Better mileage from premium gas?

[SubieMovie]

Just a question. Although the Crosstrek manual states that regular gasoline should be used, I put 4 gallons or so of premium fuel a few weeks ago and saw roughly a 6 mpg gain.

I'm running regular again and it's fallen back down to Earth.

Do the Boxer engines run better on premium or is this just imagined placebo [censored]?

I wouldn't mind throwing a blend in there if it meant a decent boost in mpg.

 

[Quattro Pete]

If you could truly gain an additional 6 mpg by running premium, Subaru would be mandating you to run premium in order to claim much better CAFE MPG numbers.

IMHO, there is no way you're gaining this much just by switching to premium.

 

[kschachn]

The energy content of gasoline is irrespective of the octane rating - assuming it is all gasoline. If it is a blend of gasoline and ethanol then that does not apply. Some regions/stations sell premium that is E0 instead of E10, that can make a difference.

 

[Reggaemon]

Both of my Subarus get the same mpg regardless of the grade or brand of gas used.

 

[SwampSurvivor]

Some vehicles can see an increase with premium fuel. Higher resistance to detonation means that the ignition timing can be advanced.

DI engines, higher compression engines will see at least a slight increase in fuel economy for sure. My parents got quite an increase in fuel economy switching their Ecoboost Escape to 91

 

[Benzadmiral]

Originally Posted By: Miller88
Some vehicles can see an increase with premium fuel. Higher resistance to detonation means that the ignition timing can be advanced.

DI engines, higher compression engines will see at least a slight increase in fuel economy for sure. My parents got quite an increase in fuel economy switching their Ecoboost Escape to 91

Last summer I tried 93 E10 in my 2.4L DI Ecotec engine. While I saw a small boost in economy, it wasn't enough to offset the extra cost of the 93. So I stick with 87 now, usually Exxon. The Regal seems to get the best mileage on it, and I get a .06/gallon "loyalty reward" rebate on each gallon I buy.

 

[dparm]

To be as scientific as possible, you'd need to keep the conditions exactly the same tank-to-tank. In the real-world, that's virtually impossible.

Only way I can think of to definitively measure this would be to either run several premium tanks and work very hard to keep conditions nearly the same, then run a few regular tanks and replicate those conditions.

Alternatively, if you have some datalogging software, check if the timing is being advanced while running premium. If it is, that means the car is taking advantage of it.

 

[FutureDoc]

A few engines can get a small bump, but it normally is not enough to offset the price-point. Often you would need one of two things, an engine that was converted to "accept" regular after being designed around premium... or a higher-compression (sometimes a turbo) DI engine that has knock sensors designed around different octane/HP abilities. One good example of the higher-compression/turbo engine is GM's GenIII Fam0 1.4T DI engines. Cruze/Encore/Spark owners see a mild bump. Something like 5-10% but not often enough to make-up the cost in gas. I have seen this when driving my mother-in-laws Buick. However, I can not completely discount error in this observation as the difference could be cause by other spurious factors. On the other hand, I think I could get a bump on my DI Hyundai as I have different EPA HP numbers for the Genesis Coupe. Have not tried it yet.

For Subaru, only the old EJ2.5 Phase I could/would see a bump. Mostly this was because the first EJ2.5 were designed to use Premium and then were later reworked for "regular". So 1995 required Premium but 96-99 could run "regular". I have tested this out with my '99 and I might see 0.5-1mpg (5%) if I am lucky. Not enough to to rule out random error.

Thus with your MPI 2.0, I would keep to regular.

 

[Jetronic]

seems the answer is: depends on the vehicle...

 

[bubbatime]

Originally Posted By: Miller88
Some vehicles can see an increase with premium fuel. Higher resistance to detonation means that the ignition timing can be advanced.

DI engines, higher compression engines will see at least a slight increase in fuel economy for sure. My parents got quite an increase in fuel economy switching their Ecoboost Escape to 91


Absolutely 100% correct. Some high compression engines will run 87 octane, but the computer retards the timing to prevent detonation. When you fill it back up with premium, the computer advances the timing to where the engine runs optimally, and thus increases fuel mileage. My Acura has 11 to 1 compression and will get about 1 mpg boost running premium fuel.

 

[WishIhadatruck]

I've never noticed a difference in fuel mileage on any vehicle I've owned in the past, but my current Cruze Eco definitely gets better mileage with mid-grade or premium. It has smoother power delivery and the calculated mileage is better by a noticeable amount. The mpg's showing in the trip computer don't show it is any better, but in warmer weather the calculated mileage is much less than the trip computer if running 87 octane. Running 89 octane or 93 octane and the calculated and trip computer are very close. You can actually feel the timing get cut back and power drops under certain circumstances when running 87 octane. In the winter I run 87, and 89 octane the rest of the year.

Each car type is different in how it responds.

 

[racer12306]

Makes me curious to do a test. I've been tracking everything about my car since I started driving it regularly again back in October.

My driving is so consistent that I could feel I could accurately test something to determine effectiveness.

Seems if I can make my drive with zero or very minimal traffic delays, it returns between 34.0 and 34.6 MPG. As soon as traffic gets thrown into the mix or more than one trip of local driving it dips to 30-32 MPG depending on the situation.

 

[pennzoil_guru]

I've only seen an improvement of 1-2 mpg using Shell V Power 91 which is E0 up here. Essa, Petro Canada, Ultramar and Husky premiums are all E10 and I've not seen any change with those. I think in the average engine, ethanol content has more of a fuel economy effect than octane rating

 

[surfstar]

^bingo

If 91+ gets you E0, vs E10, then yes, you may actually get more MPG.

 

[1000MPH]

I've been tracking the mileage on my 4Runner since new. I do a 360 mile one-way trip about 15-20 times a year. I usually stop at the same gas station and same pump. This trip is done very early in the mornings with little to no traffic. Mileage results are all over the place with the only real measurable being a consistent 10% drop in the Winter months (approx dec-feb) on the outbound leg.I've tried multiple tanks of premium but cannot track results accurately enough.
The biggest obvious factor is winds.

 

[Bamaro]

If you actually got a 6 MPG increase, most, if not all of that was for other reasons.

 

[GaleHawkins]

We better MPG and power from premium in the 1992 TBI 454 engine in the motor home but that is the only gas engine where I can measure the difference. That old system retards timing if it senses a spark knock so as a guess the premium gas prevents retarding of the timing hence better performance. Some say it can not happen but they are clueless in this case. At age 65 I can still do simple math.

 

[cmhj]

Originally Posted By: GaleHawkins
We better MPG and power from premium in the 1992 TBI 454 engine in the motor home but that is the only gas engine where I can measure the difference. That old system retards timing if it senses a spark knock so as a guess the premium gas prevents retarding of the timing hence better performance. Some say it can not happen but they are clueless in this case. At age 65 I can still do simple math.

Same here, and example is my Fiat 500. It likes premium but runs ok on regular. I'll see 3-4 MPG difference between the 2. It all depends on the setup.

 

[Wheel]

I'm curious how Subaru manages to run their engine well on regular gas having 9.3:1 or better compression ratios on their N//A 2.5 engines.

That said, I've experienced the best mileage on my last 2 Subarus running mid-grade (89) compared to regular or hi-test. Difference being only about 2 worse than hi-test, but 4-5 better than regular.

 

[surfstar]

Amazing that people get 4+mpg better with premium. That is insane. 10-15%+ mpg improvement. Sounds like snake oil! Especially when its on N/A engines!

Literally guys, it sounds like snake oil - aka, I'm not buying it. You can't convince me of this crazy phenomenon without soume great data to back it up.