So I've been back to premium for the last 3-4 fillups since my 26MPG blues, and I've gotten:

38.4MPG
39.1MPG
38.9MPG

I always shift before 3k except getting on the highway, which is a fairly consistent amount of times every tank, 90% of my driving is work/school so the routes, acceleration, and speed maintained is relatively similar.

My gas mileage has been up from my typical ~35 with premium since I've been using DFCO, increased tire PSI to 45 (hot-95F) and other economy-friendly driving style changes like coasting and drafting, for instance, I coast a total estimate of 5 miles per week; getting off highways.

I don't believe it's the higher octane rating, but lower grade just gets consistently worse mileage for me, so I don't know what to think, I'm just going to continue using it.

 

it's been my experience that i get better mileage on 87 octane than i do on 91 octane. i dont know why, but that's just how it's been going. and i seem to get the best mileage from chevron 87 than other 87s.

i'm running a fujita SRI, and a skunk2 catback, and i recently got 42 mpg in my AT sport fit.

you might want to consider getting an intercooler, because you're so hot! haha but really, give it some time. i averaged 30-ish mpg until i hit 14,000 miles, then it started going up, and now i regularly get 40.

Doug

 

My Fit get 35-39 on regular gas.
The ECU in the Fit cannot advance the timing enough to take advantage of higher octane fuel.
The knock sensor can retard timing if anything less than regular is run.
So octane boost (as always) is a complete waste of money in any engine that cannot take advantage of it.
This has been said over and over for the last 30 years.............
I have done dyno time with bikes and cars. Nearly all make more power on regular gas!

 

Is a new intake suppose to increase fuel efficiency?

 

correct me if i'm wrong, but i believe the theory is that a new intake allows the engine to breathe more easily, allowing more efficiency and increased power. i get better mileage than i did with the stock intake, i know that.

Doug

 

I was getting around 36-39MPG sticking with 94+boost since my last post, switched back to regular for my last tank and pitfalled to 28MPG--last tank of premium I had 38MPG and I had particularly maintained a same route with 90% highway 10% city for both the premium and regular tanks.

With the same route and all shifts at no more than 4k RPM for both tanks I think a 10MPG difference is way too much to only account my driving style for. I am not driving nancy with premium and harsh with regular, I have the same commutes and drive them the same, and even cut out my leisure driving entirely for the past two weeks--using my Corolla instead. (in contrast, the corolla loves regular for MPG but seems to appreciate higher octane when idling or with A/C)

I'm not complaining about my premium gas MPG, but why is that so much higher?
Does anyone know why my Fit seems to get such poor mileage on regular?

 

That's the weirdest thing I ever heard of. I tried using 93 for three tanks and just ended up wasting a couple of coffee's worth of money for no MPG gain

 

Isn't it strange? I'm back to premium and getting 35MPG+ again with very similar routes, speeds, shift times, etc.

 

if you switch tank after tank from 95 to 87.. your ecu has to relearn all the time.. I think your just confusing the ECU playing flip flop games with octane.

if the car doesnt require 95 why put it in? its just a waste

unless you got boost? put the higher octane in.

 

So what do you suggest--fill up with 87 consistently for a few fillups in a row to see if the MPG goes up?

 

you would get different results..

Maybe run 3-4 tanks and pull an avg on 87
and do the same thing with your 95 + octane.

But its ALL gonna be off because you dont drive the same path everyday.. You wait longer at some lights vs others.. You might push you foot down more than the day before..

People look too much into MPG.. Its more about how grandma'ish you drive to get good mpg..

its all a waste of time.. just drive it.. its a car... point a to b and c and home.. who cares about the rest.

 

I'll try that, will update with results.

I agree about the MPG variation, but there are only four stop lights on my 26 mile round trip daily commute, hence there isn't much room for variation in my last comparison since I cut out all other driving except my daily work commute.

After the stringent driving comparisons between 87 and 95 I'm going to stop bothering with MPG and just stick to whichever gas gives me better MPG. I'm not going to worry about MPG too much after that.

 

I don't see how higher octane is going to give you more miles per gallon. The ecu was tuned to run on 87. Octane is just a measure of resistance to detonation and higher octane ratings means it just burns slower then a lower octane rating. If you put a higher octane that 87 in your fit, the ecu will automatically retard the timing to match the slower burning, higher octane fuel.

 

if it only costs you 1$ a gallon why do you care what octane you put in? ... ;P

 

Here's what you should do, unplug the battery for 15 minutes so the ECU will reset itself. After that run 87 for about 5 tanks and it should have adjusted itself properly. Mileage will be shitty for the first few tanks because the ECU is still relearning.

 

Weird... my gas mileage, after having fun w/ the first tank, has gone up to about 35 (was getting about 30 before). This is with more conservative driving, but I don't think the intake had much to do with it.

-Mike

 

the idea is correct, but most people have a tendency to drive harder without realizing it when they have put on a performance part.

have you seen the tr1? its a freaking monster. the fits engine must feel like sticking your head out the window while going 90mph and trying to breath normally.

 

outside temps can play a big role to.
humidity etc.

 

Was your manifold installation done well" Were the ports matched? Gaskets trimmed and flushed? Was the manifold polished? These things have a monster effect on the air flow. The amount of air and fuel are established by the cylinders sucking in on the intake stroke. The amount of air/fuel that actually gets in is set by the resistance of flow into the cylinders, primarily the intake valves but also the manifold flow resistance. If your manifold is not matched, flowed and ported the engine may have to work harder to produce the needed power, whatever that may be. And thats poorer mpg. You say increased performance. the more power the less mpg. And...
And of course there's always injector problemstoo.
Out of curiosity did the manifold supplier claim more power and nothing of economy? Many manifolds designed for increased power with better flow at max rpm actually don't flow as well at lesser rpm; manifold design is a very tricky excercise. Your mpg easily can be the result of all of these.
Seems I remember some SS racer manifolds that we flowed didn't get as good street mileage. Wish I had documented that but then it didn't matter.
Last, but not least, you should go back to the high test gas for a fair assessment.

Good luck.

 

Octane boost is a complete waste, and not only that, it can do damage to your car. My previous car was a 2006 Mustang GT. I had just installed a custom Tune from a mail order speed shop on my car, and had detonation. So I decided to put in some 104+, and I got a CEL. I brought the car to a local dyno, and after a single run he noticed that the spark was going out at higher RPM's (misfire) He pulled the plugs, and found a residue that was the same color as the octane boost. Popped new plugs in, and the misfire was gone. My O2 sensors were also taking longer to go into closed loop as well, which may also have been from the octane boost gumming them up. Do a google search on Octane Boost, it is junk. Most of them don't even raise the octane a single point. As far as using Higher octane than what is needed, all this does is waste money. Higher the octane the slower the burn, so unburnt fuel is being wasted through the tailpipe.