2006 Highlander Hybrid Experiment (premium vs. regular gas)
 

According to the owner's manual, this vehicle is premium fuel recommended, but it is not required. I have been running it on regular because I am cheap. I recorded my best-ever fuel economy (35.5MPG) on the highway with nearly all premium fuel (it was an experiment).

Thus, I am deliberately trying to see if a notable difference may be had in "regular" driving with premium fuel over extended periods of time, or if I should stick to only throwing in premium when driving long distances.

Current prices: $3.37 regular, $3.57 premium (87/93 respectively)

Percent difference: 0.20 / ((3.37 + 3.57) / 2) = 5.76%

Required "MPG improvement" is equal to the percent difference to break even. Savings per mile kick in after that.

Assuming (which may not be true) that adding roughly 33% of a tank worth of premium fuel means the "desired improvement" should drop with the portion of premium vs. regular in a tank, the MPG improvement adjusts accordingly.

Fuel replaced today: 5.627 / 17.2 = 32.72% of tank

New required break-even MPG improvement (32.72% of 5.76%) - 1.88%

Unfortunately, I am well-aware that 2% changes are often lost in statistical noise with the car computer/pump introducing error. However, I hope that a 5% or more change would be noticeable.

I will update this thread as I learn whether this is a worthwhile venture.

Before anyone says "You're wasting money because premium fuel is pointless," remember this vehicle has a recommendation for higher-octane fuel--putting high-octane in an engine not tuned for it is a waste. Higher-octane in an engine that CAN take advantage of the higher octane--that has the makings of an experiment.

Not to push you in an alternative direction but on my cobalt which has a 10+:1 compression ratio motor, seems to have a sweet spot above plain 87 octain, it also coughs, bucks and chugs less at low RPM with mid grade or higher.

These cosmetic differences brought me to wonder if the higher octane of e85 and slightly lower cost would have a break even point for cost vrs fuel economy.

I tested several tanks two were with 87 octane (ethanol free) with 10-20% e85
This mix was a net savings because at the time the ethanol free was only a few cents more than E10 and the e85 was cheaper, fuel economy was about 2-5% better than expected but within noise. (but also cheaper per gallon.

I also tested 93 octane e0 mixed with 30% ethanol (e85) because it cost the same per gallon as e10 87 in this mix, and I could not find e0 87 anymore. This tank was moot (matched FE) and there was no cost savings but the car ran better in specific situations (aka low RPM) and seemed a tad peppier.

So yes you can get slightly better FE with better octane or you can use e85 and get premium like fuel at a slightly better price point without a fuel economy hit. (if you find your cars sweet spot)

Good Luck
Ryan

I can definitely try mixing E85 with regular fuel; unfortunately to my knowledge there are no straight-gas stations in my state.

I have 10.8 compression ratio--my engine definitely sounds "smoother" with high-octane fuel (I noticed that when it was ~90% premium). I'll try the high-octane route first, then try mixing E85.

UPDATE

Percent difference, gas prices: 0.30 / ((3.59 + 3.89) / 2) = 8.02%

Percent difference, MPG: 2.32 / ((20.41 + 22.73) / 2) = 10.75%

Temperatures have only gotten worse over that period, while driving patterns have remained virtually identical. By now, the regular (87) should be "out of the system" since I've put in 1/3, 1/2 and another 1/2 tank of 93.

So far, the small amount of empirical evidence suggests running this vehicle on premium costs less as the percent difference in gas price is smaller than the percent difference in MPG gained.

I mixed 87 E10 gas up to 20% ethanol with E85 (the E85 was overpriced as hell though since I'm in California) and the fuel economy seemed to be about the same as straight 87, and I think the engine was a little happier but maybe that's just placebo effect. I think the effective octane would've been about 90, my motor is 10:1 compression though. I don't have many entries in my fuel log though so it's hard to say. I'd love to do several tanks of E20 back to back sometime to get more data, but it's kind of a pain in the ass to go blend in 1.5 gallons of the stuff just to save 50 cents.