View Full Version : Fuelly findings

08-25-2010, 07:15 PM
Just figured I'd share some of my findings as far as fuel octane differences go.

I've been tracking my mpg and such through Fuelly since about Feb this year and have made some cool discoveries. Each fill-up is dated and corresponds to the points on the graphs. Included are any comments I had detailing any significant changes/occurances during the tank. I've been running 91 octane for the entire time, save the last fuel up where I filled up with 87.

As you can see, the upgrades I've made to the Celi thus far have only helped to improve my gas milage. There are 2 decent jumps when I've added something mid-tank.

My other interesting find was the significant decrease in mpg when using a lower-grade fuel. The last tank I filled up with 87 to see what differences it made. Needless to say I'm sticking with 91, unless you guys want me to do some more testing.

08-25-2010, 10:06 PM
Figure out why I average 18mpg lol my friend alltrac gets better milage than my 5s. I can't wait to chuck it. Intersting.findings though

08-26-2010, 12:13 AM
Obviously your mpg will be determined by a few things, namely your driving style, fuel system efficiency and what kind of gas you run.

Myself, I don't really stick to one style of driving through a whole tank. I'll drive all nice and responsible at times, and others I'm running like a bat out of Hell.

Now for the fuel system; I run a bottle of Prestone Fuel System Treatment (grey bottle) pretty much every fill-up and I'll run the tank down to damn near nothing. Over the few months that I've been keeping track you can see how much that has improved my mpg by cleaning out all the build-up in the tank (bigger fuel capacity from 11 gal.-15 gal.) and the rest of the system.

On the fuel, obviously switching back to 87 from 91 made a huge difference, I can't imagine what was going on when I had to run 85 for a few weeks...

08-26-2010, 01:20 AM
I actually think the octane evidence is entirely anecdotal. I am extremely dubious about 87 octane vs. 91 causing gas mileage to decrease. There is no reason (barring some kind of tampering with your ignition timing) that would show any performance improvement using 91, much less increasing gas mileage. All kinds of things can effect a tank of gas. The speed of the pump (too fast and you'll be paying for and thinking you've put in significantly more fuel than you actually get) is one such factor. To actually test this kind of thing you would need to eliminate variables: drive the same distances in the same manner (shift at the same RPM etc.) in the same weather conditions, use the same station and same pump, etc. etc. and even then you don't have a good way of measuring the exact amount of fuel put in and used.

Until I see real evidence to the contrary, I think using any octane higher than the recommended for a car is a waste of money. I do apologize for the incredulity of my post, but this question of using higher octane in a basically stock NA car comes up quite a bit and is somewhat baffling.

Also running your tank that low long term is probably going to be bad for your fuel pump.

08-26-2010, 01:49 AM
Nah, I can understand where you're coming from. I'm not saying that this is definitive proof of anything, just an observation I've made using fuelly to track it.

As far as same route, same gas station, etc., I actually do just that. I'd say about 90% of my driving is the same 10 miles to-and-from work and I almost always use the same pump on base when I fill up. My driving style might vary during a tank, but overall it's consistant. My AC doesn't work, so I don't use it so that kinda eliminates that variable as well. I don't tend to run my tank as low as I used to since I've pretty much cleaned the system out unless I feel I'm running short before payday, but since this tracks by mileage and how much I put in the numbers are consistant.

If you guys want, I'll run a few more tanks of 87 after I burn through my current one of 91, just let me know and I'll keep tracking.

08-26-2010, 03:02 AM
One thing to note on your graph is that you are starting from a cold period of the year and progressing into a warmer period. Typically vehicles will improve fuel mileage in warm weather compared to cold weather. So this alone will play a part in why the fuel mileage increased, beyond any things you were doing to do the same.

Summer blend gasoline has a higher BTU content with lower volatility than winter blends too.

But, if you look at BTU charts, there is no difference in the BTU (energy output) of gasoline based on octane. They all have the same BTU rating in summer, and they all have the same BTU rating in winter.

And a 1990 Celica GT-S 5S-FE has no way of determining a benefit higher octance has to offer, because it has no sensor capability along these lines. However, 1992 & up Celica 5S-FE's have knock sensors, and higher octane fuel tends to knock less. So a low octane fuel that starts to knock will have the ECU retard the timing a bit. A higher octane fuel therefore can run at a more advanced timing setpoint, which tends to improve combustion efficiency a little, and could improve fuel mileage. So, if you were running a 5S-FE with a knock sensor, I perhaps could understand a slight improvement in fuel mileage with higher octane fuels (although the compression ratios of these engines is low enough that engine knock isn't a significant issue, but it is an issue, otherwise Toyota would not have put knock sensors on these engines starting in 1992).

My 2 cents worth at least.

08-27-2010, 07:54 AM
Don't forget driving at night with the lights up, driving conditions. Winter fuel vs summer fuel (ethanol content). Different gas stations, different pumps, etc.

08-27-2010, 12:00 PM
Joey's right. I know the lights up can make a big difference. On a trip to Tennessee on the way down I ran in the day light all the way and on the way back I ran over half the way in the dark with the lights on. There was about a 1.5 MPG difference in the fuel mileage with the lights up. Hellious drag generation.

08-27-2010, 07:20 PM
C'mon guys, give me a little credit here. I know I ask some 'noob-ish' questions from time-to-time but I know my own car. Most of the stuff you've been bringing up I've already been considering and counted as the norm in my driving (with the shift I work I'm usually driving at the same time everyday and weekend trips are few and far between). The only thing that I didn't really know was the summer/winter fuel differences. Like I stated above, I only fuel up at one station, usually the same pump at the same time of day.

I'll keep tracking this stuff and see what happens as we near winter again, but I don't think my mpg will drop off like when I started. If you guys want me to run a few consecutive tanks of 87 to see what happens just tell me and I'll do it.

I will say though, 1/4 tank into the last 87 I could feel a difference and the mileage reflected it. I know that might only "be in my head" but I stand by it.

08-28-2010, 12:19 AM
I'll say this, I did see something similar with a 2001 Dodge Interpid that I used to have. It had the 2.7L V6. I got several miles to the gallon better fuel economy running plus (89) than regular. One thing that it could have been was that with regular (87), I was getting knock that I could fell but the electronics did and pulled timing. This could reduce the fuel mileage. Could be something like that. My wife now has a 300M with the 3.5L V-6 and I haven't noticed the issue in this car.