.
Since modern manufacture cars are required by law to run without damage on 85 octane gasoline they are in effect by law forced to be poor E85 designs (although that may change with a few creative manufactures exploring ways around that limitation).
Unfortunately the way the CAFE regulations are written, the car manufactures have no incentive to optimize the engine for E85 so in general the flex fuel cars are good regular gasoline engines and mediocre E85 engines. The manufactures get the same benefit to their fleet average fuel economy if the engine barely runs on E85 as they would if the engine was optimized for E85 so most only make the minimum necessary changes to get acceptable performance. Home conversion very often out perform factory flex fuel vehicles both in power and fuel mileage.
When driving for economy I got 92% of my long term gasoline fuel mileage on my Subaru WRX after I converted it to run on E85 and if I flogged it hard, it ran much better and made considerably more power on E85 than I could make on pump premium and it even out performed 100+ octane racing gasoline.
Most of the flex fuel setups are pretty much black box systems, so not a lot can be done to improve them, but E85 engines typically like higher operating temperatures than you would run on gasoline.
My WRX stock thermostat was a 172 deg F thermostat (gasoline pump premium peak boost 14 psi), when drag racing it on 100+ octane racing gasoline (peak boost 24 psi) I needed to run a 160 degree F thermostat to avoid detonation. After converting to E85 I had to switch to a 190 deg F thermostat so it would warm up properly in cold weather. Before I went to the colder thermostat it would never get above 185 deg F coolant temp on a 17 mile highway commute in cold weather.
If you have the means to tweak the tune, e85 fuel likes about the same timing you would run for the engine on 118 octane gasoline. Best performance on a NA engine comes with compression ratios between 12.5 and 13.2:1, and fuel air mixtures at max power mixtures richer than you would run on gasoline (lambda .76 for my car) and highway cruise best fuel mileage at lambda 1.02 which would be about 15.0:1 fuel air mixture on gasoline.
Best place I know to find out local pricing spread on E85 and gasoline is at e85prices.com, if you have an interest in doing a conversion of an older muscle car etc. drop over to my E85 forum e85forum.net (old forum still searchable e85forum.com).
Larry
