better mileage with higher octane?

Dont worry about calculating anything. Octane rating has no direct relationship to gas mileage. Antiknock properties are the key.

and just what do you think Octane rating a measure of?

The octane rating is, just as I said, a number related to antiknock properties.

It varies from car to car depending how clever the ECU is typically.

European and Japanese cars typically prefer higher octane fuels btw. In the UK I've never seen less than 95RON fuel ( that's 91 in your US PON figures ) and 98 RON is our commonly available top grade ( 94 PON ).

Graham

And good anti-knock properties ( higher octane ) allow the ignition to be advanced which *may* improve engine efficiency. It pretty much invariably improves performance.

Graham

There is qualified truth in your statement. It depends on a number of factors whether better gas mileage will result from higher octane gas. The number one factor being that the engine is designed for higher octane gas in the first place.

Most modern engines have a knock sensor that detunes the engine when it detects pinging, which is usually due to gas with too low an octane rating. When it does this the mileage and performance are compromised. Using higher octane gas in such an engine doesn't necessarily guarantee better gas mileage but rather it corrects the worse gas mileage that resulted from running it on the wrong gas.

So, if your owner's manual says you should be using higher octane gas, it should give better results if you do. If it says you should be using regular, premium gas will most likely be a waste of money.

Whether or not you get better fuel economy with higher octane gasoline is a function of the vehicle's PCM (Powertrain Control Module - aka, the engine computer). If your engine has a knock sensor (or sensors) the PCM may be able to adjust the engine parameters (mostly spark advance) to increase performance and fuel economy when you use premium fuel. I've had a couple of Fords with 5.4L engines. It was claimed that the use of premium fuel would allow for a 5 to 10 HP increase and a 1% to 3% increase in fuel economy. For both of these vehicles I ran premium fuel for a significant period of time (5K+ miles) and compared it to similar periods when I was using regular fuel. I saw no significant difference in fuel economy. Neither could I tell there was any difference in performance, although I don't think I could detect a 10HP increase without a dynometer. The results may vary for other vehicles. I have noticed that many of the Japanese manufacturers have started specifying premium fuel for their higher cost engines. I assume that the major reason for this is achieving slightly better CAFE numbers. I am confident that most of the vehicles would run OK on regular, perhaps with slightly worse fuel economy and performance (you should check the owners guide).

If you are going for maximum MONEY economy (as opposed to maximum fuel economy), then there is little chance that purchasing premium fuel is a smart decision. I doubt you will ever increase your fuel economy by more than 3% by using premium, and since premium costs at least 5% more than regular, it is a clear loser if you are trying to save money.

A word of caution - you can't really make any determination on the relative fuel economy difference between regular and premium by comparing single tank fulls of gas. Besides the inaccuracy of single tank full fuel economy calculations, you must also consider the actual octane of the fuel mix in your tank and how quickly the PCM adjusts to different fuels. The PCM will not instantaneously advance the timing if you start using premium fuel. It will make the adjustment over a period of time. So if you want to do the comparison, I'd suggest making the measurement over at least 5 tank fulls of each type of fuel. I'd also not include two tank fulls when you are transitioning from one fuel to the other.

Ed

I certainly hope so, since my Volvo dealer insists that I need "supreme" and says exactly that.

By recording the mileage between fillups, and the size of the fillup.

No, there is no truth to this at all. The higher octane ONLY has better antiknock properties.

Now, if you have a knocking problem with your car, and the knock sensor starts delaying the timing, you will get poorer gas mileage. So in that regard, if you HAVE a knocking problem, higher octane gas will give you better acceleration and better mileage. But it would strike me that, unless the car is designed to run on high octane gas, it would be better to fix the problem causing the knocking.

--scott

I expect those hopes to be dashed.

since my Volvo dealer insists that I need "supreme" and

| The octane rating is, just as I said, a number related to antiknock | properties.

Wikipedia says lower octane petrol usually produces less heat , this I find true in BP's RON92 compared with RON97 : both my mitsubishi 4G15P & honda F20A ( original cr 9.2 & 9.6, both have improved cooling via copper wires ) can use RON92 , esp in humid air here & on flat land ( no towing / climbing ), but RON92 produces less heat ( & so torque ), so its 1.56% lower price cannot justify 2-4 % less torque. My brother's mitsubishi 6G72 ( cr is just 9 ) needs just RON92 per owner's manual, but he buys RON97, & I agree as wise.

Precisely. Just think about it -- at one time, octane was increased by adding lead compounds. Why would lead increase a fuel's energy?

...and my wife does what he tells her...

I don't understand your point.

You can't just advance the timing willy nilly. In any case the ECU should control it and any advantage will be gained automatically.

Graham

It doesn't. Energy has nothing to do with it.

High octane fuel has less energy than low octane in any case.

Graham

Most ppl call it an ECU.

It depends on your engine. You need to check the gas mileage between fills to see if there is a difference.

I have a computer controlled Jeep engine in one and it makes absolutely no difference in gas mileage between 87 and 91 octanes. I have another non computer controlled Jeep engine and the gas mileage difference is close to a hundred miles per tank. It loses all top end power so has to be held to the floor on the highway. This just drinks the gas.

Mike

I have personally had vehicles that cost less to run on premium than regular - but that was back when the price difference was significantly less. My daughter's twin cam Neon gets enough better mileage on mid premium to almost pay the difference (at $0.05 per liter difference), and it doesn't ping like it does on regular. Recommendation is premium on that car.

It didn't - but it allowed the engine to extract more of the energy that the fuel contained by running at a more efficient setting.