When does higher octane gas become better value?

Factual Questions
1

As gas prices have risen around here (right now, about 4.45 for 87 octane where I live), the jump between 87 and 89, and between 89 and 93, has remained steady at about 10 cents. When gas was 2.29 for 87, it was 2.39 for 89, and 2.49 for 93.

Now it’s 4.45, 4.55, and 4.65.

Based on common sense, and some very unscientific observation that I got better gas mileage in South Dakota than I expected, where they only sold 89 octane gas, I figure higher octane leads to better mileage.

**At what point does it become cheaper to buy higher octane for the better mileage? **Does anybody have data that says about how much increase you get in mpg by going up to mid or super grade?

2

you figure wrong. Octane rating only denotes the fuel blend’s ability to resist autoignition. Nothing more. Using higher octane fuel in a car that doesn’t require it is a waste of money.

There are probably other variables in play.

3

There are cars out there that get better mileage on premium. Most of them are high performance models. They can run on regular because they have knock sensors that tell the computer to retard the timing on lower-octane gas. bup, read your owner’s manual. If it says to use premium for best results, you may get better mileage on it. If not, and it recommends regular, premium won’t make a difference.

4

The octane rating is an indication of how compressible the gasoline is before it spontaneously combusts. The octane rating says absolutely nothing at all about energy content and mileage. It really irks me that the oil companies market higher octane gas as “premium” when there is absolutely nothing inherently better about it. In fact, it could be much worse for your car.

In a typical car engine, the gasoline and air mixture is compressed. The spark plug then ignites it, and the resulting BANG is what makes your car go. If you use gasoline with too low of an octane rating for your car, what happens is that the gasoline can go BANG all by itself just from the compression. Since this happens earlier than it was supposed to, it’s called predetonation and it is a very bad thing for your car. These days, most cars have knock sensors that allow the engine computer to fiddle with the timing and prevent you from doing any permanent harm to the engine, but using too low of an octane still isn’t a good thing for your engine.

Using too high of an octane is just wasting your money, though. At best, all you are doing is wasting money on gasoline that won’t spontaneously combust at pressures that your engine doesn’t reach anyway. In other words, you are paying extra for something that makes absolutely no difference at all to your engine.

Sometimes though, higher octane gasoline burns more slowly. Since your engine is tuned to a particular octane, the higher octane can make your engine have worse performance. In other words, your mileage can actually go DOWN when using higher octane gasoline. Instead of making it better, you are paying more money to make it worse.

Want to make higher octane gas? Mix it with alcohol. Alcohol has a higher octane rating than gasoline, but it also has significantly less energy in it. Your mileage will go down as your octane rating goes up.

High performance cars often are built around high compression engines. These cars need higher octane gasoline or they’ll experience early detonation (pinging and knocking). This has led to the perception that high octane gas is for high performance cars, and the common misconception that octane equals performance. It doesn’t. A car that needs high octane gas will perform very poorly on low octane gas, but a car that needs low octane gas will not perform better with high octane gas.

Use the octane rating specified in your car’s manual. Anything lower can be harmful, and anything higher will at best be wasting money, and at worst will actually decrease performance while wasting money.

5

Thought of this after I hit submit, but some cars are designed to run on different octane ratings. My Cadillac is one of them. The manual specifies 89 or higher octane, with 91 preferred. The car can run on 89 with no fear of damage, but it will get better mileage on 91. When I figured out the cost per mile though, it was cheaper to run it on 89. The mileage difference was smaller than the cost difference. YMMV (literally).

It’s important to note that this only works with some cars, though. Our old Toyota Camry got its best mileage at 89 octane. When we put 91 in it, the mileage went down.

Things like tail winds and subtle changes in altitude can affect your mileage, so if you want to play around with your octane ratings it is best to average it over a longer time on an identical route (to and from work, for example). You should also note that gasoline with ethanol mixed in it has a lower energy content and will make your mileage go down. 89 octane gasoline without ethanol will give you better mileage than 89 octane with ethanol. The pumps will usually say “contains 15% ethanol” or some such if ethanol has been added.

It’s quite possible that you are used to a 15% ethanol blend, and you got better mileage in South Dakota because you happened to stop at pumps that didn’t have ethanol blends. It’s also possible that your car is designed for 89 octane and runs poorly on 87. Check your manual.

6

I always get great mileage in the Dakotas because they’re big flat states with wide open roads and no reason to stop.

7

engineer_comp_geek brings up alcohol, which is one of the variables that might be in play here. If you’re normally using regular, and the blend(s) you usually fill up with contain a notable percentage of ethanol, then you might see a noticeable increase in mpg if you then fill up with premium that doesn’t contain much ethanol.

8

The master provides some perspective on the topic!

9

missed the edit window- of course, had I read carefully enough, I would have seen that engineer_comp_geek already said that… go me!

10

I’ve also read from unreliable sources that higher octane fuels can increase carbon buildup. It’s also not so uncommon for carbureted vehicles to develop carbon buildup naturally which can make them detonate with lower octane gas and reduce the economy. The difference is usually pretty dramatic though.

11

If your car has no engine knock using 87 octane and your manual says it’s okay, then use that fuel. Some manuals specify 90 octane or higher meaning you would have to pay for the 93 octane but there’s a simply way around this and save money.

Fill your tank with 50% 89 and 50% 93 octane and in your area you end up with 91 octane at $4.55 per gal.

12

I’m not sure this is true anymore, but didn’t some gas companies used to put extra detergents and other stuff in their high-octane/premium gas? Not that this would directly/immediately affect the mileage, I’m just wondering if this is still true, or even if it ever was. If so, this would be some slight ROI from buying the higher octane even when your car doesn’t need it.