psteng19

I decided to try 89 octane just for the heck of it and got 34 and 33.6 mpg on my last 2 fillups (MID showed 36.8 and 36.5 - it showed as high as 39.3 mpg at one point until I hit traffic).
Did not notice the dropoff in fuel economy as many suggested would happen by switching to lower octane.

Some specifics -
-80-90% highway, but pockets of rush hour bumper-to-bumper (for those in NJ/NYC metro, you know what I mean).
-Average speed on MID was ~34 mph. On highway cruising, about 55-60 mph.
-AC usage probably 50% of the time. In the mornings, I'm able to keep the windows up, and if it starts to get hot, I'll tilt the moonroof and pull in fresh air from vents.
-On downgrades I coast in gear; on level ground I'll coast in neutral to lights/stop signs.
-Car has 28k miles on it, 6 speed manual, aftermarket wheels/tires (Falken Ziex 512 @ 40 psi)

I'm fairly certain I can make the 500 mile club and sustain 38 mpg if 100% highway with no traffic.

miner

Good news and pointer. I avg. about 30mpg with my '08 6MT nav - around 11,ooo miles. Hope it does improve with usage. City/Hwy driving. My '06 RL will get 27mpg on the same route. Using Chevron 91 in both cars.

LukeaTron

If you're driving with the intent of good mileage, you're not going to be pushing the engine to where it has to retard the timing to compensate for the lower octane very often. As such, you wouldn't be very likely to notice the difference.

Technically, it's possible to get better mileage out of a lower octane since it will burn more thoroughly at lower pressure and temperature than will a higher octane. Temperature and pressure rise significantly when you start pushing the engine but pussyfooting around like that, the conditions in the engine probably favor a lower octane. It just sucks when you need some umpf and the ECU says here's all I can give you.

moda_way

If you were to simply use 15 gallons, you'd need only 33.3mpg to make the 500 club. If you really stretch to use 16 gallons, you need only 31.25mpg, so no need for the 38mpg. Go take a trip on the weekend.

psteng19

I guess my point is that it's been repeated many times here that fuel economy will suffer with lower octane but that just hasn't been the case for me on these 2 fill ups.
I'll have to repeat the test with 93 to see if I get even better mileage next time to see if it's really true.
If not, I may just use 89 from now on.

And I've never filled up more than 13.9 gallons before. That time, the MID was well below 0 miles left and I didn't want to risk it.
The highest I ever hit was 450 miles on the tank that got me 34 mpg.