That doesn't make any sense...running better gas would not make the truck run worse.

 

It might seem odd that fuels with higher octane ratings explode less easily, yet are popularly thought of as more powerful. The misunderstanding is caused by confusing the ability of the fuel to resist compression detonation as opposed to the ability of the fuel to burn (combustion).

A simple explanation is that carbon-carbon bonds contain more energy than carbon-hydrogen bonds. Hence a fuel with a greater number of carbon bonds will carry more energy regardless of the octane rating. A premium motor fuel will often be formulated to have both higher octane as well as more energy. A counter example to this rule is that ethanol blend fuels have a higher octane rating, but carry a lower energy content by volume (per litre or per gallon). This is because ethanol is a partially oxidized hydrocarbon which can be seen by noting the presence of oxygen in the chemical formula: C2H5OH. Note the substitution of the OH hydroxyl group for a H hydrogen which transforms the gas ethane (C2H6) into ethanol. To a certain extent a fuel with a higher carbon ratio will be more dense than a fuel with a lower carbon ratio. Thus it is possible to formulate high octane fuels that carry less energy per liter than lower octane fuels. This is certainly true of ethanol blend fuels (gasohol), however fuels with no ethanol and indeed no oxygen are also possible.

 

I have a few thoughts.

I have no idea what kind of fuel filter you're running. Maybe that has something to do with it. But I doubt it.

Is your Valvoline oil weight the same as the Mobile1 you were running?

No comment on ISR except for my guess any deviation from stock will effect things down the line and only a automotive engineer could really determine what that effect is. A bunch of guys guessing on the internet isn't worth crap.

93 Octane shouldn't negatively affect your gas mileage, although unless you have your compression increased over stock its probably unnecessarily negatively affecting your wallet. {Edit - interesting post, Ian, and possible culprit} Increased octane isn't "better" for your mild engine - it allows a slower more controlled burn for ping/knock/detonation resistance in higher compression engines. If you need it for these reasons, you probably have another issue going on that needs addressed.

My last question is, how are you computing your gas mileage? How do you know all the conditions are identical (road, load, atmospheric) for an apples to apples comparison?

By your numbers, you're coming up 1mpg low. I think there are too many variables outside of a test lab to be able to accurately measure within 1mpg. I'd call it good and not worry about it.

 

I would try replacing the isr, i have heard that it causes gas mileage to go down.

I am currently getting 22-23 combined city/highway.

 

Actually it does. Higher octane gas burns slower, this is what eliminates pinging. However the slower burn can reduce power because some of the explosion happens on the down stroke of the piston, instead of at the top where more power gets generated. This is not true in every case but it does seem to affect some engines.

 

Yeah, wants some good MPG's? drive a motorcycle they get the best, dangerous as hell but they get the best.

 

Looks like you're talking about less than 10% difference.
That can easily be attributed to the switch from synthetic oil, tire pressures, alignment, a brake dragging, or the wrong octane fuel etc etc.

I consistantly get a 10% decrease in fuel mileage in all of my vehicles when gas formulations change for the winter and summer.

 

wow, i just calculated my gas mileage this morning...

i put half a tank in (20 bucks gets me half a tank) in my 3.0L and with a combination of freeway driving and city driving and LOTS OF HILLS!!!

i got 16 mpg! (pretty much 100 miles= half a tank...)


some things i dont get, is i feel like my gas pretty much drops all at once. i was at 80 miles and my guage was telling me i was barely at the quarter tank mark... then 20 miles later, the light is on... go figure. i think since the guage is "curved" that fools me lol.

x2 on the motorcycle, they get what? 50 mpg? maybe more if your not a lead... wrist? haha!

 

I think that's normal fuel economy. I think those who think they're getting better aren't really tracking it accurately or properly. I've tracked mine on fueleconomy.gov for over 2 years now. Anything about 17MPG seems a bit optimistic.

 

i should check out that website.. hmmm.

 

This is totally anecdotal but I could swear I have gotten a little lower gas mileage with premium with different vehicles over the years. Maybe it is because ethanol is added here in CA (which I'm told raises octane but contains less energy). Anyhow, if you ping, try retarding the timing a little to see if that helps and run regular. You may not like it but you could do that for a little bit just to compare gas mileage. If you have to run premium now to keep it from pinging you may have to put up with less gas mileage. That was how it was with my 85 Runner. After a motor, cam, headers and exhaust I had to run premium until I dialed the distrubutor back from where my mechanic set it. Less umph at WOT but less pinging and slightly better mileage. Other vehicles I tried premium just for power. Zilch and a little worse mileage, not enough to be totally sure though.

 

I get crappier gas mileage when running premium WTF?

 

If the car isnt designed for it, it iwll not burn it properly. Compression ratio is higher=higher octane. For instance, I cannot put anythingless than super 93 octane or above in my 84 vette, it will ping and knock like crazy. If I put 93 in my T100 it will run like crap.

 

Higher octane does not mean more energy. The octane rating is describing the fuel burn characteristics. The higher the octane, the more controlled and slow the fuel will burn.

Many cars these days adjust timing based on information from the oxygen and knock sensors. If ping is identified or lean or rich combustion happens, the ECU can compensate by changing the mixture or changing the ignition timing. If ping is detected, then the timing is retarded. This reduces power, but saves the engine.

Our 4 Runners cannot make this type of adjustment. They are designed for 87 octane gas. Thus, they cannot advance timing in response to higher octane. What probably is happening with higher octane is less efficient combustion - the fuel mix is burned over a longer interval, creating the same effect as retarding the timing. That's why higher octane is a problem.

Ethanol mixed in at the low percentage used for emissions controls has only a minor affect on gas mileage. Not 10%. The change to winter blends is a general problem that goes across gas, not just that with ethanol. Ethanol is an octane booster, but 87 octane gas or 93 octance gas is just that. The ethanol content doesn't boost the octane beyond what the mixture is rated at.

Hope this helps,
MadCityRich

 

I honestly don't know how anyone on here is getting some of the numbers they do....

My 2000 Runner with 30,000 miles got 15-16 mpg city.... with 115,000 miles it gets 15-16 mpg city.... has never changed (good maintenance over the years). My friend's has always gotten the same. Both are 3.4 ltr.

I wouldn't worry about the numbers you are getting. To me it seems like it is where it should be.

James

 

I ran super for the last two weeks because of my travel up and down the mountain. Mileage went to crap. Going back to regular after reading this. The 4runner does not have a knock sensor? Wow, I learn something knew every day.

 

New Plugs
Seafoam (per MTL 4-Runner's method)
clean the maf sensor
check PCV
replace Air filter every 10k miles
CLEAN intake manifold

 

It comes from guessing, fuzzy math, and withful thinking. Those three combined will give you a 20+MPG average 3.4l 4Runner.

 

The 4 Runner has a knock sensor. It will cause the ignition to adjust if knocking is detected. However, it will not help adjust timing based on more subtle engine operations. For example, folks running superchargers often report pinging.

If our knock sensors detect knock, they will retard timing to save the engine. However, since our engines are designed for lower octane petrol, there simply is no benefit to running higher octane.

Let's look at an engine that does prefer higher octane. I have a Subaru WRX. The owner's manual states a preference for higher octance gas. Why? A turbocharger increases the effective compression ratio of the engine. Thus, a more controlled burn is of benefit. Can I drive with lower octane petrol in the WRX? Yes. But engine performance is significantly degraded and it will damage the engine over time. The ECU can only retard timing so far.

So what it looks like is that it is possible to get a performance benefit from higher octane petrol only if the engine is designed for higher octane petrol. For such an engine, you may be able to use lower octane petrol, but only with a performance hit. There is no way to increase performance with higher octane petrol if the engine isn't designed for it. In some cases, the aftermarket supplies performance chips. That may allow for increased performance from higher octane gas. Our stcok 4Runners don't have that option.

Hope this helps. We do have knock sensors.

MadCityRich

 

You DO NOT need to run anything above regular if you're not FI. Forget 93, forget the mid-grade and just run regular. If you think it isn't running right on regular, it's either just in your head or something else is wrong with your rig.

And I don't know if someone's already mentioned this, but in Utah and in a lot of states, they run a winter formula for gas, which increases the ethynol level, causing better emmissions, but poorer gas mileage. That could be a cause as well.

But I'm 100% stock and do 75/25 Highway/City and I'm getting 20-22mpg. Straight highway 2 weeks ago (99% highway), I got 23.5MPG! 2002 Sport Edition 4x4.