Just to add a few comments here, but switching fuel types between fill-ups is not going to give you a true reading on how well your car is performing with that type of fuel.
I'm primarily talking octane ratings here, but the same can be said of ethanol blends as well.
There are several factors at work here, any of which will skew your readings:
1. The fuel left in your tank - anything left in it will dilute the new blend you put in the tank.
2. Where you buy your fuel - Different companies use different additives which can affect results. Also, if they just received a new shipment of fuel, lots of junk from the bottom of the storage tanks can be mixed in with the fuel.
3. When you buy your fuel. In the US at least, there is a "Summer" blend and a "Winter" blend. In addition to other additives, the amount of ethanol in the blend also changes in some areas.
4. The computer sensors in the engine (ECU), powertrain (PCU), Air flow sensors, O2 sensors, exhaust gas temperature sensors, etc. are all at work trying to analyze everything going on to create the perfect blend of fuel and air to burn at the best efficiency. It can take several tanks just for the fuel sensors to detect the change in octane and make the proper adjustments to the air/fuel mixture.
5. The weather - changes in air pressure, whether the road is wet/dry/slippery will certainly have an effect on the results.
6. Obviously - where (and how) you drive. If you aren't driving the same or similar routes in the same levels of traffic, etc. then your results are going to be different.
As far as whether the different octane ratings will hurt the engine, the answer is most likely not. An engine set to use regular octane, will adjust properly in most cases to a higher octane blend. Where you can do damage is by using a lower octane rating that what your engine requires as all the sensors in the world cannot add the octane required by a high-performance, high-compression engine.
Finally, I'll get to my results with my Kizashi. My city driving in the DC area is certainly pretty predictable - lots of traffic and lots of stoplights. I take regular trips to NC every couple of months that total about 600 miles per trip, almost all highway miles. What I've learned so far is that in the city, when I'm idling away in traffic and stoplights, the octane rating/ethanol rating/additive mix etc. has absolutely zero effect on mileage. If I'm not moving I am getting zero MPG. When I am moving, a 1 or 2 mpg difference really doesn't mean much.
On the Highway, however, where I can maintain a constant speed over a long period of time, I can certainly tell a difference.
I've switched from regular "89" octane to premium "93" octane over several months at a time. In the city on regular, I get on average 18mpg. On premium it jumps to 20mpg. Roughly a 10% increase. On the highway, with regular fuel, I could get 28mpg and with premium it would jump to 34mpg. Just over a 20% difference. The catch is, that in order to get the full 20% jump, I would have to run at least one full tank of premium in the city, which essentially means my net gain in combined city/highway would be about 15%. Measured over a six month period of combined city/highway, the mpg calculator would settle in at about 22.2 mpg using regular. Using premium, it settles in at around 24.8 over the same period. As I do more city driving than highway, my net gain is only 12%.
At $3.70/gallon for regular and $4.00/gallon for premium, I still come out better by paying more for the premium gas as the premium is only 8% more. To keep the calculations simple, if premium is more than 10% than regular at the pump, I'll switch to regular for that fill-up, but otherwise I get enough benefit from burning premium to make up for the difference in price.
Sorry for the long diatribe, but hopefully someone will find it useful.
)
