Exactly! Thats what GM advertises, 14-18 and thats under ideal cond. like in a controlled area or dyno. if you got a consistent 18mpg , then your ahead of the game. remember, the H3 is always in 4wd.

 

I had read online of people getting 20-22mpg highway Maybe they were 5spds though...

Ya I know that but GM tends to under-rate their power and mileage ratings. Damn.

The fulltime 4x4 is another story though. I think thats absolutely silly (even though it DOES hold heritage to a traditional humvee)

 

Octane (C8H18) is the number 8 from the first ten continuous-chain Alkanes obtained from Petroleum - Methane, Ethane, Propane, Butane, Pentane, Hexane, Heptane, Octane, Nonane, Decane

The gasoline is a mix of Octane and other few components such lead in the old times - for example: The 87 octane gasoline is supposed to contain 87% of Octane and 93 contains 93% of Octane

When it comes to fuel, there is no doubt that a 100% Octane will give you a better mileage but it might burn your valves or pistons due to the additional heat that gets produced inside of the combustion chamber

Almost all low compression gasoline engines are designed for 87 Octane, a typical case of burning valves due to the high octane gasoline, was the Jaguar V12 from the 80s that was designed for 97 Octane and burn the valves on a 97 Octane’s gasoline. The 97 Octane’s gasoline is no longer produced and since the technology has changed a lot since the 80s, you have fewer chances for burning your valves or pistons by using 93 Octane’s gasoline

Always is better to go with the higher recommendations from your manufacturer, if the user’s manual recommends 87 Octane, then you shouldn’t go any higher but if it says 87 to 93, then go with 93 especially in gasoline from Venezuela and Cuba made from a high Sulfur Petroleum. Sulfur causes premature damages to engines due to internal corrosion and if you can’t avoid it, then 7% is less than 13%

 

Did you look to see how old this thread was????????

Higher Octane is a waste of $$$$, the stock PCM tune can do virtually nothing with it.

https://www.hummerforums.com/forum/h...bumping-23221/

 

i dont know much about the overall operation,but the ign/variable valve timing seem to cope with 87 gas and 10:1 compression.
im surprised it cant be tuned better with 93.
just sayin.

 

I have a 2008 Australian H3 where the minimum recommended octane is 91. While the book states this is the minimum, it does not state what octane is "best". Generally we have 91, 95 and 98 available at the bowser. I personally run 95 in a 10% ethanol blend because it is 2 cents per litre cheaper. (average cost around AUS$1.50 /litre - you work it out) The general consensus of opinion here is that a higher octane produces more power and the extra mileage you get is more than offset by the higher price, so use the highest octane suitable for the vehicle. I have never bothered to measure my fuel consumption so cannot verify it with personal experience.
(this thread was already revived so I thought I'd add my 2 cents worth)

 

Maybe higher Octane does not produce better economy, but I believe it gives a generally better scenario for running your car. On the other hand , a better grade full syn oil will give better running and economy.. i noticed this when i switched from 5w30 to 5w40..engine temp went down and i' m probably getting 1-2 mpg more and the I5 sounds nicer.....I use 95 and sometimes even 98 Octane in all my cars..

 

To the point. (For the life of me I can't figure out why people want to debate this same thing over and over and then go throw $$ out the window.)

A factory H3 has its PCM set to burn 87 octane (AKI). It cannot "learn" a different octane grade by itself. While it can adapt to other parameters, octane is not one of them. The fuel trim and spark tables are FIXED for 87, so even with 93, it can do nothing with it.

Unless, and only if, you modify the OEM program with a PCM tune, your stock H3 cannot take advantage of anything over 87.

S. African built H3s also have their PCMs set for 87.

However, we measure octane grade different in the US than AU. the US uses AKI, AU measures by RON. 87 AKI is 91 RON. So my friend, the "best" octane to use is 87AKI/91RON, anything more and you get nothing but less cabbage in your wallet.

This thread is in a H3 section of a Hummer Forums, so while this may not apply to somebody's boat, lawnmower, or airplane, or their cousin's Alfa Romeo, it is a simple fact for these vehicles.

 

Thanks Doc.
It's starting to make sense now. I was wondering why the octane grades you guys talk about were generally less than what we have here.

 

I thought I would post up my two cents on this issue. A lot of people argue there is nothing to gain from higher octane, but what they don't understand is that there is more to just stopping ping and knock. The real reason for higher Octane is because it burns slower and more resistant to burning. While that sounds counter intuitive at first, it starts to make more sense when you really examine that idea. Now when your motor is spinning and burning fuel, low octane burns faster and quick which results in not the most even power stroke causing vibration and of course sometime not push at the last few split seconds. Where as higher octane will burn longer and more consistent through out the entire power stroke which results in a smoother, more powerful stroke of the motor. Thus why some of us claim that the motor runs better on higher octane even though we are not making more power, because we are noticing a smoother running motor that has less parasitic loss in the motor.

Now this is just stipulation on my part, but based of my mustang days, I believe that difference between 89 and 93 is not noticeable in the H3 Hummers because 93 burns too long for I5 motors that we got. And if we want to gain the advantage of that gas we would need to increase the time in the range of 3-5 degrees and we should see about 7 to 13 horses too. Keep in mind I have not done this, but I would be willing to bet on it.