It will depend a lot on the engine. The higher the compression ratio, the more it will matter. If you have a fairly pedestrian N/A engine, it might not make any difference at all. If you have a fairly high compression engine, plus turbo or super charging, and you're towing a TT, it could make a huge difference. Then there's the whole world in between those two extremes.

The engine on our F-150 is high compression, and it is turbocharged. The owner's manual recommends premium when towing. So we do that. If you have an engine monitor, you can see the difference in the ignition advance.

I decided to try the higher octane for two reasons. Out west, the "regular" gas was 85 octane and not the 87 I expect and the minimum Nissan recommends for the Frontier. Midgrade there was 87 octane and premium was 89 or 91 octane (varied with stations). As much as we were already stressing the engine in the mountains, I did not want to add detonation to the mix. Some of the stations had ethanol free gasoline in premium. A couple had cheaper ethanol "midgrade" at 87 octane but the 85 octane regular and the premium were ethanol free.

Results seemed to indicate that the higher octane did not really yield a significant increase in mileage. Ethanol free gas did but I could not say the additional octane of the premium did much good.

Contrary to layman's notion that "higher octane" equates to "more powerful, the reality is more the opposite of that. High octane gasoline is merely less susceptible to pre-ignition. The higher power part of it is that higher octane allows earlier spark (AKA more timing advance).

If you tested the highest power density fuel (diesel), it would test out well below 80 octane.

As I've said before, probably the best check on power output of your gasoline engine (not diesel, obviously), is to install an engine monitor or perhaps display it if possible (if built in), and look specifically at the spark advance. Higher octane fuels will allow much more spark advance, and thus allow more power output. If your engine is running at 10% power or less, it might not make much, if any, difference.

http://en.wikipedia.org/wiki/Octane_rating - https://en.wikipedia.org/wiki/Octane_rating

VIEWER WARNING - Mr. Cadogan is from Australia and does not share views about certain popular (or not so popular, depending on your political perspective) figures in the USA.  Someone with a more conservative orientation may have hurt feelings or be offended.  So, if you may be offended DO NOT WATCH THE VIDEO.

That was a pretty funny video lostagain. BUT, it was mostly just a dissertation of the guy's opinions.

Here is another video that does more than just spout an opinion or two. It does show pretty convincingly that the octane rating will make a difference... DEPENDING on the engine. Pedestrian engines designed to run on lower octane fuel see no advantage. Higher performance engines (High compression and/or turbo or super charging) can see significant advantages: https://www.youtube.com/watch?v=gQghB4asSnI - https://www.youtube.com/watch?v=gQghB4asSnI

What has me confused is when you go to the other side of Nowhere, in exotic places like UT and western CO you see octane ratings in the 84 and 85 range as the basic regular and 87 is a premium level.  I understand that altitude affects the octane rating but I live at the same basic altitude, and we only see 87.  So when I'm in UT do I need 85 or 87?  Beats me.  I put in 87 in the last place I gassed up in UT on US50 and when I got to civilization in just inside NV, near Baker, 87 was once again the "regular" gas.  WTF, did I get bamboozled once again by Phillips 66?  There was absolutely no discernible difference in the power and my mileage sucked because of a horrible headwind all afternoon crossing UT.  Once in civilization in NV it was like unicorns were pulling my truck in the morning sun.

There is no more energy in higher octane fuel. It is not more powerful. It is more resistant to compression ignition, or detonation. 

You get no more power from using higher octane fuel if your engine is not tuned for it. If your engine IS tuned for higher octane, then using lower octane will usually cost you power as most modern engine control modules will compensate for lower octane by retarding ignition. 

As for the grading method, it was the (R+M)/2 at all the pumps we visited on our trip.

In both UT and CO they were using the (R+M)/2 method as is pretty standard in most places I've been.  Thus, my dilema continues. 

I think next time I'll just buy the gas without ethanol.  What I need is an anemometer with a constant graph print out to measure the varying wind velocities, then compare the mileage with the wind factored in.  On second thought, I think I'll just keep cruising along and go slower when I need to.  Eventually, we'll arrive..... somewhere at any rate.   

You really do need to pay attention to the requirements of the engine. For example, the F-150 has four engine options. Two are normally aspirated (or NA); the 3.5L V6 and the 5.0L V8. Two are turbocharged (EcoBoost); the 2.7L V6 and the 3.5L V6. They all accept 87 octane fuel under normal conditions. However, the turbocharged models recommend higher octane for "severe duty", and specifically call out towing as an example.

They don't talk about high altitude 84 or 85 octane and don't state what they mean by "normal conditions" nor "some circumstances."  They do warn against allowing heavy spark knock at high speeds and suggest that it indicates service on the engine is needed.

So the the dilemma persists.  Should one buy 87 in "high altitude" states where they sell 84 or 85 octane gas?  

As for premium grade gas, looks like the Chrysler 4.7L, naturally aspirated engines don't drink it, though it'd be nice if they would clear up their ambiguities on the topic.  Seems to me the best source of octane information is the engine maker, since it knows what the engine design parameters are.

Seriously, though, do you have some reason to disagree with what the Ford and Chrysler folks have published?  

As for my nail bags, their demise relates to high humidity condition in the basement of my house in NJ.  It rotted them.  I had another pair that I bought in 1966 that lasted over 40 years.  I'm getting a new pair from Kline Tools like the old ones.  They should last until I'm 100, if I last that long, especially working on the top of patio roofs.

Originally posted by mcarter Simplify maybe. I have vintage vehicles that run on premium and need a lead additive.

I think you would be fine. The single octane they are talking about is 95 octane (they weren't clear on the method). There may be things you will have to deal with the lead issue. Likewise, the ethanol will be an issue for seals and such.

You can always go to an airport and get some 100LL....

The method I read was that the 95 was Research Method (The R in (R+M)/2). It apparently yields a higher number than the M method. I just read that the equivalent octane rating if were listed as current octane levels are would be about 91 octane.